[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
64_t nb21;
    uint64_t nb22;
    uint64_t nb23;
    int32_t  ne31;
    int32_t  ne32;
    int32_t  ne33;
    uint64_t nb31;
    uint64_t nb32;
    uint64_t nb33;
    int32_t  ne1;
    int32_t  ne2;
    int32_t  ne3;
    float    scale;
    float    max_bias;
    float    m0;
    float    m1;
    int32_t  n_head_log2;
    float    logit_softcap;
};

struct ds4_metal_args_flash_attn_ext_vec_reduce {
    int32_t nrows;
};

constant bool FC_flash_attn_ext_pad_has_mask [[function_constant(FC_FLASH_ATTN_EXT_PAD + 0)]];
constant int32_t FC_flash_attn_ext_pad_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_PAD + 25)]];

// DS4 FlashAttention padding: pads the final partial K/V/mask cache block so the
// vector FlashAttention kernel can read full 32-row chunks.
kernel void kernel_flash_attn_ext_pad(
        constant ds4_metal_args_flash_attn_ext_pad & args,
        device const char * k,
        device const char * v,
        device const char * mask,
        device       char * dst,
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiitg[[thread_index_in_threadgroup]],
        ushort3 ntg[[threads_per_threadgroup]]) {
    const int32_t C = FC_flash_attn_ext_pad_ncpsg;

    device char * k_pad    = dst;
    device char * v_pad    = k_pad + args.nb11*C*args.ne_12_2*args.ne_12_3;
    device char * mask_pad = v_pad + args.nb21*C*args.ne_12_2*args.ne_12_3;

    const int32_t icp = args.ne11 % C;
    const int32_t ic0 = args.ne11 - icp;

    const int32_t i1 = tgpig[0];
    const int32_t i2 = tgpig[1];
    const int32_t i3 = tgpig[2];

    if (i2 < args.ne_12_2 && i3 < args.ne_12_3) {
        device const char * k_src = k + args.nb11*(ic0 + i1) + args.nb12*i2 + args.nb13*i3;
        device const char * v_src = v + args.nb21*(ic0 + i1) + args.nb22*i2 + args.nb23*i3;

        device char * k_dst = k_pad + args.nb11*i1 + args.nb11*C*i2 + args.nb11*C*args.ne_12_2*i3;
        device char * v_dst = v_pad + args.nb21*i1 + args.nb21*C*i2 + args.nb21*C*args.ne_12_2*i3;

        if (i1 >= icp) {
            for (uint64_t i = tiitg; i < args.nb11; i += ntg.x) {
                k_dst[i] = 0;
            }
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = 0;
            }
        } else {
            for (uint64_t i = tiitg; i < args.nb11; i += ntg.x) {
                k_dst[i] = k_src[i];
            }
            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) {
```

<｜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
64_t nb21;
    uint64_t nb22;
    uint64_t nb23;
    int32_t  ne31;
    int32_t  ne32;
    int32_t  ne33;
    uint64_t nb31;
    uint64_t nb32;
    uint64_t nb33;
    int32_t  ne1;
    int32_t  ne2;
    int32_t  ne3;
    float    scale;
    float    max_bias;
    float    m0;
    float    m1;
    int32_t  n_head_log2;
    float    logit_softcap;
};

struct ds4_metal_args_flash_attn_ext_vec_reduce {
    int32_t nrows;
};

constant bool FC_flash_attn_ext_pad_has_mask [[function_constant(FC_FLASH_ATTN_EXT_PAD + 0)]];
constant int32_t FC_flash_attn_ext_pad_ncpsg [[function_constant(FC_FLASH_ATTN_EXT_PAD + 25)]];

// DS4 FlashAttention padding: pads the final partial K/V/mask cache block so the
// vector FlashAttention kernel can read full 32-row chunks.
kernel void kernel_flash_attn_ext_pad(
        constant ds4_metal_args_flash_attn_ext_pad & args,
        device const char * k,
        device const char * v,
        device const char * mask,
        device       char * dst,
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiitg[[thread_index_in_threadgroup]],
        ushort3 ntg[[threads_per_threadgroup]]) {
    const int32_t C = FC_flash_attn_ext_pad_ncpsg;

    device char * k_pad    = dst;
    device char * v_pad    = k_pad + args.nb11*C*args.ne_12_2*args.ne_12_3;
    device char * mask_pad = v_pad + args.nb21*C*args.ne_12_2*args.ne_12_3;

    const int32_t icp = args.ne11 % C;
    const int32_t ic0 = args.ne11 - icp;

    const int32_t i1 = tgpig[0];
    const int32_t i2 = tgpig[1];
    const int32_t i3 = tgpig[2];

    if (i2 < args.ne_12_2 && i3 < args.ne_12_3) {
        device const char * k_src = k + args.nb11*(ic0 + i1) + args.nb12*i2 + args.nb13*i3;
        device const char * v_src = v + args.nb21*(ic0 + i1) + args.nb22*i2 + args.nb23*i3;

        device char * k_dst = k_pad + args.nb11*i1 + args.nb11*C*i2 + args.nb11*C*args.ne_12_2*i3;
        device char * v_dst = v_pad + args.nb21*i1 + args.nb21*C*i2 + args.nb21*C*args.ne_12_2*i3;

        if (i1 >= icp) {
            for (uint64_t i = tiitg; i < args.nb11; i += ntg.x) {
                k_dst[i] = 0;
            }
            for (uint64_t i = tiitg; i < args.nb21; i += ntg.x) {
                v_dst[i] = 0;
            }
        } else {
            for (uint64_t i = tiitg; i < args.nb11; i += ntg.x) {
                k_dst[i] = k_src[i];
            }
            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) {
```<｜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
0, SH, 0, false);
                                simdgroup_load(vs[1], ss + 16*cc + 8, SH, 0, false);

                                FOR_UNROLL (short ii = 0; ii < NO/2; ++ii) {
                                    v8x8_t mv[4];

                                    simdgroup_load(mv[0], pv + 0*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[1], pv + 8*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[2], pv + 0*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[3], pv + 8*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);

                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[0], mv[0], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[0], mv[1], lo[2*ii + 1]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[1], mv[2], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[1], mv[3], lo[2*ii + 1]);
                                }

                                pv  += 2*8*NS20;
                            }
                        }
                    }

                    {
                        auto sot = so + 8*sgitg;

                        FOR_UNROLL (short ii = 0; ii < NO; ++ii) {
                            simdgroup_store(lo[ii], sot, PV, 0, false);

                            sot += 8*NSG;
                        }
                    }
                } else {
                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    for (short cc = 0; cc < C/8; ++cc) {
                        s8x8_t vs;
                        simdgroup_load(vs, ss + 8*cc, SH, 0, false);

                        for (short ii = 4*sgitg; ii < DV16; ii += 4*NSG) {
                            device const vd4x4_t * pv4x4 = (device const vd4x4_t *) (v + ((ic + 8*cc + ty)*args.nb21));

