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vulkan: use aligned loads for flash attention mask (#12853)

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Rewrite the stride logic for the mask tensor in the FA shader to force the
stride to be aligned, to allow using more efficient loads.
This commit is contained in:
Jeff Bolz
2025-04-12 03:44:48 -05:00
committed by GitHub
parent e59ea539b8
commit a4837577aa
1 changed files with 7 additions and 4 deletions
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11
ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_cm2.comp
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@@ -201,6 +201,11 @@ void main() {
uint32_t q_stride = p.gqa_ratio > 1 ? (p.nb02 / 4) : p.nb01;
uint32_t k_stride = p.nb11;
uint32_t v_stride = p.nb21;
// When using grouped query attention, all rows use the same mask (stride 0).
// "p.gqa_ratio >> 16" is just a roundabout way of writing zero
// that prevents the compiler from folding the "&" through the select
// and breaking the alignment detection.
uint32_t m_stride = (p.gqa_ratio > 1) ? (p.gqa_ratio >> 16) : KV;
// hint to the compiler that strides are aligned for the aligned variant of the shader
if (Clamp != gl_CooperativeMatrixClampModeConstantNV)
{
@@ -209,6 +214,7 @@ void main() {
k_stride &= ~7;
v_stride &= ~7;
#endif
m_stride &= ~7;
}
tensorLayoutQ = setTensorLayoutStrideNV(tensorLayoutQ, q_stride, 1);
tensorLayoutK = setTensorLayoutStrideNV(tensorLayoutK, k_stride, 1);
@@ -261,10 +267,7 @@ void main() {
if (p.mask != 0) {
tensorLayoutNV<2, Clamp> tensorLayoutM = createTensorLayoutNV(2, Clamp);
tensorLayoutM = setTensorLayoutDimensionNV(tensorLayoutM, p.nem1, KV);
// When using grouped query attention, all rows use the same mask.
if (p.gqa_ratio > 1) {
tensorLayoutM = setTensorLayoutStrideNV(tensorLayoutM, 0, 1);
}
tensorLayoutM = setTensorLayoutStrideNV(tensorLayoutM, m_stride, 1);
coopmat<float16_t, gl_ScopeWorkgroup, Br, Bc, gl_MatrixUseAccumulator> mv;