[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            lo[ii] += o4_t(float4(pv4[cc*NE*NS20/4 + ii*NL])*float4(sst[cc*NE]));
                        }
                    }
                } else {
                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        device const vd4_t * pv4 = (device const vd4_t *) (v + ((ic + NE*cc + ty)*args.nb21));

                        FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            v4_t mv;
                            deq_v_t4(pv4 + i/nl_v, i%nl_v, mv);

                            lo[ii] += o4_t(float4(mv)*float4(ss[NE*cc + ty]));
                        }
                    }
                }

                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    if (NE > 1) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0], 16);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1], 16);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2], 16);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3], 16);
                    }

                    if (NE > 2) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  8);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  8);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  8);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  8);
                    }

                    if (NE > 4) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  4);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  4);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  4);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  4);
                    }

                    if (NE > 8) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  2);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  2);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  2);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  2);
                    }

                    if (NE > 16) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  1);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  1);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            lo[ii] += o4_t(float4(pv4[cc*NE*NS20/4 + ii*NL])*float4(sst[cc*NE]));
                        }
                    }
                } else {
                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        device const vd4_t * pv4 = (device const vd4_t *) (v + ((ic + NE*cc + ty)*args.nb21));

                        FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            v4_t mv;
                            deq_v_t4(pv4 + i/nl_v, i%nl_v, mv);

                            lo[ii] += o4_t(float4(mv)*float4(ss[NE*cc + ty]));
                        }
                    }
                }

                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    if (NE > 1) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0], 16);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1], 16);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2], 16);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3], 16);
                    }

                    if (NE > 2) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  8);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  8);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  8);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  8);
                    }

                    if (NE > 4) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  4);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  4);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  4);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  4);
                    }

                    if (NE > 8) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  2);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  2);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  2);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  2);
                    }

                    if (NE > 16) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  1);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  1);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            lo[ii] += o4_t(float4(pv4[cc*NE*NS20/4 + ii*NL])*float4(sst[cc*NE]));
                        }
                    }
                } else {
                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        device const vd4_t * pv4 = (device const vd4_t *) (v + ((ic + NE*cc + ty)*args.nb21));

                        FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            v4_t mv;
                            deq_v_t4(pv4 + i/nl_v, i%nl_v, mv);

                            lo[ii] += o4_t(float4(mv)*float4(ss[NE*cc + ty]));
                        }
                    }
                }

                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    if (NE > 1) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0], 16);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1], 16);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2], 16);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3], 16);
                    }

                    if (NE > 2) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  8);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  8);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  8);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  8);
                    }

                    if (NE > 4) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  4);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  4);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  4);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  4);
                    }

                    if (NE > 8) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  2);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  2);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  2);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  2);
                    }

                    if (NE > 16) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  1);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  1);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            lo[ii] += o4_t(float4(pv4[cc*NE*NS20/4 + ii*NL])*float4(sst[cc*NE]));
                        }
                    }
                } else {
                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        device const vd4_t * pv4 = (device const vd4_t *) (v + ((ic + NE*cc + ty)*args.nb21));

                        FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            v4_t mv;
                            deq_v_t4(pv4 + i/nl_v, i%nl_v, mv);

                            lo[ii] += o4_t(float4(mv)*float4(ss[NE*cc + ty]));
                        }
                    }
                }

                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    if (NE > 1) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0], 16);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1], 16);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2], 16);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3], 16);
                    }

                    if (NE > 2) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  8);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  8);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  8);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  8);
                    }

                    if (NE > 4) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  4);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  4);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  4);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  4);
                    }

                    if (NE > 8) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  2);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  2);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  2);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  2);
                    }

                    if (NE > 16) {
                        lo[ii][0] += simd_shuffle_down(lo[ii][0],  1);
                        lo[ii][1] += simd_shuffle_down(lo[ii][1],  1);
                        lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }

                if ((DV4/NL % NW == 0) || ty == 0) {
                    FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                        so4[ii*NL] += lo[ii];
                    }
                }
            }
        }

        if (FC_flash_attn_ext_vec_has_sinks && sgitg == 0 && iwg == 0) {
            const float m = M;
            const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

            M = simd_max(max(M, s));

            const float ms = exp(m - M);
            const float vs = exp(s - M);

            S = S*ms + simd_sum(vs);

            if ((DV4/NL % NW == 0) || ty == 0) {
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    so4[ii*NL] *= ms;
                }
            }
        }

        if (tiisg == 0) {
            ss[0] = (s_t) S;
            ss[1] = (s_t) M;
        }
    }

    so4 -= tiisg;

    threadgroup_barrier(mem_flags::mem_threadgroup);

    for (short r = NSG/2; r > 0; r >>= 1) {
        if (sgitg < r) {
            const float S0 = ss[           0];
            const float S1 = ss[r*(SH/2) + 0];

            const float M0 = ss[           1];
            const float M1 = ss[r*(SH/2) + 1];

            const float M = max(M0, M1);

            const float ms0 = exp(M0 - M);
            const float ms1 = exp(M1 - M);

            const float S = S0*ms0 + S1*ms1;

            if (tiisg == 0) {
                ss[0] = S;
                ss[1] = M;
            }

            for (short i = tiisg; i < DV4; i += NW) {
                so4[i] = so4[i]*ms0 + so4[i + r*PV4]*ms1;
            }
        }

        threadgroup_barrier(mem_flags::mem_threadgroup);
    }

    if (sgitg == 0) {
        const int64_t nrows = args.ne3*args.ne2*args.ne1;
        const int64_t rid   = iq3*args.ne2*args.ne1 + iq2 + iq1*args.ne1;

        device float4 * dst4 = (device float4 *) dst;
        device float  * dst1 = (device float  *) dst + nrows*DV*NWG;

        const float S = NWG == 1 ? (ss[0] == 0.0f ? 0.0f : 1.0f/ss[0]) : 1.0f;

        for (short i = tiisg; i < DV4; i += NW) {
            dst4[rid*DV4*NWG + NWG*i + iwg] = (float4) so4[i]*S;
        }

        if (NWG > 1) {
            if (tiisg == 0) {
                dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }

                if ((DV4/NL % NW == 0) || ty == 0) {
                    FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                        so4[ii*NL] += lo[ii];
                    }
                }
            }
        }

        if (FC_flash_attn_ext_vec_has_sinks && sgitg == 0 && iwg == 0) {
            const float m = M;
            const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

            M = simd_max(max(M, s));

            const float ms = exp(m - M);
            const float vs = exp(s - M);

            S = S*ms + simd_sum(vs);

            if ((DV4/NL % NW == 0) || ty == 0) {
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    so4[ii*NL] *= ms;
                }
            }
        }

        if (tiisg == 0) {
            ss[0] = (s_t) S;
            ss[1] = (s_t) M;
        }
    }

    so4 -= tiisg;

    threadgroup_barrier(mem_flags::mem_threadgroup);

    for (short r = NSG/2; r > 0; r >>= 1) {
        if (sgitg < r) {
            const float S0 = ss[           0];
            const float S1 = ss[r*(SH/2) + 0];

            const float M0 = ss[           1];
            const float M1 = ss[r*(SH/2) + 1];

            const float M = max(M0, M1);

            const float ms0 = exp(M0 - M);
            const float ms1 = exp(M1 - M);

            const float S = S0*ms0 + S1*ms1;

            if (tiisg == 0) {
                ss[0] = S;
                ss[1] = M;
            }

            for (short i = tiisg; i < DV4; i += NW) {
                so4[i] = so4[i]*ms0 + so4[i + r*PV4]*ms1;
            }
        }

        threadgroup_barrier(mem_flags::mem_threadgroup);
    }

    if (sgitg == 0) {
        const int64_t nrows = args.ne3*args.ne2*args.ne1;
        const int64_t rid   = iq3*args.ne2*args.ne1 + iq2 + iq1*args.ne1;

        device float4 * dst4 = (device float4 *) dst;
        device float  * dst1 = (device float  *) dst + nrows*DV*NWG;

        const float S = NWG == 1 ? (ss[0] == 0.0f ? 0.0f : 1.0f/ss[0]) : 1.0f;

        for (short i = tiisg; i < DV4; i += NW) {
            dst4[rid*DV4*NWG + NWG*i + iwg] = (float4) so4[i]*S;
        }

        if (NWG > 1) {
            if (tiisg == 0) {
                dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }

                if ((DV4/NL % NW == 0) || ty == 0) {
                    FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                        so4[ii*NL] += lo[ii];
                    }
                }
            }
        }

        if (FC_flash_attn_ext_vec_has_sinks && sgitg == 0 && iwg == 0) {
            const float m = M;
            const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

            M = simd_max(max(M, s));

            const float ms = exp(m - M);
            const float vs = exp(s - M);