                            if (DV16%4 == 0) {
                                {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[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
0, SH, 0, false);
                                simdgroup_load(vs[1], ss + 16*cc + 8, SH, 0, false);

                                FOR_UNROLL (short ii = 0; ii < NO/2; ++ii) {
                                    v8x8_t mv[4];

                                    simdgroup_load(mv[0], pv + 0*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[1], pv + 8*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[2], pv + 0*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[3], pv + 8*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);

                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[0], mv[0], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[0], mv[1], lo[2*ii + 1]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[1], mv[2], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[1], mv[3], lo[2*ii + 1]);
                                }

                                pv  += 2*8*NS20;
                            }
                        }
                    }

                    {
                        auto sot = so + 8*sgitg;

                        FOR_UNROLL (short ii = 0; ii < NO; ++ii) {
                            simdgroup_store(lo[ii], sot, PV, 0, false);

                            sot += 8*NSG;
                        }
                    }
                } else {
                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    for (short cc = 0; cc < C/8; ++cc) {
                        s8x8_t vs;
                        simdgroup_load(vs, ss + 8*cc, SH, 0, false);

                        for (short ii = 4*sgitg; ii < DV16; ii += 4*NSG) {
                            device const vd4x4_t * pv4x4 = (device const vd4x4_t *) (v + ((ic + 8*cc + ty)*args.nb21));

                            if (DV16%4 == 0) {
                                {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[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
0, SH, 0, false);
                                simdgroup_load(vs[1], ss + 16*cc + 8, SH, 0, false);

                                FOR_UNROLL (short ii = 0; ii < NO/2; ++ii) {
                                    v8x8_t mv[4];

                                    simdgroup_load(mv[0], pv + 0*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[1], pv + 8*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[2], pv + 0*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[3], pv + 8*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);

                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[0], mv[0], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[0], mv[1], lo[2*ii + 1]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[1], mv[2], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[1], mv[3], lo[2*ii + 1]);
                                }

                                pv  += 2*8*NS20;
                            }
                        }
                    }

                    {
                        auto sot = so + 8*sgitg;

                        FOR_UNROLL (short ii = 0; ii < NO; ++ii) {
                            simdgroup_store(lo[ii], sot, PV, 0, false);

                            sot += 8*NSG;
                        }
                    }
                } else {
                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    for (short cc = 0; cc < C/8; ++cc) {
                        s8x8_t vs;
                        simdgroup_load(vs, ss + 8*cc, SH, 0, false);

                        for (short ii = 4*sgitg; ii < DV16; ii += 4*NSG) {
                            device const vd4x4_t * pv4x4 = (device const vd4x4_t *) (v + ((ic + 8*cc + ty)*args.nb21));

                            if (DV16%4 == 0) {
                                {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[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
0, SH, 0, false);
                                simdgroup_load(vs[1], ss + 16*cc + 8, SH, 0, false);

                                FOR_UNROLL (short ii = 0; ii < NO/2; ++ii) {
                                    v8x8_t mv[4];

                                    simdgroup_load(mv[0], pv + 0*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[1], pv + 8*NSG + 16*ii*NSG + 0*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[2], pv + 0*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);
                                    simdgroup_load(mv[3], pv + 8*NSG + 16*ii*NSG + 1*8*NS20, NS20, 0, false);

                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[0], mv[0], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[0], mv[1], lo[2*ii + 1]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 0], vs[1], mv[2], lo[2*ii + 0]);
                                    simdgroup_multiply_accumulate(lo[2*ii + 1], vs[1], mv[3], lo[2*ii + 1]);
                                }

                                pv  += 2*8*NS20;
                            }
                        }
                    }

                    {
                        auto sot = so + 8*sgitg;

                        FOR_UNROLL (short ii = 0; ii < NO; ++ii) {
                            simdgroup_store(lo[ii], sot, PV, 0, false);

                            sot += 8*NSG;
                        }
                    }
                } else {
                    const short tx = tiisg%4;
                    const short ty = tiisg/4;

                    for (short cc = 0; cc < C/8; ++cc) {
                        s8x8_t vs;
                        simdgroup_load(vs, ss + 8*cc, SH, 0, false);

                        for (short ii = 4*sgitg; ii < DV16; ii += 4*NSG) {
                            device const vd4x4_t * pv4x4 = (device const vd4x4_t *) (v + ((ic + 8*cc + ty)*args.nb21));

                            if (DV16%4 == 0) {
                                {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[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_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            } else {
                                if (ii + tx < DV16) {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                for (short k = 0; k < 4 && ii + k < DV16; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            }
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }
```

<｜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_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            } else {
                                if (ii + tx < DV16) {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                for (short k = 0; k < 4 && ii + k < DV16; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            }
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }
```<｜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_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            } else {
                                if (ii + tx < DV16) {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                for (short k = 0; k < 4 && ii + k < DV16; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            }
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }
```

<｜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_barrier(mem_flags::mem_threadgroup);

                                FOR_UNROLL (short k = 0; k < 4; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            } else {
                                if (ii + tx < DV16) {
                                    v4x4_t tmp;
                                    deq_v(pv4x4 + (ii + tx)/nl_v, (ii + tx)%nl_v, tmp);
                                    sv4x4[4*ty + tx] = tmp;
                                }

                                simdgroup_barrier(mem_flags::mem_threadgroup);

                                for (short k = 0; k < 4 && ii + k < DV16; ++k) {
                                    v8x8_t mv[2];
                                    o8x8_t lo[2];

                                    simdgroup_load(mv[0], sv + 16*k + 0*8, 4*16, 0, false);
                                    simdgroup_load(mv[1], sv + 16*k + 1*8, 4*16, 0, false);
                                    simdgroup_load(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_load(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);

                                    simdgroup_multiply_accumulate(lo[0], vs, mv[0], lo[0]);
                                    simdgroup_multiply_accumulate(lo[1], vs, mv[1], lo[1]);

                                    simdgroup_store(lo[0], so + 8*(2*(ii + k) + 0), PV, 0, false);
                                    simdgroup_store(lo[1], so + 8*(2*(ii + k) + 1), PV, 0, false);
                                }
                            }
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }
```<｜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
}
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }

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

                const float m = M[jj];
                const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

                M[jj] = simd_max(max(M[jj], s));

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

                S[jj] = S[jj]*ms + simd_sum(vs);

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

    for (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;
        if (iq1 + j >= args.ne01) {
            break;
        }

        device float4 * dst4 = (device float4 *) dst + ((uint64_t)iq3*args.ne2*args.ne1 + iq2 + (uint64_t)(iq1 + j)*args.ne1)*DV4;

        const float scale = S[jj] == 0.0 ? 0.0f : 1.0f/S[jj];

        if (DV4 % NW == 0) {
            FOR_UNROLL (short ii = 0; ii < DV4/NW; ++ii) {
                const short i = ii*NW + tiisg;

