flash: Initialize rowmax/rowsum cache in sharedmem

tests/regression/flash_attention/kernel.cpp

@@ -14,12 +14,34 @@ using float_type = float16_t;
 #define B_ROW BM
 #define B_COL BN
 
-inline void thread_block_flashattn(float *S,
-                                   const uint32_t tid_in_threadblock,
+inline void thread_block_init_sharedmem(const uint32_t tid_in_threadblock,
+                                   const uint32_t threads_per_threadblock,
+                                   float *sharedmem_scratchpad,
+                                   float *sharedmem_rowmax,
+                                   float *sharedmem_rowsum) {
+  const uint32_t tid_in_warp = tid_in_threadblock % NUM_THREADS;
+  const uint32_t warp_id = tid_in_threadblock / NUM_THREADS;
+
+  static_assert((B_ROW % NUM_THREADS) == 0,
+                "B_ROW must be a multiple of NUM_THREADS");
+  // FIXME: this shouldn't be necessary
+  static_assert(B_ROW < (NUM_THREADS * CORES_PER_CLUSTER * NUM_WARPS),
+                "Not enough warps to initialize rowmax/rowsum");
+
+  constexpr uint32_t num_warps = B_ROW / NUM_THREADS;
+  if (warp_id < num_warps) {
+    uint32_t offset = NUM_THREADS * warp_id + tid_in_warp;
+    sharedmem_rowmax[offset] = FLT_MIN;
+    sharedmem_rowsum[offset] = 0.0f;
+  }
+}
+
+inline void thread_block_flashattn(float *S, const uint32_t tid_in_threadblock,
                                    const uint32_t threads_per_threadblock,
                                    const uint32_t threadblock_id_in_cluster,
                                    float *sharedmem_scratchpad,
-                                   float *sharedmem_row_max_sum) {
+                                   float *sharedmem_rowmax,
+                                   float *sharedmem_rowsum) {
   asm volatile("thread_block_flashattn_start_%=:" ::);
 
   const uint32_t tid_in_warp = tid_in_threadblock % NUM_THREADS;
@@ -63,7 +85,7 @@ inline void thread_block_flashattn(float *S,
                      : "=f"(max)
                      : "f"(max), "f"(S[first_thread_offset + i]));
       }
-      sharedmem_row_max_sum[row] = max;
+      sharedmem_rowmax[row] = max;
       gmem_tmp0[row] = max;
     }
 
@@ -92,14 +114,23 @@ inline void thread_block_flashattn(float *S,
 
     // elect 0-th thread to reduce all other thread's values in the warp
     if (tid_in_warp == 0) {
+      float rowmax = per_thread_max;
       for (int iter = 1; iter < NUM_THREADS; iter++) {
         float other = warp_smem[iter];
         asm volatile("fmax.s %0, %1, %2"
-                     : "=f"(per_thread_max)
-                     : "f"(per_thread_max), "f"(other));
+                     : "=f"(rowmax)
+                     : "f"(rowmax), "f"(other));
       }
-      sharedmem_row_max_sum[row] = per_thread_max;
-      gmem_tmp0[row] = per_thread_max;
+
+      // update previous rowsum
+      // i.e. mi_new = max(mi, mij)
+      float prev_rowmax = sharedmem_rowmax[row];
+      asm volatile("fmax.s %0, %1, %2"
+                   : "=f"(rowmax)
+                   : "f"(rowmax), "f"(prev_rowmax));
+
+      sharedmem_rowmax[row] = rowmax;
+      gmem_tmp0[row] = rowmax;
     }
 #endif
 
@@ -114,7 +145,7 @@ inline void thread_block_flashattn(float *S,
     //     (exp_elem_per_thread * threads_per_threadblock) / B_COL;
 
     // broadcast rowmax to all threads in the warp
-    const float row_max = sharedmem_row_max_sum[row];
+    const float row_max = sharedmem_rowmax[row];
 
     thread_offset = first_thread_offset + tid_in_warp;
 #pragma GCC unroll
@@ -160,7 +191,9 @@ inline void thread_block_flashattn(float *S,
         float other = warp_smem[iter];
         per_thread_sum += other;
       }
-      sharedmem_row_max_sum[row] = per_thread_sum;
+
+      // TODO: update previous rowsum here
+      sharedmem_rowsum[row] = per_thread_sum;
       gmem_tmp2[row] = per_thread_sum;
     }
 
@@ -212,17 +245,26 @@ void kernel_body(int task_id, kernel_arg_t *__UNIFORM__ arg) {
                                 (2 * BM * BK) * threadblock_id_in_cluster);
 
   uint8_t *smem_S = sharedmem_per_threadblock;
-  constexpr uint32_t sharedmem_row_max_sum_size = 2 * sizeof(float) * B_ROW;
+  constexpr uint32_t sharedmem_rowmax_size = sizeof(float) * B_ROW;
+  constexpr uint32_t sharedmem_rowsum_size = sizeof(float) * B_ROW;
   // sharedmem area to store rowmax/rowsum values in softmax
-  uint8_t *sharedmem_row_max_sum =
-      reinterpret_cast<uint8_t *>(SMEM_ADDR_END) - sharedmem_row_max_sum_size;
+  uint8_t *sharedmem_rowmax =
+      reinterpret_cast<uint8_t *>(SMEM_ADDR_END) - sharedmem_rowmax_size;
+  uint8_t *sharedmem_rowsum = sharedmem_rowmax - sharedmem_rowsum_size;
+
   // sharedmem "scratchpad" area to put temporary data, e.g. for tree reduction
   // in rowsum
   // NOTE: out-of bounds is not checked
   constexpr uint32_t sharedmem_scratchpad_size =
       sizeof(float) * B_ROW * NUM_THREADS * 2 /*arbitrary slack*/;
   uint8_t *sharedmem_scratchpad =
-      sharedmem_row_max_sum - sharedmem_scratchpad_size;
+      sharedmem_rowmax - sharedmem_scratchpad_size;
+
+  // initialize rowmax/rowsum values in sharedmem
+  thread_block_init_sharedmem(tid_in_threadblock, threads_per_threadblock,
+                              (float *)sharedmem_scratchpad,
+                              (float *)sharedmem_rowmax,
+                              (float *)sharedmem_rowsum);
 
   // thread_block_gemm<float_type, /*write_to_gmem=*/true>(
   //     (const float_type *)arg->addr_a, (const float_type *)arg->addr_b,
@@ -240,7 +282,7 @@ void kernel_body(int task_id, kernel_arg_t *__UNIFORM__ arg) {
   thread_block_flashattn((float *)arg->addr_a /* smem_S, */, tid_in_threadblock,
                          threads_per_threadblock, threadblock_id_in_cluster,
                          (float *)sharedmem_scratchpad,
-                         (float *)sharedmem_row_max_sum);
+                         (float *)sharedmem_rowmax, (float *)sharedmem_rowsum);
 }
 
 int main() {