            S = S*ms + simd_sum(vs);

            if ((DV4/NL % NW == 0) || ty == 0) {
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    so4[ii*NL] *= ms;
                }
            }
        }

        if (tiisg == 0) {
            ss[0] = (s_t) S;
            ss[1] = (s_t) M;
        }
    }

    so4 -= tiisg;

    threadgroup_barrier(mem_flags::mem_threadgroup);

    for (short r = NSG/2; r > 0; r >>= 1) {
        if (sgitg < r) {
            const float S0 = ss[           0];
            const float S1 = ss[r*(SH/2) + 0];

            const float M0 = ss[           1];
            const float M1 = ss[r*(SH/2) + 1];

            const float M = max(M0, M1);

            const float ms0 = exp(M0 - M);
            const float ms1 = exp(M1 - M);

            const float S = S0*ms0 + S1*ms1;

            if (tiisg == 0) {
                ss[0] = S;
                ss[1] = M;
            }

            for (short i = tiisg; i < DV4; i += NW) {
                so4[i] = so4[i]*ms0 + so4[i + r*PV4]*ms1;
            }
        }

        threadgroup_barrier(mem_flags::mem_threadgroup);
    }

    if (sgitg == 0) {
        const int64_t nrows = args.ne3*args.ne2*args.ne1;
        const int64_t rid   = iq3*args.ne2*args.ne1 + iq2 + iq1*args.ne1;

        device float4 * dst4 = (device float4 *) dst;
        device float  * dst1 = (device float  *) dst + nrows*DV*NWG;

        const float S = NWG == 1 ? (ss[0] == 0.0f ? 0.0f : 1.0f/ss[0]) : 1.0f;

        for (short i = tiisg; i < DV4; i += NW) {
            dst4[rid*DV4*NWG + NWG*i + iwg] = (float4) so4[i]*S;
        }

        if (NWG > 1) {
            if (tiisg == 0) {
                dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
lo[ii][2] += simd_shuffle_down(lo[ii][2],  1);
                        lo[ii][3] += simd_shuffle_down(lo[ii][3],  1);
                    }
                }

                if ((DV4/NL % NW == 0) || ty == 0) {
                    FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                        so4[ii*NL] += lo[ii];
                    }
                }
            }
        }

        if (FC_flash_attn_ext_vec_has_sinks && sgitg == 0 && iwg == 0) {
            const float m = M;
            const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

            M = simd_max(max(M, s));

            const float ms = exp(m - M);
            const float vs = exp(s - M);

            S = S*ms + simd_sum(vs);

            if ((DV4/NL % NW == 0) || ty == 0) {
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    so4[ii*NL] *= ms;
                }
            }
        }

        if (tiisg == 0) {
            ss[0] = (s_t) S;
            ss[1] = (s_t) M;
        }
    }

    so4 -= tiisg;

    threadgroup_barrier(mem_flags::mem_threadgroup);

    for (short r = NSG/2; r > 0; r >>= 1) {
        if (sgitg < r) {
            const float S0 = ss[           0];
            const float S1 = ss[r*(SH/2) + 0];

            const float M0 = ss[           1];
            const float M1 = ss[r*(SH/2) + 1];

            const float M = max(M0, M1);

            const float ms0 = exp(M0 - M);
            const float ms1 = exp(M1 - M);

            const float S = S0*ms0 + S1*ms1;

            if (tiisg == 0) {
                ss[0] = S;
                ss[1] = M;
            }

            for (short i = tiisg; i < DV4; i += NW) {
                so4[i] = so4[i]*ms0 + so4[i + r*PV4]*ms1;
            }
        }

        threadgroup_barrier(mem_flags::mem_threadgroup);
    }

    if (sgitg == 0) {
        const int64_t nrows = args.ne3*args.ne2*args.ne1;
        const int64_t rid   = iq3*args.ne2*args.ne1 + iq2 + iq1*args.ne1;

        device float4 * dst4 = (device float4 *) dst;
        device float  * dst1 = (device float  *) dst + nrows*DV*NWG;

        const float S = NWG == 1 ? (ss[0] == 0.0f ? 0.0f : 1.0f/ss[0]) : 1.0f;

        for (short i = tiisg; i < DV4; i += NW) {
            dst4[rid*DV4*NWG + NWG*i + iwg] = (float4) so4[i]*S;
        }

        if (NWG > 1) {
            if (tiisg == 0) {
                dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
}
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = v_src[i];
            }
        }
    }

    if (FC_flash_attn_ext_pad_has_mask) {
        if (i2 < args.ne32 && i3 < args.ne33) {
            for (int ib = i1; ib < args.ne31; ib += C) {
                device const half * mask_src = (device const half *)(mask      + args.nb31*ib + args.nb32*i2 + args.nb33*i3) + ic0;
                device       half * mask_dst = (device       half *)(mask_pad) + C*ib + C*args.ne31*i2 + C*args.ne31*args.ne32*i3;

                for (int i = tiitg; i < C; i += ntg.x) {
                    if (i >= icp) {
                        mask_dst[i] = -MAXHALF;
                    } else {
                        mask_dst[i] = mask_src[i];
                    }
                }
            }
        }
    }
}

constant int32_t FC_flash_attn_ext_blk_nqptg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 24)]];
constant int32_t FC_flash_attn_ext_blk_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 25)]];

// DS4 FlashAttention mask scan: marks blocks so the non-vector kernel can skip
// blocks that are entirely masked or entirely zero.
kernel void kernel_flash_attn_ext_blk(
        constant ds4_metal_args_flash_attn_ext_blk & args,
        device const char * mask,
        device       char * dst,
        uint3  tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]]) {
    const int32_t Q = FC_flash_attn_ext_blk_nqptg;
    const int32_t C = FC_flash_attn_ext_blk_ncpsg;

    constexpr short NW  = N_SIMDWIDTH;

    const int32_t i3 = tgpig[2]/args.ne32;
    const int32_t i2 = tgpig[2]%args.ne32;
    const int32_t i1 = tgpig[1];
    const int32_t i0 = tgpig[0];

    char res = i0*C + C > args.ne30 ? 1 : 0;

    device const half * mask_src = (device const half *) (mask + (i1*Q)*args.nb31 + i2*args.nb32 + i3*args.nb33) + i0*C + tiisg;

    if ((C > NW || Q > 1) && res == 0) {
        half mmin =  MAXHALF;
        half mmax = -MAXHALF;

        FOR_UNROLL (short j = 0; j < Q; ++j) {
            FOR_UNROLL (short ii = 0; ii < C/NW; ++ii) {
                mmin = min(mmin, mask_src[ii*NW]);
                mmax = max(mmax, mask_src[ii*NW]);
            }

            mask_src += args.nb31/2;
        }

        mmin = simd_min(mmin);
        mmax = simd_max(mmax);

        if (mmax > -MAXHALF) {
            if (mmin == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
}
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = v_src[i];
            }
        }
    }

    if (FC_flash_attn_ext_pad_has_mask) {
        if (i2 < args.ne32 && i3 < args.ne33) {
            for (int ib = i1; ib < args.ne31; ib += C) {
                device const half * mask_src = (device const half *)(mask      + args.nb31*ib + args.nb32*i2 + args.nb33*i3) + ic0;
                device       half * mask_dst = (device       half *)(mask_pad) + C*ib + C*args.ne31*i2 + C*args.ne31*args.ne32*i3;

                for (int i = tiitg; i < C; i += ntg.x) {
                    if (i >= icp) {
                        mask_dst[i] = -MAXHALF;
                    } else {
                        mask_dst[i] = mask_src[i];
                    }
                }
            }
        }
    }
}

constant int32_t FC_flash_attn_ext_blk_nqptg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 24)]];
constant int32_t FC_flash_attn_ext_blk_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 25)]];

// DS4 FlashAttention mask scan: marks blocks so the non-vector kernel can skip
// blocks that are entirely masked or entirely zero.
kernel void kernel_flash_attn_ext_blk(
        constant ds4_metal_args_flash_attn_ext_blk & args,
        device const char * mask,
        device       char * dst,
        uint3  tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]]) {
    const int32_t Q = FC_flash_attn_ext_blk_nqptg;
    const int32_t C = FC_flash_attn_ext_blk_ncpsg;

    constexpr short NW  = N_SIMDWIDTH;

    const int32_t i3 = tgpig[2]/args.ne32;
    const int32_t i2 = tgpig[2]%args.ne32;
    const int32_t i1 = tgpig[1];
    const int32_t i0 = tgpig[0];

    char res = i0*C + C > args.ne30 ? 1 : 0;

    device const half * mask_src = (device const half *) (mask + (i1*Q)*args.nb31 + i2*args.nb32 + i3*args.nb33) + i0*C + tiisg;

    if ((C > NW || Q > 1) && res == 0) {
        half mmin =  MAXHALF;
        half mmax = -MAXHALF;

        FOR_UNROLL (short j = 0; j < Q; ++j) {
            FOR_UNROLL (short ii = 0; ii < C/NW; ++ii) {
                mmin = min(mmin, mask_src[ii*NW]);
                mmax = max(mmax, mask_src[ii*NW]);
            }

            mask_src += args.nb31/2;
        }

        mmin = simd_min(mmin);
        mmax = simd_max(mmax);

        if (mmax > -MAXHALF) {
            if (mmin == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
}
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = v_src[i];
            }
        }
    }

    if (FC_flash_attn_ext_pad_has_mask) {
        if (i2 < args.ne32 && i3 < args.ne33) {
            for (int ib = i1; ib < args.ne31; ib += C) {
                device const half * mask_src = (device const half *)(mask      + args.nb31*ib + args.nb32*i2 + args.nb33*i3) + ic0;
                device       half * mask_dst = (device       half *)(mask_pad) + C*ib + C*args.ne31*i2 + C*args.ne31*args.ne32*i3;

                for (int i = tiitg; i < C; i += ntg.x) {
                    if (i >= icp) {
                        mask_dst[i] = -MAXHALF;
                    } else {
                        mask_dst[i] = mask_src[i];
                    }
                }
            }
        }
    }
}

constant int32_t FC_flash_attn_ext_blk_nqptg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 24)]];
constant int32_t FC_flash_attn_ext_blk_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 25)]];