                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        } else {
            for (short i = tiisg; i < DV4; i += NW) {
                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        }
    }

#undef NS10
#undef NS20
}

// Batched FlashAttention for prompt/prefill rows. It computes QK, applies mask,
// sinks, ALiBi/softcap options when enabled, and multiplies by V without
// materializing the full attention matrix.
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  = OP_FLASH_ATTN_EXT_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_NCPSG>
kernel void kernel_flash_attn_ext(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
```

<｜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
}
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }

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

                const float m = M[jj];
                const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

                M[jj] = simd_max(max(M[jj], s));

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

                S[jj] = S[jj]*ms + simd_sum(vs);

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

    for (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;
        if (iq1 + j >= args.ne01) {
            break;
        }

        device float4 * dst4 = (device float4 *) dst + ((uint64_t)iq3*args.ne2*args.ne1 + iq2 + (uint64_t)(iq1 + j)*args.ne1)*DV4;

        const float scale = S[jj] == 0.0 ? 0.0f : 1.0f/S[jj];

        if (DV4 % NW == 0) {
            FOR_UNROLL (short ii = 0; ii < DV4/NW; ++ii) {
                const short i = ii*NW + tiisg;

                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        } else {
            for (short i = tiisg; i < DV4; i += NW) {
                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        }
    }

#undef NS10
#undef NS20
}

// Batched FlashAttention for prompt/prefill rows. It computes QK, applies mask,
// sinks, ALiBi/softcap options when enabled, and multiplies by V without
// materializing the full attention matrix.
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  = OP_FLASH_ATTN_EXT_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_NCPSG>
kernel void kernel_flash_attn_ext(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
```<｜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
}
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }

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

                const float m = M[jj];
                const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

                M[jj] = simd_max(max(M[jj], s));

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

                S[jj] = S[jj]*ms + simd_sum(vs);

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

    for (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;
        if (iq1 + j >= args.ne01) {
            break;
        }

        device float4 * dst4 = (device float4 *) dst + ((uint64_t)iq3*args.ne2*args.ne1 + iq2 + (uint64_t)(iq1 + j)*args.ne1)*DV4;

        const float scale = S[jj] == 0.0 ? 0.0f : 1.0f/S[jj];

        if (DV4 % NW == 0) {
            FOR_UNROLL (short ii = 0; ii < DV4/NW; ++ii) {
                const short i = ii*NW + tiisg;

                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        } else {
            for (short i = tiisg; i < DV4; i += NW) {
                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        }
    }

#undef NS10
#undef NS20
}

// Batched FlashAttention for prompt/prefill rows. It computes QK, applies mask,
// sinks, ALiBi/softcap options when enabled, and multiplies by V without
// materializing the full attention matrix.
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  = OP_FLASH_ATTN_EXT_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_NCPSG>
kernel void kernel_flash_attn_ext(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
```

<｜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
}
                        }
                    }
                }
            }

            threadgroup_barrier(mem_flags::mem_threadgroup);
        }

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

                const float m = M[jj];
                const float s = tiisg == 0 ? ((device const float *) sinks)[iq2] : -FLT_MAX/2;

                M[jj] = simd_max(max(M[jj], s));

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

                S[jj] = S[jj]*ms + simd_sum(vs);

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

    for (short jj = 0; jj < NQ; ++jj) {
        const short j = jj*NSG + sgitg;
        if (iq1 + j >= args.ne01) {
            break;
        }

        device float4 * dst4 = (device float4 *) dst + ((uint64_t)iq3*args.ne2*args.ne1 + iq2 + (uint64_t)(iq1 + j)*args.ne1)*DV4;

        const float scale = S[jj] == 0.0 ? 0.0f : 1.0f/S[jj];

        if (DV4 % NW == 0) {
            FOR_UNROLL (short ii = 0; ii < DV4/NW; ++ii) {
                const short i = ii*NW + tiisg;

                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        } else {
            for (short i = tiisg; i < DV4; i += NW) {
                dst4[i] = (float4) so4[j*PV4 + i]*scale;
            }
        }
    }

#undef NS10
#undef NS20
}

// Batched FlashAttention for prompt/prefill rows. It computes QK, applies mask,
// sinks, ALiBi/softcap options when enabled, and multiplies by V without
// materializing the full attention matrix.
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  = OP_FLASH_ATTN_EXT_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_NCPSG>
kernel void kernel_flash_attn_ext(
        constant ds4_metal_args_flash_attn_ext & args,
        device const char * q,
        device const char * k,
        device const char * v,
```<｜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
rnel void kernel_flash_attn_ext(
        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 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
#define FWD_TMPL q_t, q4_t, q8x8_t, k_t, k4x4_t, k8x8_t, v_t, v4x4_t, v8x8_t, qk_t, qk8x8_t, s_t, s2_t, s8x8_t, o_t, o4_t, o8x8_t, kd4x4_t, nl_k, deq_k, vd4x4_t, nl_v, deq_v, DK, DV, Q, C
#define FWD_ARGS args, q, k, v, mask, sinks, pad, blk, dst, shmem_f16, tgpig, tiisg, sgitg
    switch (FC_flash_attn_ext_nsg) {
        case 4: kernel_flash_attn_ext_impl<FWD_TMPL, 4>(FWD_ARGS); break;
        case 8: kernel_flash_attn_ext_impl<FWD_TMPL, 8>(FWD_ARGS); break;
    }
#undef FWD_TMPL
#undef FWD_ARGS
}

#define FA_NONVEC_TYPES \
    half,   half4,     simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    float,             simdgroup_float8x8, \
    float,  float2,    simdgroup_float8x8, \
    float,  float4,    simdgroup_float8x8

typedef decltype(kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>) flash_attn_ext_dk512_t;

// Host-visible prefill FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_f16_dk512_dv512")]]
kernel flash_attn_ext_dk512_t kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>;

