[ET-VK][ez] Align SDPA attention weights S dim to the next multiple of 4 (#15599)

Title says it all!

Why?

* Technically, this is should not be needed but SDPA op was producing incorrect output on Samsung S24 with buffer input tensors. The exact root cause is unclear, but it appears to be an issue specific to the Adreno 750 since it does not reproduce on any other GPU. The best guess at the moment is that we need to ensure that there is no possibility of multiple threads writing to the same memory location.

Differential Revision: [D86226134](https://our.internmc.facebook.com/intern/diff/D86226134/)

Author: pytorchbot

backends/vulkan/runtime/graph/ops/glsl/sdpa_attn_weights_softmax.glsl

@@ -76,6 +76,7 @@ void main() {
   const int Q_H = q_projected_sizes.y;
   // sequence length
   const int S = q_projected_sizes.z;
+  const int S_aligned = align_up_4(S);
   // manually determine size of the context_len dim of the attention weight.
   // The "actual" tensor sizes may have been aligned to a multiple of 4 to allow
   // memory loads to be aligned to texel boundaries.
@@ -96,7 +97,7 @@ void main() {
   // number of threads in the work group.
   for (int c4 = worker_id; c4 < C4_limit; c4 += NUM_WORKERS_PER_WG) {
     VEC4_T in_texel = load_attn_weights_c4(
-        c4, s, q_h, context_texel_len, S, Q_H);
+        c4, s, q_h, context_texel_len, S_aligned, Q_H);
 
     for (int comp = 0; comp < 4; comp++) {
       local_exp_sum += exp(in_texel[comp]);
@@ -108,7 +109,7 @@ void main() {
     for (int c4 = C4_limit; c4 < context_texel_len; ++c4) {
       const int c_base = mul_4(c4);
       VEC4_T in_texel = load_attn_weights_c4(
-          c4, s, q_h, context_texel_len, S, Q_H);
+          c4, s, q_h, context_texel_len, S_aligned, Q_H);
 
       [[unroll]] for (int comp = 0; comp < 4; comp++) {
         if (c_base + comp < context_len) {
@@ -138,19 +139,19 @@ void main() {
   // Now go back through each element in the row and normalize
   for (int c4 = worker_id; c4 < C4_limit; c4 += NUM_WORKERS_PER_WG) {
     VEC4_T in_texel = load_attn_weights_c4(
-        c4, s, q_h, context_texel_len, S, Q_H);
+        c4, s, q_h, context_texel_len, S_aligned, Q_H);
 
     VEC4_T out_texel = exp(in_texel) / local_exp_sum;
     store_attn_weights_softmax_c4(
-        out_texel, c4, s, q_h, context_texel_len, S, Q_H);
+        out_texel, c4, s, q_h, context_texel_len, S_aligned, Q_H);
   }
   // First thread in the work group responsible for handling last texel if it
   // contains any padded elements
   if (worker_id == 0) {
     for (int c4 = C4_limit; c4 < context_texel_len; ++c4) {
       const int c_base = mul_4(c4);
       VEC4_T in_texel = load_attn_weights_c4(
-          c4, s, q_h, context_texel_len, S, Q_H);
+          c4, s, q_h, context_texel_len, S_aligned, Q_H);
 
       // Ensure that padding elements are set to 0.
       VEC4_T out_texel = VEC4_T(0);
@@ -160,7 +161,7 @@ void main() {
         }
       }
       store_attn_weights_softmax_c4(
-          out_texel, c4, s, q_h, context_texel_len, S, Q_H);
+          out_texel, c4, s, q_h, context_texel_len, S_aligned, Q_H);
     }
   }
 }

backends/vulkan/runtime/graph/ops/glsl/sdpa_compute_attn_weights_coop.glsl

@@ -81,6 +81,7 @@ void main() {
   const int Q_H = q_projected_sizes.y;
   // sequence length
   const int S = q_projected_sizes.z;
+  const int S_aligned = align_up_4(S);
 
   // number of K/V heads
   const int KV_H = k_cache_sizes.y;
@@ -205,7 +206,7 @@ void main() {
       s,
       q_h,
       context_texel_len,
-      S,
+      S_aligned,
       Q_H);
   }
 }

backends/vulkan/runtime/graph/ops/glsl/sdpa_compute_attn_weights_tiled.glsl

@@ -93,6 +93,7 @@ void main() {
   const int Q_H = q_projected_sizes.y;
   // sequence length
   const int S = q_projected_sizes.z;
+  const int S_aligned = align_up_4(S);
 
   // number of K/V heads
   const int KV_H = k_cache_sizes.y;
@@ -196,6 +197,6 @@ void main() {
     s,
     q_h,
     context_texel_len,
-    S,
+    S_aligned,
     Q_H);
 }

backends/vulkan/runtime/graph/ops/glsl/sdpa_compute_out_coop.glsl

@@ -81,6 +81,7 @@ void main() {
   const int Q_H = q_projected_sizes.y;
   // sequence length
   const int S = q_projected_sizes.z;
+  const int S_aligned = align_up_4(S);
 
   // number of K/V heads
   const int KV_H = v_cache_sizes.y;
@@ -120,7 +121,7 @@ void main() {
       s,
       q_h,
       context_texel_len,
-      S,
+      S_aligned,
       Q_H);
 
     load_v_cache_tile_no_checks(
@@ -146,7 +147,7 @@ void main() {
         s,
         q_h,
         context_texel_len,
-        S,
+        S_aligned,
         Q_H);
 
       load_v_cache_tile_with_checks(

backends/vulkan/runtime/graph/ops/glsl/sdpa_compute_out_tiled.glsl

@@ -75,6 +75,7 @@ void main() {
   const int Q_H = q_projected_sizes.y;
   // sequence length
   const int S = q_projected_sizes.z;
+  const int S_aligned = align_up_4(S);
 
   // number of K/V heads
   const int KV_H = v_cache_sizes.y;
@@ -113,7 +114,7 @@ void main() {
       s,
       q_h,
       context_texel_len,
-      S,
+      S_aligned,
       Q_H);
 
     load_v_cache_tile_no_checks(
@@ -136,7 +137,7 @@ void main() {
       s,
       q_h,
       context_texel_len,
-      S,
+      S_aligned,
       Q_H);
 
     load_v_cache_tile_with_checks(

backends/vulkan/runtime/graph/ops/impl/SDPA.cpp

@@ -50,7 +50,7 @@ void resize_compute_attn_weights_node(
   std::vector<int64_t> out_sizes = {
       1, // batch
       num_q_heads,
-      seq_len,
+      utils::align_up_4(seq_len),
       utils::align_up_4(context_len)};
 
   graph->virtual_resize(attn_weights, out_sizes);