// DS4 FlashAttention mask scan: marks blocks so the non-vector kernel can skip
// blocks that are entirely masked or entirely zero.
kernel void kernel_flash_attn_ext_blk(
        constant ds4_metal_args_flash_attn_ext_blk & args,
        device const char * mask,
        device       char * dst,
        uint3  tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]]) {
    const int32_t Q = FC_flash_attn_ext_blk_nqptg;
    const int32_t C = FC_flash_attn_ext_blk_ncpsg;

    constexpr short NW  = N_SIMDWIDTH;

    const int32_t i3 = tgpig[2]/args.ne32;
    const int32_t i2 = tgpig[2]%args.ne32;
    const int32_t i1 = tgpig[1];
    const int32_t i0 = tgpig[0];

    char res = i0*C + C > args.ne30 ? 1 : 0;

    device const half * mask_src = (device const half *) (mask + (i1*Q)*args.nb31 + i2*args.nb32 + i3*args.nb33) + i0*C + tiisg;

    if ((C > NW || Q > 1) && res == 0) {
        half mmin =  MAXHALF;
        half mmax = -MAXHALF;

        FOR_UNROLL (short j = 0; j < Q; ++j) {
            FOR_UNROLL (short ii = 0; ii < C/NW; ++ii) {
                mmin = min(mmin, mask_src[ii*NW]);
                mmax = max(mmax, mask_src[ii*NW]);
            }

            mask_src += args.nb31/2;
        }

        mmin = simd_min(mmin);
        mmax = simd_max(mmax);

        if (mmax > -MAXHALF) {
            if (mmin == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
}
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = v_src[i];
            }
        }
    }

    if (FC_flash_attn_ext_pad_has_mask) {
        if (i2 < args.ne32 && i3 < args.ne33) {
            for (int ib = i1; ib < args.ne31; ib += C) {
                device const half * mask_src = (device const half *)(mask      + args.nb31*ib + args.nb32*i2 + args.nb33*i3) + ic0;
                device       half * mask_dst = (device       half *)(mask_pad) + C*ib + C*args.ne31*i2 + C*args.ne31*args.ne32*i3;

                for (int i = tiitg; i < C; i += ntg.x) {
                    if (i >= icp) {
                        mask_dst[i] = -MAXHALF;
                    } else {
                        mask_dst[i] = mask_src[i];
                    }
                }
            }
        }
    }
}

constant int32_t FC_flash_attn_ext_blk_nqptg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 24)]];
constant int32_t FC_flash_attn_ext_blk_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_BLK + 25)]];

// DS4 FlashAttention mask scan: marks blocks so the non-vector kernel can skip
// blocks that are entirely masked or entirely zero.
kernel void kernel_flash_attn_ext_blk(
        constant ds4_metal_args_flash_attn_ext_blk & args,
        device const char * mask,
        device       char * dst,
        uint3  tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]]) {
    const int32_t Q = FC_flash_attn_ext_blk_nqptg;
    const int32_t C = FC_flash_attn_ext_blk_ncpsg;

    constexpr short NW  = N_SIMDWIDTH;

    const int32_t i3 = tgpig[2]/args.ne32;
    const int32_t i2 = tgpig[2]%args.ne32;
    const int32_t i1 = tgpig[1];
    const int32_t i0 = tgpig[0];

    char res = i0*C + C > args.ne30 ? 1 : 0;

    device const half * mask_src = (device const half *) (mask + (i1*Q)*args.nb31 + i2*args.nb32 + i3*args.nb33) + i0*C + tiisg;

    if ((C > NW || Q > 1) && res == 0) {
        half mmin =  MAXHALF;
        half mmax = -MAXHALF;

        FOR_UNROLL (short j = 0; j < Q; ++j) {
            FOR_UNROLL (short ii = 0; ii < C/NW; ++ii) {
                mmin = min(mmin, mask_src[ii*NW]);
                mmax = max(mmax, mask_src[ii*NW]);
            }

            mask_src += args.nb31/2;
        }

        mmin = simd_min(mmin);
        mmax = simd_max(mmax);

        if (mmax > -MAXHALF) {
            if (mmin == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}

#define FA_TYPES \
           half4,  \
           half4,  \
           half4,  \
    float,         \
    float, float4, \
           float4

#define FA_TYPES_F32 \
           half4,  \
           float4, \
           float4, \
    float,         \
    float, float4, \
           float4

typedef decltype(kernel_flash_attn_ext_vec<FA_TYPES, half4, 1, dequantize_f16_t4, half4, 1, dequantize_f16_t4, 128, 128, 4>) flash_attn_ext_vec_t;

// Host-visible decode FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_vec_f16_dk512_dv512")]]  kernel flash_attn_ext_vec_t kernel_flash_attn_ext_vec<FA_TYPES,     half4,  1, dequantize_f16_t4, half4,  1, dequantize_f16_t4, 512, 512, 1>;

#undef FA_TYPES
#undef FA_TYPES_F32

constant int32_t FC_flash_attn_ext_vec_reduce_DV  [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 0)]];
constant int32_t FC_flash_attn_ext_vec_reduce_NWG [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 1)]];

// Reduces split-K decode FlashAttention partials. It combines each workgroup's
// output vector and softmax (sum,max) pair into the final attention result.
kernel void kernel_flash_attn_ext_vec_reduce(
        constant ds4_metal_args_flash_attn_ext_vec_reduce & args,
        device  const char * htmp,
        device        char * dst,
        uint   tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]],
        ushort sgitg[[simdgroup_index_in_threadgroup]]) {
#define NWG (FC_flash_attn_ext_vec_reduce_NWG)
#define DV  (FC_flash_attn_ext_vec_reduce_DV)

    const uint64_t rid = tgpig;

    const short iwg = tiisg;

    device const float  * ss    = (device const float  *) htmp + (uint64_t)args.nrows*DV*NWG;

    float S = ss[rid*(2*NWG) + 2*iwg + 0];
    float M = ss[rid*(2*NWG) + 2*iwg + 1];

    const float m  = simd_max(M);
    const float ms = exp(M - m);

    S = simd_sum(S*ms);
    S = S == 0.0f ? 0.0f : 1.0f/S;

    const short DV4 = DV/4;

    device const float4 * htmp4 = (device const float4 *) htmp + rid*DV4*NWG;
    device       float4 * dst4  = (device       float4 *) dst  + rid*DV4;

    for (short i = sgitg; i < DV4; i += NWG) {
        const float4 v = simd_sum(htmp4[i*NWG + iwg]*ms);

        if (iwg == 0) {
            dst4[i] = v*S;
        }
    }

#undef NWG
#undef DV
}
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}

#define FA_TYPES \
           half4,  \
           half4,  \
           half4,  \
    float,         \
    float, float4, \
           float4

#define FA_TYPES_F32 \
           half4,  \
           float4, \
           float4, \
    float,         \
    float, float4, \
           float4

typedef decltype(kernel_flash_attn_ext_vec<FA_TYPES, half4, 1, dequantize_f16_t4, half4, 1, dequantize_f16_t4, 128, 128, 4>) flash_attn_ext_vec_t;

// Host-visible decode FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_vec_f16_dk512_dv512")]]  kernel flash_attn_ext_vec_t kernel_flash_attn_ext_vec<FA_TYPES,     half4,  1, dequantize_f16_t4, half4,  1, dequantize_f16_t4, 512, 512, 1>;

#undef FA_TYPES
#undef FA_TYPES_F32

constant int32_t FC_flash_attn_ext_vec_reduce_DV  [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 0)]];
constant int32_t FC_flash_attn_ext_vec_reduce_NWG [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 1)]];

// Reduces split-K decode FlashAttention partials. It combines each workgroup's
// output vector and softmax (sum,max) pair into the final attention result.
kernel void kernel_flash_attn_ext_vec_reduce(
        constant ds4_metal_args_flash_attn_ext_vec_reduce & args,
        device  const char * htmp,
        device        char * dst,
        uint   tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]],
        ushort sgitg[[simdgroup_index_in_threadgroup]]) {
#define NWG (FC_flash_attn_ext_vec_reduce_NWG)
#define DV  (FC_flash_attn_ext_vec_reduce_DV)

    const uint64_t rid = tgpig;

    const short iwg = tiisg;

    device const float  * ss    = (device const float  *) htmp + (uint64_t)args.nrows*DV*NWG;

    float S = ss[rid*(2*NWG) + 2*iwg + 0];
    float M = ss[rid*(2*NWG) + 2*iwg + 1];

    const float m  = simd_max(M);
    const float ms = exp(M - m);