#undef FA_NONVEC_TYPES

constant bool FC_flash_attn_ext_vec_has_mask  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 0)]];
constant bool FC_flash_attn_ext_vec_has_sinks [[function_constant(FC_FLASH_ATTN_EXT_VEC + 1)]];
constant bool FC_flash_attn_ext_vec_has_bias  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 2)]];
constant bool FC_flash_attn_ext_vec_has_scap  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 3)]];
constant bool FC_flash_attn_ext_vec_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT_VEC + 4)]];
constant int32_t FC_flash_attn_ext_vec_ns10 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 20)]];
constant int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
```

<｜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
rnel void kernel_flash_attn_ext(
        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 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
#define FWD_TMPL q_t, q4_t, q8x8_t, k_t, k4x4_t, k8x8_t, v_t, v4x4_t, v8x8_t, qk_t, qk8x8_t, s_t, s2_t, s8x8_t, o_t, o4_t, o8x8_t, kd4x4_t, nl_k, deq_k, vd4x4_t, nl_v, deq_v, DK, DV, Q, C
#define FWD_ARGS args, q, k, v, mask, sinks, pad, blk, dst, shmem_f16, tgpig, tiisg, sgitg
    switch (FC_flash_attn_ext_nsg) {
        case 4: kernel_flash_attn_ext_impl<FWD_TMPL, 4>(FWD_ARGS); break;
        case 8: kernel_flash_attn_ext_impl<FWD_TMPL, 8>(FWD_ARGS); break;
    }
#undef FWD_TMPL
#undef FWD_ARGS
}

#define FA_NONVEC_TYPES \
    half,   half4,     simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    float,             simdgroup_float8x8, \
    float,  float2,    simdgroup_float8x8, \
    float,  float4,    simdgroup_float8x8

typedef decltype(kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>) flash_attn_ext_dk512_t;

// Host-visible prefill FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_f16_dk512_dv512")]]
kernel flash_attn_ext_dk512_t kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>;

#undef FA_NONVEC_TYPES

constant bool FC_flash_attn_ext_vec_has_mask  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 0)]];
constant bool FC_flash_attn_ext_vec_has_sinks [[function_constant(FC_FLASH_ATTN_EXT_VEC + 1)]];
constant bool FC_flash_attn_ext_vec_has_bias  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 2)]];
constant bool FC_flash_attn_ext_vec_has_scap  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 3)]];
constant bool FC_flash_attn_ext_vec_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT_VEC + 4)]];
constant int32_t FC_flash_attn_ext_vec_ns10 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 20)]];
constant int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
```<｜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
rnel void kernel_flash_attn_ext(
        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 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
#define FWD_TMPL q_t, q4_t, q8x8_t, k_t, k4x4_t, k8x8_t, v_t, v4x4_t, v8x8_t, qk_t, qk8x8_t, s_t, s2_t, s8x8_t, o_t, o4_t, o8x8_t, kd4x4_t, nl_k, deq_k, vd4x4_t, nl_v, deq_v, DK, DV, Q, C
#define FWD_ARGS args, q, k, v, mask, sinks, pad, blk, dst, shmem_f16, tgpig, tiisg, sgitg
    switch (FC_flash_attn_ext_nsg) {
        case 4: kernel_flash_attn_ext_impl<FWD_TMPL, 4>(FWD_ARGS); break;
        case 8: kernel_flash_attn_ext_impl<FWD_TMPL, 8>(FWD_ARGS); break;
    }
#undef FWD_TMPL
#undef FWD_ARGS
}

#define FA_NONVEC_TYPES \
    half,   half4,     simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    float,             simdgroup_float8x8, \
    float,  float2,    simdgroup_float8x8, \
    float,  float4,    simdgroup_float8x8

typedef decltype(kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>) flash_attn_ext_dk512_t;

// Host-visible prefill FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_f16_dk512_dv512")]]
kernel flash_attn_ext_dk512_t kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>;

#undef FA_NONVEC_TYPES

constant bool FC_flash_attn_ext_vec_has_mask  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 0)]];
constant bool FC_flash_attn_ext_vec_has_sinks [[function_constant(FC_FLASH_ATTN_EXT_VEC + 1)]];
constant bool FC_flash_attn_ext_vec_has_bias  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 2)]];
constant bool FC_flash_attn_ext_vec_has_scap  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 3)]];
constant bool FC_flash_attn_ext_vec_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT_VEC + 4)]];
constant int32_t FC_flash_attn_ext_vec_ns10 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 20)]];
constant int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
```

<｜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
rnel void kernel_flash_attn_ext(
        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 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
#define FWD_TMPL q_t, q4_t, q8x8_t, k_t, k4x4_t, k8x8_t, v_t, v4x4_t, v8x8_t, qk_t, qk8x8_t, s_t, s2_t, s8x8_t, o_t, o4_t, o8x8_t, kd4x4_t, nl_k, deq_k, vd4x4_t, nl_v, deq_v, DK, DV, Q, C
#define FWD_ARGS args, q, k, v, mask, sinks, pad, blk, dst, shmem_f16, tgpig, tiisg, sgitg
    switch (FC_flash_attn_ext_nsg) {
        case 4: kernel_flash_attn_ext_impl<FWD_TMPL, 4>(FWD_ARGS); break;
        case 8: kernel_flash_attn_ext_impl<FWD_TMPL, 8>(FWD_ARGS); break;
    }
#undef FWD_TMPL
#undef FWD_ARGS
}

#define FA_NONVEC_TYPES \
    half,   half4,     simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    half,   half4x4,   simdgroup_half8x8,  \
    float,             simdgroup_float8x8, \
    float,  float2,    simdgroup_float8x8, \
    float,  float4,    simdgroup_float8x8

typedef decltype(kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>) flash_attn_ext_dk512_t;

// Host-visible prefill FlashAttention variant for DS4's 512-wide F16 K/V rows.
template [[host_name("kernel_flash_attn_ext_f16_dk512_dv512")]]
kernel flash_attn_ext_dk512_t kernel_flash_attn_ext<FA_NONVEC_TYPES, half4x4, 1, dequantize_f16, half4x4, 1, dequantize_f16, 512, 512>;

#undef FA_NONVEC_TYPES

constant bool FC_flash_attn_ext_vec_has_mask  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 0)]];
constant bool FC_flash_attn_ext_vec_has_sinks [[function_constant(FC_FLASH_ATTN_EXT_VEC + 1)]];
constant bool FC_flash_attn_ext_vec_has_bias  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 2)]];
constant bool FC_flash_attn_ext_vec_has_scap  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 3)]];
constant bool FC_flash_attn_ext_vec_has_kvpad [[function_constant(FC_FLASH_ATTN_EXT_VEC + 4)]];
constant int32_t FC_flash_attn_ext_vec_ns10 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 20)]];
constant int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
```<｜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
int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
constant int32_t FC_flash_attn_ext_vec_nwg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 23)]];

// Decode FlashAttention for one query row. DS4 uses this in generation to scan
// raw and compressed KV cache chunks, optionally splitting long contexts across
// workgroups and writing partial softmax state for a later reduction.
template<
    typename q4_t,
    typename k4_t,
    typename v4_t,
    typename qk_t,
    typename s_t,
    typename s4_t,
    typename o4_t,
    typename kd4_t,
    short nl_k,
    void (*deq_k_t4)(device const kd4_t *, short, thread k4_t &),
    typename vd4_t,
    short nl_v,
    void (*deq_v_t4)(device const vd4_t *, short, thread v4_t &),
    short DK,
    short DV,
    short NE = 4,
    short Q  = OP_FLASH_ATTN_EXT_VEC_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_VEC_NCPSG>
kernel void kernel_flash_attn_ext_vec(
        constant ds4_metal_args_flash_attn_ext_vec & 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       char * dst,
        threadgroup  half * shmem_f16 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
    static_assert(DK % 32 == 0, "DK must be divisible by 32");
    static_assert(DV % 32 == 0, "DV must be divisible by 32");

