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vulkan: support arbitrary KV dimension in flash attention #16160

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Merged
merged 1 commit into from
Sep 27, 2025
Merged

vulkan: support arbitrary KV dimension in flash attention #16160

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20 changes: 18 additions & 2 deletions ggml/src/ggml-vulkan/vulkan-shaders/flash_attn.comp
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Original file line number Diff line number Diff line change
Expand Up @@ -117,6 +117,9 @@ void main() {


[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
[[unroll]] for (uint32_t d = 0; d < HSK_per_thread / 4; ++d) {
#if BLOCK_SIZE > 1
uint coord = (j * Bc + c * cols_per_iter + col_tid) * k_stride * BLOCK_SIZE + 4 * (d * D_split + d_tid);
Expand Down Expand Up @@ -155,7 +158,11 @@ void main() {
uint32_t c = (idx + tid) % Bc;
uint32_t r = (idx + tid) / Bc;
if (idx + tid < Bc * Br) {
masksh[c][r] = float(data_m[m_offset + (i * Br + r) * m_stride + (j * Bc + c)]);
if (!KV_bounds_check || j * Bc + c < KV) {
masksh[c][r] = float(data_m[m_offset + (i * Br + r) * m_stride + (j * Bc + c)]);
} else {
masksh[c][r] = float(0);
}
}
}
barrier();
Expand All @@ -172,8 +179,11 @@ void main() {

float rowmaxf[Br], Pf[Br][cols_per_thread], rowsumf[Br], eMf[Br], Moldf[Br];
[[unroll]] for (uint32_t r = 0; r < Br; ++r) {
rowmaxf[r] = Sf[r][0];
rowmaxf[r] = NEG_FLT_MAX_OVER_2;
[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
rowmaxf[r] = max(rowmaxf[r], Sf[r][c]);
}
Moldf[r] = Mf[r];
Expand All @@ -190,6 +200,9 @@ void main() {
// Compute sum across row of P
rowsumf[r] = 0.0;
[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
rowsumf[r] += Pf[r][c];
}

Expand All @@ -203,6 +216,9 @@ void main() {
}

[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
[[unroll]] for (uint32_t d = 0; d < HSV_per_thread / 4; ++d) {
#if BLOCK_SIZE > 1
uint coord = (j * Bc + c * cols_per_iter + col_tid) * v_stride * BLOCK_SIZE + 4 * (d * D_split + d_tid);
Expand Down
2 changes: 2 additions & 0 deletions ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_base.comp
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,8 @@ layout (constant_id = 6) const uint32_t D_split = 16;
const uint32_t HSK_pad = (HSK + 15) & ~15;
const uint32_t HSV_pad = (HSV + 15) & ~15;

const bool KV_bounds_check = Clamp != 0;

layout (push_constant) uniform parameter {
uint32_t N;
uint32_t KV;
Expand Down
25 changes: 18 additions & 7 deletions ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_cm1.comp
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Original file line number Diff line number Diff line change
Expand Up @@ -152,14 +152,17 @@ void main() {
uint32_t d = (idx + tid) % (HSK / 4);
uint32_t c = (idx + tid) / (HSK / 4);
if (c < Bc && d < HSK / 4) {
f16vec4 K_Tf = f16vec4(0);
if (!KV_bounds_check || j * Bc + c < KV) {
#if BLOCK_SIZE > 1
uint coord = (j * Bc + c) * k_stride * BLOCK_SIZE + 4 * d;
uint ib = coord / BLOCK_SIZE;
uint iqs = (coord % BLOCK_SIZE);
f16vec4 K_Tf = f16vec4(dequantize4(ib, iqs, k_offset, BINDING_IDX_K));
uint coord = (j * Bc + c) * k_stride * BLOCK_SIZE + 4 * d;
uint ib = coord / BLOCK_SIZE;
uint iqs = (coord % BLOCK_SIZE);
K_Tf = f16vec4(dequantize4(ib, iqs, k_offset, BINDING_IDX_K));
#else
f16vec4 K_Tf = f16vec4(data_kv4[k_offset / 4 + (j * Bc + c) * k_stride / 4 + d]);
K_Tf = f16vec4(data_kv4[k_offset / 4 + (j * Bc + c) * k_stride / 4 + d]);
#endif
}

ksh[c * kshstride + d] = K_Tf;
}
Expand Down Expand Up @@ -202,16 +205,21 @@ void main() {
uint32_t c = (idx + tid) % Bc;
uint32_t r = (idx + tid) / Bc;
if (idx + tid < Bc * Br || idx + gl_WorkGroupSize.x <= Bc * Br) {
sfsh[c * sfshstride + r] += ACC_TYPE(slope[r] * float(data_m[m_offset + (i * Br + r) * m_stride + (j * Bc + c)]));
if (!KV_bounds_check || j * Bc + c < KV) {
sfsh[c * sfshstride + r] += ACC_TYPE(slope[r] * float(data_m[m_offset + (i * Br + r) * m_stride + (j * Bc + c)]));
}
}
}
barrier();
}

float eMf[rows_per_thread];
[[unroll]] for (uint32_t r = 0; r < rows_per_thread; ++r) {
float rowmaxf = sfsh[tile_row(r) + (0 * cols_per_iter + col_tid) * sfshstride];
float rowmaxf = NEG_FLT_MAX_OVER_2;
[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
rowmaxf = max(rowmaxf, float(sfsh[tile_row(r) + (c * cols_per_iter + col_tid) * sfshstride]));
}
float Moldf = Mf[r];
Expand All @@ -233,6 +241,9 @@ void main() {
}

[[unroll]] for (uint32_t c = 0; c < cols_per_thread; ++c) {
if (KV_bounds_check && j * Bc + c * cols_per_iter + col_tid >= KV) {
continue;
}
float Pf[rows_per_thread];
[[unroll]] for (uint32_t r = 0; r < rows_per_thread; ++r) {
Pf[r] = exp(sfsh[tile_row(r) + (c * cols_per_iter + col_tid) * sfshstride] - Mf[r]);
Expand Down
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