    S = simd_sum(S*ms);
    S = S == 0.0f ? 0.0f : 1.0f/S;

    const short DV4 = DV/4;

    device const float4 * htmp4 = (device const float4 *) htmp + rid*DV4*NWG;
    device       float4 * dst4  = (device       float4 *) dst  + rid*DV4;

    for (short i = sgitg; i < DV4; i += NWG) {
        const float4 v = simd_sum(htmp4[i*NWG + iwg]*ms);

        if (iwg == 0) {
            dst4[i] = v*S;
        }
    }

#undef NWG
#undef DV
}
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}

#define FA_TYPES \
           half4,  \
           half4,  \
           half4,  \
    float,         \
    float, float4, \
           float4

#define FA_TYPES_F32 \
           half4,  \
           float4, \
           float4, \
    float,         \
    float, float4, \
           float4

typedef decltype(kernel_flash_attn_ext_vec<FA_TYPES, half4, 1, dequantize_f16_t4, half4, 1, dequantize_f16_t4, 128, 128, 4>) flash_attn_ext_vec_t;

// Host-visible decode FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_vec_f16_dk512_dv512")]]  kernel flash_attn_ext_vec_t kernel_flash_attn_ext_vec<FA_TYPES,     half4,  1, dequantize_f16_t4, half4,  1, dequantize_f16_t4, 512, 512, 1>;

#undef FA_TYPES
#undef FA_TYPES_F32

constant int32_t FC_flash_attn_ext_vec_reduce_DV  [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 0)]];
constant int32_t FC_flash_attn_ext_vec_reduce_NWG [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 1)]];

// Reduces split-K decode FlashAttention partials. It combines each workgroup's
// output vector and softmax (sum,max) pair into the final attention result.
kernel void kernel_flash_attn_ext_vec_reduce(
        constant ds4_metal_args_flash_attn_ext_vec_reduce & args,
        device  const char * htmp,
        device        char * dst,
        uint   tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]],
        ushort sgitg[[simdgroup_index_in_threadgroup]]) {
#define NWG (FC_flash_attn_ext_vec_reduce_NWG)
#define DV  (FC_flash_attn_ext_vec_reduce_DV)

    const uint64_t rid = tgpig;

    const short iwg = tiisg;

    device const float  * ss    = (device const float  *) htmp + (uint64_t)args.nrows*DV*NWG;

    float S = ss[rid*(2*NWG) + 2*iwg + 0];
    float M = ss[rid*(2*NWG) + 2*iwg + 1];

    const float m  = simd_max(M);
    const float ms = exp(M - m);

    S = simd_sum(S*ms);
    S = S == 0.0f ? 0.0f : 1.0f/S;

    const short DV4 = DV/4;

    device const float4 * htmp4 = (device const float4 *) htmp + rid*DV4*NWG;
    device       float4 * dst4  = (device       float4 *) dst  + rid*DV4;

    for (short i = sgitg; i < DV4; i += NWG) {
        const float4 v = simd_sum(htmp4[i*NWG + iwg]*ms);

        if (iwg == 0) {
            dst4[i] = v*S;
        }
    }

#undef NWG
#undef DV
}
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
dst1[rid*(2*NWG) + 2*iwg + 0] = ss[0];
                dst1[rid*(2*NWG) + 2*iwg + 1] = ss[1];
            }
        }
    }

#undef NWG
#undef NSG
#undef NS10
#undef NS20
}

#define FA_TYPES \
           half4,  \
           half4,  \
           half4,  \
    float,         \
    float, float4, \
           float4

#define FA_TYPES_F32 \
           half4,  \
           float4, \
           float4, \
    float,         \
    float, float4, \
           float4

typedef decltype(kernel_flash_attn_ext_vec<FA_TYPES, half4, 1, dequantize_f16_t4, half4, 1, dequantize_f16_t4, 128, 128, 4>) flash_attn_ext_vec_t;

// Host-visible decode FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_vec_f16_dk512_dv512")]]  kernel flash_attn_ext_vec_t kernel_flash_attn_ext_vec<FA_TYPES,     half4,  1, dequantize_f16_t4, half4,  1, dequantize_f16_t4, 512, 512, 1>;

#undef FA_TYPES
#undef FA_TYPES_F32

constant int32_t FC_flash_attn_ext_vec_reduce_DV  [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 0)]];
constant int32_t FC_flash_attn_ext_vec_reduce_NWG [[function_constant(FC_FLASH_ATTN_EXT_VEC_REDUCE + 1)]];

// Reduces split-K decode FlashAttention partials. It combines each workgroup's
// output vector and softmax (sum,max) pair into the final attention result.
kernel void kernel_flash_attn_ext_vec_reduce(
        constant ds4_metal_args_flash_attn_ext_vec_reduce & args,
        device  const char * htmp,
        device        char * dst,
        uint   tgpig[[threadgroup_position_in_grid]],
        ushort tiisg[[thread_index_in_simdgroup]],
        ushort sgitg[[simdgroup_index_in_threadgroup]]) {
#define NWG (FC_flash_attn_ext_vec_reduce_NWG)
#define DV  (FC_flash_attn_ext_vec_reduce_DV)

    const uint64_t rid = tgpig;

    const short iwg = tiisg;

    device const float  * ss    = (device const float  *) htmp + (uint64_t)args.nrows*DV*NWG;

    float S = ss[rid*(2*NWG) + 2*iwg + 0];
    float M = ss[rid*(2*NWG) + 2*iwg + 1];

    const float m  = simd_max(M);
    const float ms = exp(M - m);

    S = simd_sum(S*ms);
    S = S == 0.0f ? 0.0f : 1.0f/S;

    const short DV4 = DV/4;

    device const float4 * htmp4 = (device const float4 *) htmp + rid*DV4*NWG;
    device       float4 * dst4  = (device       float4 *) dst  + rid*DV4;

    for (short i = sgitg; i < DV4; i += NWG) {
        const float4 v = simd_sum(htmp4[i*NWG + iwg]*ms);

        if (iwg == 0) {
            dst4[i] = v*S;
        }
    }

#undef NWG
#undef DV
}
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
in == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
    const int32_t nblk0 = ((args.ne30 + C - 1)/C);

    if (tiisg == 0) {
        dst[((i3*args.ne32 + i2)*nblk1 + i1)*nblk0 + i0] = res;
    }
}

constant bool FC_flash_attn_ext_has_mask  [[function_constant(FC_FLASH_ATTN_EXT + 0)]];
constant bool FC_flash_attn_ext_has_sinks [[function_constant(FC_FLASH_ATTN_EXT + 1)]];
constant bool FC_flash_attn_ext_has_bias  [[function_constant(FC_FLASH_ATTN_EXT + 2)]];
constant bool FC_flash_attn_ext_has_scap  [[function_constant(FC_FLASH_ATTN_EXT + 3)]];
constant bool FC_flash_attn_ext_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT + 4)]];

constant bool FC_flash_attn_ext_bc_mask [[function_constant(FC_FLASH_ATTN_EXT + 10)]];

constant int32_t FC_flash_attn_ext_ns10 [[function_constant(FC_FLASH_ATTN_EXT + 20)]];
constant int32_t FC_flash_attn_ext_ns20 [[function_constant(FC_FLASH_ATTN_EXT + 21)]];
constant int32_t FC_flash_attn_ext_nsg  [[function_constant(FC_FLASH_ATTN_EXT + 22)]];

// DS4 non-vector FlashAttention. The only exported instance uses the model's
// 512-wide F16 K/V rows; keeping the template body generic preserves the same
// arithmetic for dense and compressed-attention prefill.
template<
    typename q_t,
    typename q4_t,
    typename q8x8_t,
    typename k_t,
    typename k4x4_t,
    typename k8x8_t,
    typename v_t,
    typename v4x4_t,
    typename v8x8_t,
    typename qk_t,
    typename qk8x8_t,
    typename s_t,
    typename s2_t,
    typename s8x8_t,
    typename o_t,
    typename o4_t,
    typename o8x8_t,
    typename kd4x4_t,
    short nl_k,
    void (*deq_k)(device const kd4x4_t *, short, thread k4x4_t &),
    typename vd4x4_t,
    short nl_v,
    void (*deq_v)(device const vd4x4_t *, short, thread v4x4_t &),
    short DK,
    short DV,
    short Q,
    short C,
    short NSG>
void kernel_flash_attn_ext_impl(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
        device const char * mask,
        device const char * sinks,
        device const char * pad,
        device const char * blk,
        device       char * dst,
        threadgroup  half * shmem_f16,
        uint3   tgpig,
        ushort  tiisg,
        ushort  sgitg) {
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
in == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
    const int32_t nblk0 = ((args.ne30 + C - 1)/C);

    if (tiisg == 0) {
        dst[((i3*args.ne32 + i2)*nblk1 + i1)*nblk0 + i0] = res;
    }
}

constant bool FC_flash_attn_ext_has_mask  [[function_constant(FC_FLASH_ATTN_EXT + 0)]];
constant bool FC_flash_attn_ext_has_sinks [[function_constant(FC_FLASH_ATTN_EXT + 1)]];
constant bool FC_flash_attn_ext_has_bias  [[function_constant(FC_FLASH_ATTN_EXT + 2)]];
constant bool FC_flash_attn_ext_has_scap  [[function_constant(FC_FLASH_ATTN_EXT + 3)]];
constant bool FC_flash_attn_ext_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT + 4)]];

constant bool FC_flash_attn_ext_bc_mask [[function_constant(FC_FLASH_ATTN_EXT + 10)]];

constant int32_t FC_flash_attn_ext_ns10 [[function_constant(FC_FLASH_ATTN_EXT + 20)]];
constant int32_t FC_flash_attn_ext_ns20 [[function_constant(FC_FLASH_ATTN_EXT + 21)]];
constant int32_t FC_flash_attn_ext_nsg  [[function_constant(FC_FLASH_ATTN_EXT + 22)]];