#define NWG  (FC_flash_attn_ext_vec_nwg)
#define NSG  (FC_flash_attn_ext_vec_nsg)
#define NS10 (FC_flash_attn_ext_vec_ns10)
#define NS20 (FC_flash_attn_ext_vec_ns20)

    const short iwg = tgpig[2]%NWG;

    const ushort iq3 = tgpig[2]/NWG;
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0];

    constexpr short DK4 = DK/4;
    constexpr short DV4 = DV/4;
    constexpr short PK  = PAD2(DK, 128);
    constexpr short PK4 = PK/4;
    constexpr short PV  = PAD2(DV, 128);
    constexpr short PV4 = PV/4;
    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NL  = NW/NE;
    constexpr short SH  = 4*C;

    static_assert(DK4 % NL == 0, "DK4 must be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
```

<｜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
int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
constant int32_t FC_flash_attn_ext_vec_nwg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 23)]];

// Decode FlashAttention for one query row. DS4 uses this in generation to scan
// raw and compressed KV cache chunks, optionally splitting long contexts across
// workgroups and writing partial softmax state for a later reduction.
template<
    typename q4_t,
    typename k4_t,
    typename v4_t,
    typename qk_t,
    typename s_t,
    typename s4_t,
    typename o4_t,
    typename kd4_t,
    short nl_k,
    void (*deq_k_t4)(device const kd4_t *, short, thread k4_t &),
    typename vd4_t,
    short nl_v,
    void (*deq_v_t4)(device const vd4_t *, short, thread v4_t &),
    short DK,
    short DV,
    short NE = 4,
    short Q  = OP_FLASH_ATTN_EXT_VEC_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_VEC_NCPSG>
kernel void kernel_flash_attn_ext_vec(
        constant ds4_metal_args_flash_attn_ext_vec & 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       char * dst,
        threadgroup  half * shmem_f16 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
    static_assert(DK % 32 == 0, "DK must be divisible by 32");
    static_assert(DV % 32 == 0, "DV must be divisible by 32");

#define NWG  (FC_flash_attn_ext_vec_nwg)
#define NSG  (FC_flash_attn_ext_vec_nsg)
#define NS10 (FC_flash_attn_ext_vec_ns10)
#define NS20 (FC_flash_attn_ext_vec_ns20)

    const short iwg = tgpig[2]%NWG;

    const ushort iq3 = tgpig[2]/NWG;
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0];

    constexpr short DK4 = DK/4;
    constexpr short DV4 = DV/4;
    constexpr short PK  = PAD2(DK, 128);
    constexpr short PK4 = PK/4;
    constexpr short PV  = PAD2(DV, 128);
    constexpr short PV4 = PV/4;
    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NL  = NW/NE;
    constexpr short SH  = 4*C;

    static_assert(DK4 % NL == 0, "DK4 must be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
```<｜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
int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
constant int32_t FC_flash_attn_ext_vec_nwg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 23)]];

// Decode FlashAttention for one query row. DS4 uses this in generation to scan
// raw and compressed KV cache chunks, optionally splitting long contexts across
// workgroups and writing partial softmax state for a later reduction.
template<
    typename q4_t,
    typename k4_t,
    typename v4_t,
    typename qk_t,
    typename s_t,
    typename s4_t,
    typename o4_t,
    typename kd4_t,
    short nl_k,
    void (*deq_k_t4)(device const kd4_t *, short, thread k4_t &),
    typename vd4_t,
    short nl_v,
    void (*deq_v_t4)(device const vd4_t *, short, thread v4_t &),
    short DK,
    short DV,
    short NE = 4,
    short Q  = OP_FLASH_ATTN_EXT_VEC_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_VEC_NCPSG>
kernel void kernel_flash_attn_ext_vec(
        constant ds4_metal_args_flash_attn_ext_vec & 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       char * dst,
        threadgroup  half * shmem_f16 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
    static_assert(DK % 32 == 0, "DK must be divisible by 32");
    static_assert(DV % 32 == 0, "DV must be divisible by 32");

#define NWG  (FC_flash_attn_ext_vec_nwg)
#define NSG  (FC_flash_attn_ext_vec_nsg)
#define NS10 (FC_flash_attn_ext_vec_ns10)
#define NS20 (FC_flash_attn_ext_vec_ns20)

    const short iwg = tgpig[2]%NWG;

    const ushort iq3 = tgpig[2]/NWG;
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0];

    constexpr short DK4 = DK/4;
    constexpr short DV4 = DV/4;
    constexpr short PK  = PAD2(DK, 128);
    constexpr short PK4 = PK/4;
    constexpr short PV  = PAD2(DV, 128);
    constexpr short PV4 = PV/4;
    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NL  = NW/NE;
    constexpr short SH  = 4*C;

    static_assert(DK4 % NL == 0, "DK4 must be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
```

<｜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
int32_t FC_flash_attn_ext_vec_ns20 [[function_constant(FC_FLASH_ATTN_EXT_VEC + 21)]];
constant int32_t FC_flash_attn_ext_vec_nsg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 22)]];
constant int32_t FC_flash_attn_ext_vec_nwg  [[function_constant(FC_FLASH_ATTN_EXT_VEC + 23)]];

// Decode FlashAttention for one query row. DS4 uses this in generation to scan
// raw and compressed KV cache chunks, optionally splitting long contexts across
// workgroups and writing partial softmax state for a later reduction.
template<
    typename q4_t,
    typename k4_t,
    typename v4_t,
    typename qk_t,
    typename s_t,
    typename s4_t,
    typename o4_t,
    typename kd4_t,
    short nl_k,
    void (*deq_k_t4)(device const kd4_t *, short, thread k4_t &),
    typename vd4_t,
    short nl_v,
    void (*deq_v_t4)(device const vd4_t *, short, thread v4_t &),
    short DK,
    short DV,
    short NE = 4,
    short Q  = OP_FLASH_ATTN_EXT_VEC_NQPSG,
    short C  = OP_FLASH_ATTN_EXT_VEC_NCPSG>
kernel void kernel_flash_attn_ext_vec(
        constant ds4_metal_args_flash_attn_ext_vec & 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       char * dst,
        threadgroup  half * shmem_f16 [[threadgroup(0)]],
        uint3   tgpig[[threadgroup_position_in_grid]],
        ushort  tiisg[[thread_index_in_simdgroup]],
        ushort  sgitg[[simdgroup_index_in_threadgroup]]) {
    static_assert(DK % 32 == 0, "DK must be divisible by 32");
    static_assert(DV % 32 == 0, "DV must be divisible by 32");

#define NWG  (FC_flash_attn_ext_vec_nwg)
#define NSG  (FC_flash_attn_ext_vec_nsg)
#define NS10 (FC_flash_attn_ext_vec_ns10)
#define NS20 (FC_flash_attn_ext_vec_ns20)

    const short iwg = tgpig[2]%NWG;

    const ushort iq3 = tgpig[2]/NWG;
    const ushort iq2 = tgpig[1];
    const ushort iq1 = tgpig[0];

    constexpr short DK4 = DK/4;
    constexpr short DV4 = DV/4;
    constexpr short PK  = PAD2(DK, 128);
    constexpr short PK4 = PK/4;
    constexpr short PV  = PAD2(DV, 128);
    constexpr short PV4 = PV/4;
    constexpr short NW  = N_SIMDWIDTH;
    constexpr short NL  = NW/NE;
    constexpr short SH  = 4*C;

    static_assert(DK4 % NL == 0, "DK4 must be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
```<｜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
be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
    threadgroup s_t   * ss  = (threadgroup s_t   *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup s4_t  * ss4 = (threadgroup s4_t  *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup half  * sm  = (threadgroup half  *) (shmem_f16 +   sgitg*SH + 2*C + NSG*PK);
    threadgroup o4_t  * so4 = (threadgroup o4_t  *) (shmem_f16 + 2*sgitg*PV       + NSG*PK + NSG*SH);