// DS4 non-vector FlashAttention. The only exported instance uses the model's
// 512-wide F16 K/V rows; keeping the template body generic preserves the same
// arithmetic for dense and compressed-attention prefill.
template<
    typename q_t,
    typename q4_t,
    typename q8x8_t,
    typename k_t,
    typename k4x4_t,
    typename k8x8_t,
    typename v_t,
    typename v4x4_t,
    typename v8x8_t,
    typename qk_t,
    typename qk8x8_t,
    typename s_t,
    typename s2_t,
    typename s8x8_t,
    typename o_t,
    typename o4_t,
    typename o8x8_t,
    typename kd4x4_t,
    short nl_k,
    void (*deq_k)(device const kd4x4_t *, short, thread k4x4_t &),
    typename vd4x4_t,
    short nl_v,
    void (*deq_v)(device const vd4x4_t *, short, thread v4x4_t &),
    short DK,
    short DV,
    short Q,
    short C,
    short NSG>
void kernel_flash_attn_ext_impl(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
        device const char * mask,
        device const char * sinks,
        device const char * pad,
        device const char * blk,
        device       char * dst,
        threadgroup  half * shmem_f16,
        uint3   tgpig,
        ushort  tiisg,
        ushort  sgitg) {
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
in == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
    const int32_t nblk0 = ((args.ne30 + C - 1)/C);

    if (tiisg == 0) {
        dst[((i3*args.ne32 + i2)*nblk1 + i1)*nblk0 + i0] = res;
    }
}

constant bool FC_flash_attn_ext_has_mask  [[function_constant(FC_FLASH_ATTN_EXT + 0)]];
constant bool FC_flash_attn_ext_has_sinks [[function_constant(FC_FLASH_ATTN_EXT + 1)]];
constant bool FC_flash_attn_ext_has_bias  [[function_constant(FC_FLASH_ATTN_EXT + 2)]];
constant bool FC_flash_attn_ext_has_scap  [[function_constant(FC_FLASH_ATTN_EXT + 3)]];
constant bool FC_flash_attn_ext_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT + 4)]];

constant bool FC_flash_attn_ext_bc_mask [[function_constant(FC_FLASH_ATTN_EXT + 10)]];

constant int32_t FC_flash_attn_ext_ns10 [[function_constant(FC_FLASH_ATTN_EXT + 20)]];
constant int32_t FC_flash_attn_ext_ns20 [[function_constant(FC_FLASH_ATTN_EXT + 21)]];
constant int32_t FC_flash_attn_ext_nsg  [[function_constant(FC_FLASH_ATTN_EXT + 22)]];

// DS4 non-vector FlashAttention. The only exported instance uses the model's
// 512-wide F16 K/V rows; keeping the template body generic preserves the same
// arithmetic for dense and compressed-attention prefill.
template<
    typename q_t,
    typename q4_t,
    typename q8x8_t,
    typename k_t,
    typename k4x4_t,
    typename k8x8_t,
    typename v_t,
    typename v4x4_t,
    typename v8x8_t,
    typename qk_t,
    typename qk8x8_t,
    typename s_t,
    typename s2_t,
    typename s8x8_t,
    typename o_t,
    typename o4_t,
    typename o8x8_t,
    typename kd4x4_t,
    short nl_k,
    void (*deq_k)(device const kd4x4_t *, short, thread k4x4_t &),
    typename vd4x4_t,
    short nl_v,
    void (*deq_v)(device const vd4x4_t *, short, thread v4x4_t &),
    short DK,
    short DV,
    short Q,
    short C,
    short NSG>
void kernel_flash_attn_ext_impl(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
        device const char * mask,
        device const char * sinks,
        device const char * pad,
        device const char * blk,
        device       char * dst,
        threadgroup  half * shmem_f16,
        uint3   tgpig,
        ushort  tiisg,
        ushort  sgitg) {
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
in == 0.0 && mmax == 0.0) {
                res = 2;
            } else {
                res = 1;
            }
        }
    }

    const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
    const int32_t nblk0 = ((args.ne30 + C - 1)/C);

    if (tiisg == 0) {
        dst[((i3*args.ne32 + i2)*nblk1 + i1)*nblk0 + i0] = res;
    }
}

constant bool FC_flash_attn_ext_has_mask  [[function_constant(FC_FLASH_ATTN_EXT + 0)]];
constant bool FC_flash_attn_ext_has_sinks [[function_constant(FC_FLASH_ATTN_EXT + 1)]];
constant bool FC_flash_attn_ext_has_bias  [[function_constant(FC_FLASH_ATTN_EXT + 2)]];
constant bool FC_flash_attn_ext_has_scap  [[function_constant(FC_FLASH_ATTN_EXT + 3)]];
constant bool FC_flash_attn_ext_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT + 4)]];

constant bool FC_flash_attn_ext_bc_mask [[function_constant(FC_FLASH_ATTN_EXT + 10)]];

constant int32_t FC_flash_attn_ext_ns10 [[function_constant(FC_FLASH_ATTN_EXT + 20)]];
constant int32_t FC_flash_attn_ext_ns20 [[function_constant(FC_FLASH_ATTN_EXT + 21)]];
constant int32_t FC_flash_attn_ext_nsg  [[function_constant(FC_FLASH_ATTN_EXT + 22)]];

// DS4 non-vector FlashAttention. The only exported instance uses the model's
// 512-wide F16 K/V rows; keeping the template body generic preserves the same
// arithmetic for dense and compressed-attention prefill.
template<
    typename q_t,
    typename q4_t,
    typename q8x8_t,
    typename k_t,
    typename k4x4_t,
    typename k8x8_t,
    typename v_t,
    typename v4x4_t,
    typename v8x8_t,
    typename qk_t,
    typename qk8x8_t,
    typename s_t,
    typename s2_t,
    typename s8x8_t,
    typename o_t,
    typename o4_t,
    typename o8x8_t,
    typename kd4x4_t,
    short nl_k,
    void (*deq_k)(device const kd4x4_t *, short, thread k4x4_t &),
    typename vd4x4_t,
    short nl_v,
    void (*deq_v)(device const vd4x4_t *, short, thread v4x4_t &),
    short DK,
    short DV,
    short Q,
    short C,
    short NSG>
void kernel_flash_attn_ext_impl(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
        device const char * mask,
        device const char * sinks,
        device const char * pad,
        device const char * blk,
        device       char * dst,
        threadgroup  half * shmem_f16,
        uint3   tgpig,
        ushort  tiisg,
        ushort  sgitg) {
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
{
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)

    constexpr short KV   = 8;

    constexpr short DK4  = DK/4;
    constexpr short DK8  = DK/8;
    constexpr short DK16 = DK/16;
    constexpr short DV4  = DV/4;
    constexpr short DV16 = DV/16;

    constexpr short PV   = PAD2(DV, 64);
    constexpr short PV4  = PV/4;
    constexpr short PV8  = PV/8;

    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NQ  = Q/NSG;
    constexpr short SH  = 2*C;

    constexpr short TS = 2*SH;
    constexpr short T  = DK + 2*PV;

    threadgroup q_t  * sq  = (threadgroup q_t  *) (shmem_f16 + 0*T);
    threadgroup q4_t * sq4 = (threadgroup q4_t *) (shmem_f16 + 0*T);
    threadgroup o_t  * so  = (threadgroup o_t  *) (shmem_f16 + 0*T + Q*DK);
    threadgroup o4_t * so4 = (threadgroup o4_t *) (shmem_f16 + 0*T + Q*DK);
    threadgroup s_t  * ss  = (threadgroup s_t  *) (shmem_f16 + Q*T);
    threadgroup s2_t * ss2 = (threadgroup s2_t *) (shmem_f16 + Q*T);

    threadgroup k_t    * sk    = (threadgroup k_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup k4x4_t * sk4x4 = (threadgroup k4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup v_t    * sv    = (threadgroup v_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup v4x4_t * sv4x4 = (threadgroup v4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup half2 * sm2 = (threadgroup half2 *) (shmem_f16 + Q*T + 2*C);

    device const half2 * pm2[NQ];

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        pm2[jj] = (device const half2 *) ((device const char *) mask + (iq1 + j)*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);
    }

    {
        const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
        const int32_t nblk0 = ((args.ne11 + C - 1)/C);

        blk += (((iq3%args.ne33)*args.ne32 + (iq2%args.ne32))*nblk1 + iq1/Q)*nblk0;
    }

    {
        q += iq1*args.nb01 + iq2*args.nb02 + iq3*args.nb03;

        const short ikv2 = iq2/(args.ne02/args.ne_12_2);
        const short ikv3 = iq3/(args.ne03/args.ne_12_3);

        k += ikv2*args.nb12 + ikv3*args.nb13;
        v += ikv2*args.nb22 + ikv3*args.nb23;
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
{
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)

    constexpr short KV   = 8;

    constexpr short DK4  = DK/4;
    constexpr short DK8  = DK/8;
    constexpr short DK16 = DK/16;
    constexpr short DV4  = DV/4;
    constexpr short DV16 = DV/16;

    constexpr short PV   = PAD2(DV, 64);
    constexpr short PV4  = PV/4;
    constexpr short PV8  = PV/8;

    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NQ  = Q/NSG;
    constexpr short SH  = 2*C;

    constexpr short TS = 2*SH;
    constexpr short T  = DK + 2*PV;

    threadgroup q_t  * sq  = (threadgroup q_t  *) (shmem_f16 + 0*T);
    threadgroup q4_t * sq4 = (threadgroup q4_t *) (shmem_f16 + 0*T);
    threadgroup o_t  * so  = (threadgroup o_t  *) (shmem_f16 + 0*T + Q*DK);
    threadgroup o4_t * so4 = (threadgroup o4_t *) (shmem_f16 + 0*T + Q*DK);
    threadgroup s_t  * ss  = (threadgroup s_t  *) (shmem_f16 + Q*T);
    threadgroup s2_t * ss2 = (threadgroup s2_t *) (shmem_f16 + Q*T);

    threadgroup k_t    * sk    = (threadgroup k_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup k4x4_t * sk4x4 = (threadgroup k4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup v_t    * sv    = (threadgroup v_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup v4x4_t * sv4x4 = (threadgroup v4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup half2 * sm2 = (threadgroup half2 *) (shmem_f16 + Q*T + 2*C);

    device const half2 * pm2[NQ];

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        pm2[jj] = (device const half2 *) ((device const char *) mask + (iq1 + j)*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);
    }

    {
        const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
        const int32_t nblk0 = ((args.ne11 + C - 1)/C);

        blk += (((iq3%args.ne33)*args.ne32 + (iq2%args.ne32))*nblk1 + iq1/Q)*nblk0;
    }

    {
        q += iq1*args.nb01 + iq2*args.nb02 + iq3*args.nb03;

        const short ikv2 = iq2/(args.ne02/args.ne_12_2);
        const short ikv3 = iq3/(args.ne03/args.ne_12_3);

        k += ikv2*args.nb12 + ikv3*args.nb13;
        v += ikv2*args.nb22 + ikv3*args.nb23;
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
{
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)

    constexpr short KV   = 8;

    constexpr short DK4  = DK/4;
    constexpr short DK8  = DK/8;
    constexpr short DK16 = DK/16;
    constexpr short DV4  = DV/4;
    constexpr short DV16 = DV/16;

    constexpr short PV   = PAD2(DV, 64);
    constexpr short PV4  = PV/4;
    constexpr short PV8  = PV/8;

    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NQ  = Q/NSG;
    constexpr short SH  = 2*C;

    constexpr short TS = 2*SH;
    constexpr short T  = DK + 2*PV;

    threadgroup q_t  * sq  = (threadgroup q_t  *) (shmem_f16 + 0*T);
    threadgroup q4_t * sq4 = (threadgroup q4_t *) (shmem_f16 + 0*T);
    threadgroup o_t  * so  = (threadgroup o_t  *) (shmem_f16 + 0*T + Q*DK);
    threadgroup o4_t * so4 = (threadgroup o4_t *) (shmem_f16 + 0*T + Q*DK);
    threadgroup s_t  * ss  = (threadgroup s_t  *) (shmem_f16 + Q*T);
    threadgroup s2_t * ss2 = (threadgroup s2_t *) (shmem_f16 + Q*T);

    threadgroup k_t    * sk    = (threadgroup k_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup k4x4_t * sk4x4 = (threadgroup k4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup v_t    * sv    = (threadgroup v_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup v4x4_t * sv4x4 = (threadgroup v4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup half2 * sm2 = (threadgroup half2 *) (shmem_f16 + Q*T + 2*C);

    device const half2 * pm2[NQ];

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        pm2[jj] = (device const half2 *) ((device const char *) mask + (iq1 + j)*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);
    }

    {
        const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
        const int32_t nblk0 = ((args.ne11 + C - 1)/C);

        blk += (((iq3%args.ne33)*args.ne32 + (iq2%args.ne32))*nblk1 + iq1/Q)*nblk0;
    }

    {
        q += iq1*args.nb01 + iq2*args.nb02 + iq3*args.nb03;

        const short ikv2 = iq2/(args.ne02/args.ne_12_2);
        const short ikv3 = iq3/(args.ne03/args.ne_12_3);

        k += ikv2*args.nb12 + ikv3*args.nb13;
        v += ikv2*args.nb22 + ikv3*args.nb23;
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Suggest a minimal correctness-preserving optimization for this excerpt.

File: metal/flash_attn.metal

```metal
{
    const ushort iq3 = tgpig[2];
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0]*Q;

#define NS10 (FC_flash_attn_ext_ns10)
#define NS20 (FC_flash_attn_ext_ns20)

    constexpr short KV   = 8;

    constexpr short DK4  = DK/4;
    constexpr short DK8  = DK/8;
    constexpr short DK16 = DK/16;
    constexpr short DV4  = DV/4;
    constexpr short DV16 = DV/16;

    constexpr short PV   = PAD2(DV, 64);
    constexpr short PV4  = PV/4;
    constexpr short PV8  = PV/8;

    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NQ  = Q/NSG;
    constexpr short SH  = 2*C;

    constexpr short TS = 2*SH;
    constexpr short T  = DK + 2*PV;

    threadgroup q_t  * sq  = (threadgroup q_t  *) (shmem_f16 + 0*T);
    threadgroup q4_t * sq4 = (threadgroup q4_t *) (shmem_f16 + 0*T);
    threadgroup o_t  * so  = (threadgroup o_t  *) (shmem_f16 + 0*T + Q*DK);
    threadgroup o4_t * so4 = (threadgroup o4_t *) (shmem_f16 + 0*T + Q*DK);
    threadgroup s_t  * ss  = (threadgroup s_t  *) (shmem_f16 + Q*T);
    threadgroup s2_t * ss2 = (threadgroup s2_t *) (shmem_f16 + Q*T);

    threadgroup k_t    * sk    = (threadgroup k_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup k4x4_t * sk4x4 = (threadgroup k4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup v_t    * sv    = (threadgroup v_t    *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);
    threadgroup v4x4_t * sv4x4 = (threadgroup v4x4_t *) (shmem_f16 + sgitg*(4*16*KV) + Q*T + Q*TS);

    threadgroup half2 * sm2 = (threadgroup half2 *) (shmem_f16 + Q*T + 2*C);

    device const half2 * pm2[NQ];

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        pm2[jj] = (device const half2 *) ((device const char *) mask + (iq1 + j)*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);
    }

    {
        const int32_t nblk1 = ((args.ne01 + Q - 1)/Q);
        const int32_t nblk0 = ((args.ne11 + C - 1)/C);

        blk += (((iq3%args.ne33)*args.ne32 + (iq2%args.ne32))*nblk1 + iq1/Q)*nblk0;
    }

    {
        q += iq1*args.nb01 + iq2*args.nb02 + iq3*args.nb03;

        const short ikv2 = iq2/(args.ne02/args.ne_12_2);
        const short ikv3 = iq3/(args.ne03/args.ne_12_3);

        k += ikv2*args.nb12 + ikv3*args.nb13;
        v += ikv2*args.nb22 + ikv3*args.nb23;
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Trova bug sottili e spiega quali invarianti dell'inferenza possono rompersi.

File: metal/flash_attn.metal

```metal
st short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
            if (iq1 + j < args.ne01) {
                sq4[j*DK4 + i] = (q4_t) q4[i];
            } else {
                sq4[j*DK4 + i] = 0;
            }
        }
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        for (short i = tiisg; i < DV4; i += NW) {
            so4[j*PV4 + i] = 0;
        }

        for (short i = tiisg; i < SH; i += NW) {
            ss[j*SH + i] = 0.0f;
        }
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    float S[NQ] = { [0 ... NQ-1] = 0.0f };

    {
        float M[NQ] = { [0 ... NQ-1] = -FLT_MAX/2 };

        float slope = 1.0f;

        if (FC_flash_attn_ext_has_bias) {
            const short h = iq2;

            const float base = h < args.n_head_log2 ? args.m0 : args.m1;
            const short exph = h < args.n_head_log2 ? h + 1 : 2*(h - args.n_head_log2) + 1;

            slope = pow(base, exph);
        }

        for (int ic0 = 0; ; ++ic0) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_has_kvpad && ic + C > args.ne11) {
                k    = pad;
                v    = k + args.nb11*C*args.ne_12_2*args.ne_12_3;
                mask = v + args.nb21*C*args.ne_12_2*args.ne_12_3;

                const short ikv2 = iq2/(args.ne02/args.ne_12_2);
                const short ikv3 = iq3/(args.ne03/args.ne_12_3);

                k += (ikv2 + ikv3*args.ne_12_2)*args.nb11*C;
                v += (ikv2 + ikv3*args.ne_12_2)*args.nb21*C;

                if (!FC_flash_attn_ext_has_mask) {
                    threadgroup half * sm = (threadgroup half *) (sm2);

                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        for (short i = tiisg; i < C; i += NW) {
                            if (ic + i >= args.ne11) {
                                sm[2*j*SH + i] = -MAXHALF;
                            }
                        }
                    }
                } else {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Trova bug sottili e spiega quali invarianti dell'inferenza possono rompersi.