    so4 += tiisg;

    {
        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;
    }

    device const float4 * q4 = (device const float4 *) ((device const char *) q);

    if (iq1 < args.ne01) {
        for (short i = tiisg; i < PK4; i += NW) {
            if (i < DK4) {
                sq4[i] = (q4_t) q4[i];
            } else {
                sq4[i] = (q4_t) 0.0f;
            }
        }
    }

    for (short i = 0; i < DV4/NL; ++i) {
        so4[i*NL] = (o4_t) 0.0f;
    }

    for (short i = tiisg; i < SH/4; i += NW) {
        ss4[i] = (s4_t) 0.0f;
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    {
        float S = 0.0f;
        float M = -FLT_MAX/2;

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

        device const half * pm = (device const half *) (mask + iq1*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);

        float slope = 1.0f;

        if (FC_flash_attn_ext_vec_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 = iwg*NSG + sgitg; ; ic0 += NWG*NSG) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_vec_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;
```

<｜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
be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
    threadgroup s_t   * ss  = (threadgroup s_t   *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup s4_t  * ss4 = (threadgroup s4_t  *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup half  * sm  = (threadgroup half  *) (shmem_f16 +   sgitg*SH + 2*C + NSG*PK);
    threadgroup o4_t  * so4 = (threadgroup o4_t  *) (shmem_f16 + 2*sgitg*PV       + NSG*PK + NSG*SH);

    so4 += tiisg;

    {
        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;
    }

    device const float4 * q4 = (device const float4 *) ((device const char *) q);

    if (iq1 < args.ne01) {
        for (short i = tiisg; i < PK4; i += NW) {
            if (i < DK4) {
                sq4[i] = (q4_t) q4[i];
            } else {
                sq4[i] = (q4_t) 0.0f;
            }
        }
    }

    for (short i = 0; i < DV4/NL; ++i) {
        so4[i*NL] = (o4_t) 0.0f;
    }

    for (short i = tiisg; i < SH/4; i += NW) {
        ss4[i] = (s4_t) 0.0f;
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    {
        float S = 0.0f;
        float M = -FLT_MAX/2;

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

        device const half * pm = (device const half *) (mask + iq1*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);

        float slope = 1.0f;

        if (FC_flash_attn_ext_vec_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 = iwg*NSG + sgitg; ; ic0 += NWG*NSG) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_vec_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;
```<｜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
be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
    threadgroup s_t   * ss  = (threadgroup s_t   *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup s4_t  * ss4 = (threadgroup s4_t  *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup half  * sm  = (threadgroup half  *) (shmem_f16 +   sgitg*SH + 2*C + NSG*PK);
    threadgroup o4_t  * so4 = (threadgroup o4_t  *) (shmem_f16 + 2*sgitg*PV       + NSG*PK + NSG*SH);

    so4 += tiisg;

    {
        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;
    }

    device const float4 * q4 = (device const float4 *) ((device const char *) q);

    if (iq1 < args.ne01) {
        for (short i = tiisg; i < PK4; i += NW) {
            if (i < DK4) {
                sq4[i] = (q4_t) q4[i];
            } else {
                sq4[i] = (q4_t) 0.0f;
            }
        }
    }

    for (short i = 0; i < DV4/NL; ++i) {
        so4[i*NL] = (o4_t) 0.0f;
    }

    for (short i = tiisg; i < SH/4; i += NW) {
        ss4[i] = (s4_t) 0.0f;
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    {
        float S = 0.0f;
        float M = -FLT_MAX/2;

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

        device const half * pm = (device const half *) (mask + iq1*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);

        float slope = 1.0f;

        if (FC_flash_attn_ext_vec_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 = iwg*NSG + sgitg; ; ic0 += NWG*NSG) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_vec_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;
```

<｜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
be divisible by NL");
    static_assert(DV4 % NL == 0, "DV4 must be divisible by NL");

    threadgroup q4_t  * sq4 = (threadgroup q4_t  *) (shmem_f16 +                      0*PK);
    threadgroup s_t   * ss  = (threadgroup s_t   *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup s4_t  * ss4 = (threadgroup s4_t  *) (shmem_f16 +   sgitg*SH       + NSG*PK);
    threadgroup half  * sm  = (threadgroup half  *) (shmem_f16 +   sgitg*SH + 2*C + NSG*PK);
    threadgroup o4_t  * so4 = (threadgroup o4_t  *) (shmem_f16 + 2*sgitg*PV       + NSG*PK + NSG*SH);

    so4 += tiisg;

    {
        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;
    }

    device const float4 * q4 = (device const float4 *) ((device const char *) q);

    if (iq1 < args.ne01) {
        for (short i = tiisg; i < PK4; i += NW) {
            if (i < DK4) {
                sq4[i] = (q4_t) q4[i];
            } else {
                sq4[i] = (q4_t) 0.0f;
            }
        }
    }

    for (short i = 0; i < DV4/NL; ++i) {
        so4[i*NL] = (o4_t) 0.0f;
    }

    for (short i = tiisg; i < SH/4; i += NW) {
        ss4[i] = (s4_t) 0.0f;
    }

    threadgroup_barrier(mem_flags::mem_threadgroup);

    {
        float S = 0.0f;
        float M = -FLT_MAX/2;

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

        device const half * pm = (device const half *) (mask + iq1*args.nb31 + (iq2%args.ne32)*args.nb32 + (iq3%args.ne33)*args.nb33);

        float slope = 1.0f;

        if (FC_flash_attn_ext_vec_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 = iwg*NSG + sgitg; ; ic0 += NWG*NSG) {
            int ic = ic0*C;
            if (ic >= args.ne11) {
                break;
            }

            if (FC_flash_attn_ext_vec_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;
```<｜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 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_vec_has_mask) {
                    if (ic + tiisg >= args.ne11) {
                        sm[tiisg] = -MAXHALF;
                    }
                } else {
                    pm = (device const half *) (mask) +
                        iq1*C +
                        (iq2%args.ne32)*(C*args.ne31) +
                        (iq3%args.ne33)*(C*args.ne31*args.ne32);
                }

                ic = 0;
            }

            if (FC_flash_attn_ext_vec_has_mask) {
                sm[tiisg] = pm[ic + tiisg];
            }

            if (simd_max(sm[tiisg]) <= -MAXHALF) {
                continue;
            }

            {
                device      const k4_t * pk4 = (device const k4_t *) (k + ic*args.nb11);
                threadgroup const q4_t * pq4 = sq4;

                pk4 += ty*NS10/4 + tx;
                pq4 += tx;

                qk_t mqk[C/NE] = { [ 0 ... C/NE - 1] = 0.0f };

                FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                    if (is_same<kd4_t, k4_t>::value) {
                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            mqk[cc] += dot((float4) pk4[cc*NE*NS10/4 +  ii*NL], (float4) pq4[ii*NL]);
                        }
                    } else {
                        device const kd4_t * pk = (device const kd4_t *) (k + ((ic + NE*cc + ty)*args.nb11));

                        k4_t mk;

                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            deq_k_t4(pk + i/nl_k, i%nl_k, mk);

                            mqk[cc] += dot((float4) mk, (float4) sq4[i]);
                        }
                    }

                    if (NE == 1) {
                        mqk[cc] = simd_sum(mqk[cc]);
                    } else {
                        if (NE <= 1) {
                            mqk[cc] += simd_shuffle_down(mqk[cc], 16);
                        }
                        if (NE <= 2) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  8);
                        }
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
```

<｜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 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_vec_has_mask) {
                    if (ic + tiisg >= args.ne11) {
                        sm[tiisg] = -MAXHALF;
                    }
                } else {
                    pm = (device const half *) (mask) +
                        iq1*C +
                        (iq2%args.ne32)*(C*args.ne31) +
                        (iq3%args.ne33)*(C*args.ne31*args.ne32);
                }

                ic = 0;
            }

            if (FC_flash_attn_ext_vec_has_mask) {
                sm[tiisg] = pm[ic + tiisg];
            }

            if (simd_max(sm[tiisg]) <= -MAXHALF) {
                continue;
            }

            {
                device      const k4_t * pk4 = (device const k4_t *) (k + ic*args.nb11);
                threadgroup const q4_t * pq4 = sq4;

                pk4 += ty*NS10/4 + tx;
                pq4 += tx;

                qk_t mqk[C/NE] = { [ 0 ... C/NE - 1] = 0.0f };

                FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                    if (is_same<kd4_t, k4_t>::value) {
                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            mqk[cc] += dot((float4) pk4[cc*NE*NS10/4 +  ii*NL], (float4) pq4[ii*NL]);
                        }
                    } else {
                        device const kd4_t * pk = (device const kd4_t *) (k + ((ic + NE*cc + ty)*args.nb11));

                        k4_t mk;

                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            deq_k_t4(pk + i/nl_k, i%nl_k, mk);

                            mqk[cc] += dot((float4) mk, (float4) sq4[i]);
                        }
                    }

                    if (NE == 1) {
                        mqk[cc] = simd_sum(mqk[cc]);
                    } else {
                        if (NE <= 1) {
                            mqk[cc] += simd_shuffle_down(mqk[cc], 16);
                        }
                        if (NE <= 2) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  8);
                        }
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
```<｜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 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_vec_has_mask) {
                    if (ic + tiisg >= args.ne11) {
                        sm[tiisg] = -MAXHALF;
                    }
                } else {
                    pm = (device const half *) (mask) +
                        iq1*C +
                        (iq2%args.ne32)*(C*args.ne31) +
                        (iq3%args.ne33)*(C*args.ne31*args.ne32);
                }

                ic = 0;
            }

            if (FC_flash_attn_ext_vec_has_mask) {
                sm[tiisg] = pm[ic + tiisg];
            }

            if (simd_max(sm[tiisg]) <= -MAXHALF) {
                continue;
            }

            {
                device      const k4_t * pk4 = (device const k4_t *) (k + ic*args.nb11);
                threadgroup const q4_t * pq4 = sq4;

                pk4 += ty*NS10/4 + tx;
                pq4 += tx;

                qk_t mqk[C/NE] = { [ 0 ... C/NE - 1] = 0.0f };

                FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                    if (is_same<kd4_t, k4_t>::value) {
                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            mqk[cc] += dot((float4) pk4[cc*NE*NS10/4 +  ii*NL], (float4) pq4[ii*NL]);
                        }
                    } else {
                        device const kd4_t * pk = (device const kd4_t *) (k + ((ic + NE*cc + ty)*args.nb11));

                        k4_t mk;

                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            deq_k_t4(pk + i/nl_k, i%nl_k, mk);

                            mqk[cc] += dot((float4) mk, (float4) sq4[i]);
                        }
                    }

                    if (NE == 1) {
                        mqk[cc] = simd_sum(mqk[cc]);
                    } else {
                        if (NE <= 1) {
                            mqk[cc] += simd_shuffle_down(mqk[cc], 16);
                        }
                        if (NE <= 2) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  8);
                        }
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
```

<｜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 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_vec_has_mask) {
                    if (ic + tiisg >= args.ne11) {
                        sm[tiisg] = -MAXHALF;
                    }
                } else {
                    pm = (device const half *) (mask) +
                        iq1*C +
                        (iq2%args.ne32)*(C*args.ne31) +
                        (iq3%args.ne33)*(C*args.ne31*args.ne32);
                }

                ic = 0;
            }

            if (FC_flash_attn_ext_vec_has_mask) {
                sm[tiisg] = pm[ic + tiisg];
            }

            if (simd_max(sm[tiisg]) <= -MAXHALF) {
                continue;
            }

            {
                device      const k4_t * pk4 = (device const k4_t *) (k + ic*args.nb11);
                threadgroup const q4_t * pq4 = sq4;

                pk4 += ty*NS10/4 + tx;
                pq4 += tx;

                qk_t mqk[C/NE] = { [ 0 ... C/NE - 1] = 0.0f };

                FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                    if (is_same<kd4_t, k4_t>::value) {
                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            mqk[cc] += dot((float4) pk4[cc*NE*NS10/4 +  ii*NL], (float4) pq4[ii*NL]);
                        }
                    } else {
                        device const kd4_t * pk = (device const kd4_t *) (k + ((ic + NE*cc + ty)*args.nb11));

                        k4_t mk;

                        FOR_UNROLL (short ii = 0; ii < DK4/NL; ++ii) {
                            const short i = ii*NL + tx;

                            deq_k_t4(pk + i/nl_k, i%nl_k, mk);

                            mqk[cc] += dot((float4) mk, (float4) sq4[i]);
                        }
                    }

                    if (NE == 1) {
                        mqk[cc] = simd_sum(mqk[cc]);
                    } else {
                        if (NE <= 1) {
                            mqk[cc] += simd_shuffle_down(mqk[cc], 16);
                        }
                        if (NE <= 2) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  8);
                        }
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
```<｜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
}
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  2);
                        }
                        if (NE <= 16) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  1);
                        }