File: metal/flash_attn.metal

```metal
st short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
            if (iq1 + j < args.ne01) {
                sq4[j*DK4 + i] = (q4_t) q4[i];
            } else {
                sq4[j*DK4 + i] = 0;
            }
        }
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        for (short i = tiisg; i < DV4; i += NW) {
            so4[j*PV4 + i] = 0;
        }

        for (short i = tiisg; i < SH; i += NW) {
            ss[j*SH + i] = 0.0f;
        }
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    float S[NQ] = { [0 ... NQ-1] = 0.0f };

    {
        float M[NQ] = { [0 ... NQ-1] = -FLT_MAX/2 };

        float slope = 1.0f;

        if (FC_flash_attn_ext_has_bias) {
            const short h = iq2;

            const float base = h < args.n_head_log2 ? args.m0 : args.m1;
            const short exph = h < args.n_head_log2 ? h + 1 : 2*(h - args.n_head_log2) + 1;

            slope = pow(base, exph);
        }

        for (int ic0 = 0; ; ++ic0) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_has_kvpad && ic + C > args.ne11) {
                k    = pad;
                v    = k + args.nb11*C*args.ne_12_2*args.ne_12_3;
                mask = v + args.nb21*C*args.ne_12_2*args.ne_12_3;

                const short ikv2 = iq2/(args.ne02/args.ne_12_2);
                const short ikv3 = iq3/(args.ne03/args.ne_12_3);

                k += (ikv2 + ikv3*args.ne_12_2)*args.nb11*C;
                v += (ikv2 + ikv3*args.ne_12_2)*args.nb21*C;

                if (!FC_flash_attn_ext_has_mask) {
                    threadgroup half * sm = (threadgroup half *) (sm2);

                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        for (short i = tiisg; i < C; i += NW) {
                            if (ic + i >= args.ne11) {
                                sm[2*j*SH + i] = -MAXHALF;
                            }
                        }
                    }
                } else {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Trova bug sottili e spiega quali invarianti dell'inferenza possono rompersi.

File: metal/flash_attn.metal

```metal
st short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
            if (iq1 + j < args.ne01) {
                sq4[j*DK4 + i] = (q4_t) q4[i];
            } else {
                sq4[j*DK4 + i] = 0;
            }
        }
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        for (short i = tiisg; i < DV4; i += NW) {
            so4[j*PV4 + i] = 0;
        }

        for (short i = tiisg; i < SH; i += NW) {
            ss[j*SH + i] = 0.0f;
        }
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    float S[NQ] = { [0 ... NQ-1] = 0.0f };

    {
        float M[NQ] = { [0 ... NQ-1] = -FLT_MAX/2 };

        float slope = 1.0f;

        if (FC_flash_attn_ext_has_bias) {
            const short h = iq2;

            const float base = h < args.n_head_log2 ? args.m0 : args.m1;
            const short exph = h < args.n_head_log2 ? h + 1 : 2*(h - args.n_head_log2) + 1;

            slope = pow(base, exph);
        }

        for (int ic0 = 0; ; ++ic0) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_has_kvpad && ic + C > args.ne11) {
                k    = pad;
                v    = k + args.nb11*C*args.ne_12_2*args.ne_12_3;
                mask = v + args.nb21*C*args.ne_12_2*args.ne_12_3;

                const short ikv2 = iq2/(args.ne02/args.ne_12_2);
                const short ikv3 = iq3/(args.ne03/args.ne_12_3);

                k += (ikv2 + ikv3*args.ne_12_2)*args.nb11*C;
                v += (ikv2 + ikv3*args.ne_12_2)*args.nb21*C;

                if (!FC_flash_attn_ext_has_mask) {
                    threadgroup half * sm = (threadgroup half *) (sm2);

                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        for (short i = tiisg; i < C; i += NW) {
                            if (ic + i >= args.ne11) {
                                sm[2*j*SH + i] = -MAXHALF;
                            }
                        }
                    }
                } else {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Trova bug sottili e spiega quali invarianti dell'inferenza possono rompersi.

File: metal/flash_attn.metal

```metal
st short j = jj*NSG + sgitg;

        device const float4 * q4 = (device const float4 *) ((device const char *) q + j*args.nb01);

        for (short i = tiisg; i < DK4; i += NW) {
            if (iq1 + j < args.ne01) {
                sq4[j*DK4 + i] = (q4_t) q4[i];
            } else {
                sq4[j*DK4 + i] = 0;
            }
        }
    }

    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;

        for (short i = tiisg; i < DV4; i += NW) {
            so4[j*PV4 + i] = 0;
        }

        for (short i = tiisg; i < SH; i += NW) {
            ss[j*SH + i] = 0.0f;
        }
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    float S[NQ] = { [0 ... NQ-1] = 0.0f };

    {
        float M[NQ] = { [0 ... NQ-1] = -FLT_MAX/2 };

        float slope = 1.0f;

        if (FC_flash_attn_ext_has_bias) {
            const short h = iq2;

            const float base = h < args.n_head_log2 ? args.m0 : args.m1;
            const short exph = h < args.n_head_log2 ? h + 1 : 2*(h - args.n_head_log2) + 1;

            slope = pow(base, exph);
        }

        for (int ic0 = 0; ; ++ic0) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_has_kvpad && ic + C > args.ne11) {
                k    = pad;
                v    = k + args.nb11*C*args.ne_12_2*args.ne_12_3;
                mask = v + args.nb21*C*args.ne_12_2*args.ne_12_3;

                const short ikv2 = iq2/(args.ne02/args.ne_12_2);
                const short ikv3 = iq3/(args.ne03/args.ne_12_3);

                k += (ikv2 + ikv3*args.ne_12_2)*args.nb11*C;
                v += (ikv2 + ikv3*args.ne_12_2)*args.nb21*C;

                if (!FC_flash_attn_ext_has_mask) {
                    threadgroup half * sm = (threadgroup half *) (sm2);

                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        for (short i = tiisg; i < C; i += NW) {
                            if (ic + i >= args.ne11) {
                                sm[2*j*SH + i] = -MAXHALF;
                            }
                        }
                    }
                } else {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
                                (iq3%args.ne33)*(C*args.ne31*args.ne32));
                    }
                }

                ic = 0;
            }

            char blk_cur = 1;

            if (FC_flash_attn_ext_has_mask) {
                blk_cur = blk[ic0];

                if (blk_cur == 0) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }

                    continue;
                }

                if (blk_cur == 1) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        if (FC_flash_attn_ext_bc_mask) {
                            sm2[j*SH + tiisg] = (iq1 + j) < args.ne31 ? pm2[jj][tiisg] : half2(-MAXHALF, -MAXHALF);
                        } else {
                            sm2[j*SH + tiisg] = pm2[jj][tiisg];
                        }

                        pm2[jj] += NW;
                    }
                } else if (blk_cur == 2) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }
                }
            }

            if (is_same<kd4x4_t, k4x4_t>::value) {
                device      const k_t * pk = (device const k_t *) (k + ic*args.nb11);
                threadgroup const q_t * pq = sq;
                threadgroup       s_t * ps = ss;

                pk += sgitg*(8*NS10);
                ps += sgitg*(8*1);

                static_assert((C/8) % NSG == 0, "");

                constexpr short NC = (C/8)/NSG;

                FOR_UNROLL (short cc = 0; cc < NC; ++cc) {
                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    if (DK % 16 != 0) {
                        k8x8_t mk;
                        q8x8_t mq;

                        FOR_UNROLL (short i = 0; i < DK8; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mk, pk + 8*i, NS10, 0, true);
                            simdgroup_load(mq, pq + 8*i, DK);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
                                (iq3%args.ne33)*(C*args.ne31*args.ne32));
                    }
                }

                ic = 0;
            }

            char blk_cur = 1;

            if (FC_flash_attn_ext_has_mask) {
                blk_cur = blk[ic0];

                if (blk_cur == 0) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }

                    continue;
                }

                if (blk_cur == 1) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        if (FC_flash_attn_ext_bc_mask) {
                            sm2[j*SH + tiisg] = (iq1 + j) < args.ne31 ? pm2[jj][tiisg] : half2(-MAXHALF, -MAXHALF);
                        } else {
                            sm2[j*SH + tiisg] = pm2[jj][tiisg];
                        }

                        pm2[jj] += NW;
                    }
                } else if (blk_cur == 2) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }
                }
            }

            if (is_same<kd4x4_t, k4x4_t>::value) {
                device      const k_t * pk = (device const k_t *) (k + ic*args.nb11);
                threadgroup const q_t * pq = sq;
                threadgroup       s_t * ps = ss;

                pk += sgitg*(8*NS10);
                ps += sgitg*(8*1);

                static_assert((C/8) % NSG == 0, "");

                constexpr short NC = (C/8)/NSG;

                FOR_UNROLL (short cc = 0; cc < NC; ++cc) {
                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    if (DK % 16 != 0) {
                        k8x8_t mk;
                        q8x8_t mq;

                        FOR_UNROLL (short i = 0; i < DK8; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mk, pk + 8*i, NS10, 0, true);
                            simdgroup_load(mq, pq + 8*i, DK);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];
```<｜Assistant｜></think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
                                (iq3%args.ne33)*(C*args.ne31*args.ne32));
                    }
                }

                ic = 0;
            }

            char blk_cur = 1;

            if (FC_flash_attn_ext_has_mask) {
                blk_cur = blk[ic0];

                if (blk_cur == 0) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }

                    continue;
                }

                if (blk_cur == 1) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        if (FC_flash_attn_ext_bc_mask) {
                            sm2[j*SH + tiisg] = (iq1 + j) < args.ne31 ? pm2[jj][tiisg] : half2(-MAXHALF, -MAXHALF);
                        } else {
                            sm2[j*SH + tiisg] = pm2[jj][tiisg];
                        }

                        pm2[jj] += NW;
                    }
                } else if (blk_cur == 2) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }
                }
            }

            if (is_same<kd4x4_t, k4x4_t>::value) {
                device      const k_t * pk = (device const k_t *) (k + ic*args.nb11);
                threadgroup const q_t * pq = sq;
                threadgroup       s_t * ps = ss;

                pk += sgitg*(8*NS10);
                ps += sgitg*(8*1);

                static_assert((C/8) % NSG == 0, "");

                constexpr short NC = (C/8)/NSG;

                FOR_UNROLL (short cc = 0; cc < NC; ++cc) {
                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    if (DK % 16 != 0) {
                        k8x8_t mk;
                        q8x8_t mq;

                        FOR_UNROLL (short i = 0; i < DK8; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mk, pk + 8*i, NS10, 0, true);
                            simdgroup_load(mq, pq + 8*i, DK);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Review this excerpt for correctness risks, memory lifetime issues, and performance bottlenecks.

File: metal/flash_attn.metal

```metal
pm2[jj] = (device const half2 *) ((device const half *) mask +
                                (iq1 + j)*C +
                                (iq2%args.ne32)*(C*args.ne31) +
                                (iq3%args.ne33)*(C*args.ne31*args.ne32));
                    }
                }

                ic = 0;
            }

            char blk_cur = 1;

            if (FC_flash_attn_ext_has_mask) {
                blk_cur = blk[ic0];

                if (blk_cur == 0) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }

                    continue;
                }

                if (blk_cur == 1) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        const short j = jj*NSG + sgitg;

                        if (FC_flash_attn_ext_bc_mask) {
                            sm2[j*SH + tiisg] = (iq1 + j) < args.ne31 ? pm2[jj][tiisg] : half2(-MAXHALF, -MAXHALF);
                        } else {
                            sm2[j*SH + tiisg] = pm2[jj][tiisg];
                        }

                        pm2[jj] += NW;
                    }
                } else if (blk_cur == 2) {
                    FOR_UNROLL (short jj = 0; jj < NQ; ++jj) {
                        pm2[jj] += NW;
                    }
                }
            }

            if (is_same<kd4x4_t, k4x4_t>::value) {
                device      const k_t * pk = (device const k_t *) (k + ic*args.nb11);
                threadgroup const q_t * pq = sq;
                threadgroup       s_t * ps = ss;

                pk += sgitg*(8*NS10);
                ps += sgitg*(8*1);

                static_assert((C/8) % NSG == 0, "");

                constexpr short NC = (C/8)/NSG;

                FOR_UNROLL (short cc = 0; cc < NC; ++cc) {
                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    if (DK % 16 != 0) {
                        k8x8_t mk;
                        q8x8_t mq;

                        FOR_UNROLL (short i = 0; i < DK8; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mk, pk + 8*i, NS10, 0, true);
                            simdgroup_load(mq, pq + 8*i, DK);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];
```<｜Assistant｜><think>


[System] You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.

[User]
Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];

                        #pragma unroll (MIN(DK8/2, 4*NSG))
                        for (short i = 0; i < DK8/2; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mq[0], pq + 0*8 + 16*i, DK);
                            simdgroup_load(mq[1], pq + 1*8 + 16*i, DK);

                            simdgroup_load(mk[0], pk + 0*8 + 16*i, NS10, 0, true);
                            simdgroup_load(mk[1], pk + 1*8 + 16*i, NS10, 0, true);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq[0], mk[0], mqk);
                            simdgroup_multiply_accumulate(mqk, mq[1], mk[1], mqk);
                        }
                    }

                    simdgroup_store(mqk, ps, SH, 0, false);

                    pk += 8*(NSG*NS10);
                    ps += 8*(NSG);
                }
            } else {
                for (short ccc = 0; ccc < (C/8)/NSG; ++ccc) {
                    const short cc = ccc*NSG + sgitg;

                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    for (short ii = 0; ii < DK16; ii += 4) {
                        device const kd4x4_t * pk4x4 = (device const kd4x4_t *) (k + ((ic + 8*cc + ty)*args.nb11));

                        if (DK16%4 == 0) {
                            {
                                k4x4_t tmp;
                                deq_k(pk4x4 + (ii + tx)/nl_k, (ii + tx)%nl_k, tmp);
                                sk4x4[4*ty + tx] = tmp;
                            }

                            simdgroup_barrier(mem_flags::mem_threadgroup);

                            FOR_UNROLL (short k = 0; k < 4; ++k) {
                                k8x8_t mk;
                                q8x8_t mq;

                                simdgroup_load(mk, sk + 16*k + 0*8, 4*16, 0, true);
                                simdgroup_load(mq, sq + (2*(ii + k) + 0)*8, DK);
                                simdgroup_multiply_accumulate(mqk, mq, mk, mqk);

                                simdgroup_load(mk, sk + 16*k + 1*8, 4*16, 0, true);
                                simdgroup_load(mq, sq + (2*(ii + k) + 1)*8, DK);
```

<｜begin▁of▁sentence｜>You are DeepSeek V4 Flash running locally. Answer accurately, preserve technical details, and use tools only when the prompt asks for tool use.<｜User｜>Explain what this code does and identify the inference stage it belongs to.

File: metal/flash_attn.metal

```metal
simdgroup_multiply_accumulate(mqk, mq, mk, mqk);
                        }
                    } else {
                        k8x8_t mk[2];
                        q8x8_t mq[2];

                        #pragma unroll (MIN(DK8/2, 4*NSG))
                        for (short i = 0; i < DK8/2; ++i) {
                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_load(mq[0], pq + 0*8 + 16*i, DK);
                            simdgroup_load(mq[1], pq + 1*8 + 16*i, DK);

                            simdgroup_load(mk[0], pk + 0*8 + 16*i, NS10, 0, true);
                            simdgroup_load(mk[1], pk + 1*8 + 16*i, NS10, 0, true);

                            simdgroup_barrier(mem_flags::mem_none);

                            simdgroup_multiply_accumulate(mqk, mq[0], mk[0], mqk);
                            simdgroup_multiply_accumulate(mqk, mq[1], mk[1], mqk);
                        }
                    }

                    simdgroup_store(mqk, ps, SH, 0, false);

                    pk += 8*(NSG*NS10);
                    ps += 8*(NSG);
                }
            } else {
                for (short ccc = 0; ccc < (C/8)/NSG; ++ccc) {
                    const short cc = ccc*NSG + sgitg;

                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    qk8x8_t mqk = make_filled_simdgroup_matrix<qk_t, 8>((qk_t) 0.0f);

                    for (short ii = 0; ii < DK16; ii += 4) {
                        device const kd4x4_t * pk4x4 = (device const kd4x4_t *) (k + ((ic + 8*cc + ty)*args.nb11));

                        if (DK16%4 == 0) {
                            {
                                k4x4_t tmp;
                                deq_k(pk4x4 + (ii + tx)/nl_k, (ii + tx)%nl_k, tmp);
                                sk4x4[4*ty + tx] = tmp;
                            }

                            simdgroup_barrier(mem_flags::mem_threadgroup);

                            FOR_UNROLL (short k = 0; k < 4; ++k) {
                                k8x8_t mk;
                                q8x8_t mq;

                                simdgroup_load(mk, sk + 16*k + 0*8, 4*16, 0, true);
                                simdgroup_load(mq, sq + (2*(ii + k) + 0)*8, DK);
                                simdgroup_multiply_accumulate(mqk, mq, mk, mqk);

                                simdgroup_load(mk, sk + 16*k + 1*8, 4*16, 0, true);
                                simdgroup_load(mq, sq + (2*(ii + k) + 1)*8, DK);
```<｜Assistant｜></think>