                        mqk[cc] = simd_shuffle(mqk[cc], NL*ty);
                    }
                }

                if (FC_flash_attn_ext_vec_has_mask &&
                   !FC_flash_attn_ext_vec_has_scap &&
                   !FC_flash_attn_ext_vec_has_bias) {
                    ss[NE*tx + ty] = fma(mqk[tx], args.scale, (qk_t) sm[NE*tx + ty]);
                } else {
                    mqk[tx] *= args.scale;

                    if (FC_flash_attn_ext_vec_has_scap) {
                        mqk[tx] = args.logit_softcap*precise::tanh(mqk[tx]);
                    }

                    if (FC_flash_attn_ext_vec_has_bias) {
                        mqk[tx] += (qk_t) sm[NE*tx + ty]*slope;
                    } else {
                        mqk[tx] += (qk_t) sm[NE*tx + ty];
                    }

                    ss[NE*tx + ty] = mqk[tx];
                }
            }

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                const float m = M;
                const float s = ss[tiisg];

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

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

                S = S*ms + simd_sum(vs);

                ss[tiisg] = vs;

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

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                o4_t lo[DV4/NL];
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    lo[ii] = 0.0f;
                }

                if (is_same<vd4_t, v4_t>::value) {
                    device const v4_t * pv4 = (device const v4_t *) (v + ic*args.nb21);

                    pv4 += ty*NS20/4 + tx;

                    const auto sst = ss + ty;

                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        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]));
                        }
```

<｜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
}
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  2);
                        }
                        if (NE <= 16) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  1);
                        }

                        mqk[cc] = simd_shuffle(mqk[cc], NL*ty);
                    }
                }

                if (FC_flash_attn_ext_vec_has_mask &&
                   !FC_flash_attn_ext_vec_has_scap &&
                   !FC_flash_attn_ext_vec_has_bias) {
                    ss[NE*tx + ty] = fma(mqk[tx], args.scale, (qk_t) sm[NE*tx + ty]);
                } else {
                    mqk[tx] *= args.scale;

                    if (FC_flash_attn_ext_vec_has_scap) {
                        mqk[tx] = args.logit_softcap*precise::tanh(mqk[tx]);
                    }

                    if (FC_flash_attn_ext_vec_has_bias) {
                        mqk[tx] += (qk_t) sm[NE*tx + ty]*slope;
                    } else {
                        mqk[tx] += (qk_t) sm[NE*tx + ty];
                    }

                    ss[NE*tx + ty] = mqk[tx];
                }
            }

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                const float m = M;
                const float s = ss[tiisg];

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

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

                S = S*ms + simd_sum(vs);

                ss[tiisg] = vs;

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

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                o4_t lo[DV4/NL];
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    lo[ii] = 0.0f;
                }

                if (is_same<vd4_t, v4_t>::value) {
                    device const v4_t * pv4 = (device const v4_t *) (v + ic*args.nb21);

                    pv4 += ty*NS20/4 + tx;

                    const auto sst = ss + ty;

                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        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]));
                        }
```<｜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
}
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  2);
                        }
                        if (NE <= 16) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  1);
                        }

                        mqk[cc] = simd_shuffle(mqk[cc], NL*ty);
                    }
                }

                if (FC_flash_attn_ext_vec_has_mask &&
                   !FC_flash_attn_ext_vec_has_scap &&
                   !FC_flash_attn_ext_vec_has_bias) {
                    ss[NE*tx + ty] = fma(mqk[tx], args.scale, (qk_t) sm[NE*tx + ty]);
                } else {
                    mqk[tx] *= args.scale;

                    if (FC_flash_attn_ext_vec_has_scap) {
                        mqk[tx] = args.logit_softcap*precise::tanh(mqk[tx]);
                    }

                    if (FC_flash_attn_ext_vec_has_bias) {
                        mqk[tx] += (qk_t) sm[NE*tx + ty]*slope;
                    } else {
                        mqk[tx] += (qk_t) sm[NE*tx + ty];
                    }

                    ss[NE*tx + ty] = mqk[tx];
                }
            }

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                const float m = M;
                const float s = ss[tiisg];

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

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

                S = S*ms + simd_sum(vs);

                ss[tiisg] = vs;

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

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                o4_t lo[DV4/NL];
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    lo[ii] = 0.0f;
                }

                if (is_same<vd4_t, v4_t>::value) {
                    device const v4_t * pv4 = (device const v4_t *) (v + ic*args.nb21);

                    pv4 += ty*NS20/4 + tx;

                    const auto sst = ss + ty;

                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        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]));
                        }
```

<｜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
}
                        if (NE <= 4) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  4);
                        }
                        if (NE <= 8) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  2);
                        }
                        if (NE <= 16) {
                            mqk[cc] += simd_shuffle_down(mqk[cc],  1);
                        }

                        mqk[cc] = simd_shuffle(mqk[cc], NL*ty);
                    }
                }

                if (FC_flash_attn_ext_vec_has_mask &&
                   !FC_flash_attn_ext_vec_has_scap &&
                   !FC_flash_attn_ext_vec_has_bias) {
                    ss[NE*tx + ty] = fma(mqk[tx], args.scale, (qk_t) sm[NE*tx + ty]);
                } else {
                    mqk[tx] *= args.scale;

                    if (FC_flash_attn_ext_vec_has_scap) {
                        mqk[tx] = args.logit_softcap*precise::tanh(mqk[tx]);
                    }

                    if (FC_flash_attn_ext_vec_has_bias) {
                        mqk[tx] += (qk_t) sm[NE*tx + ty]*slope;
                    } else {
                        mqk[tx] += (qk_t) sm[NE*tx + ty];
                    }

                    ss[NE*tx + ty] = mqk[tx];
                }
            }

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                const float m = M;
                const float s = ss[tiisg];

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

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

                S = S*ms + simd_sum(vs);

                ss[tiisg] = vs;

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

            simdgroup_barrier(mem_flags::mem_threadgroup);

            {
                o4_t lo[DV4/NL];
                FOR_UNROLL (short ii = 0; ii < DV4/NL; ++ii) {
                    lo[ii] = 0.0f;
                }

                if (is_same<vd4_t, v4_t>::value) {
                    device const v4_t * pv4 = (device const v4_t *) (v + ic*args.nb21);

                    pv4 += ty*NS20/4 + tx;

                    const auto sst = ss + ty;

                    FOR_UNROLL (short cc = 0; cc < C/NE; ++cc) {
                        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]));
                        }
```<｜Assistant｜><think>