sgemm_wg: Document tiling parameter constraints

tests/regression/sgemm_wg/kernel.cpp

@@ -4,11 +4,20 @@
 #include <vx_spawn.h>
 #include "common.h"
 
+// Constraints on parameters:
+// * Memory:
+//   (BM + BN) * BK * sizeof(float) <= sharedmem size.
+//   BM * BK == BN * BK >= threadblock size >= NT * CORES_PER_CLUSTER
+//     When larger, the kernel runs a sequential loop to read into sharedmem;
+//     but smaller case is not handled.
+// * Compute:
+//   ( M* N) / (TM*TN) == grid size >= NC*NW*NT
+//   (BM*BN) / (TM*TN) == threadblock size >= NT * CORES_PER_CLUSTER
+// * Combining BM * BK >= (BM*BN) / (TM*TN) == threadblock yields
+//   BM <= BK*TM*TN.
 #define BM 8
 #define BN BM
 #define BK 2
-// #define TM (BM/BK)
-// #define TN (BN/BK)
 #define TM 2
 #define TN 2
 
@@ -82,7 +91,9 @@ void thread_block_gemm(kernel_arg_t *__UNIFORM__ arg,
     threadblock_barrier(tid_in_threadblock, threadblock_id_in_core,
                         threadblock_dim_y);
 
+    // Compute single tile*tile matmul
     for (uint32_t local_k = 0; local_k < BK; local_k++) {
+      // First, pump data from SMEM->RF
 #pragma GCC unroll TM
       for (uint32_t res_idx_m = 0; res_idx_m < TM; res_idx_m++) {
         reg_a[res_idx_m] =
@@ -94,7 +105,7 @@ void thread_block_gemm(kernel_arg_t *__UNIFORM__ arg,
             local_b[BN * local_k + (TN * local_c_col + res_idx_n)];
       }
 
-      // Compute multiple result elements (TM) per thread
+      // Next, compute multiple result elements (TM*TN) by reusing data in RF
 #pragma GCC unroll TM
       for (uint32_t res_idx_m = 0; res_idx_m < TM; res_idx_m++) {
 #pragma GCC unroll TN
@@ -113,6 +124,7 @@ void thread_block_gemm(kernel_arg_t *__UNIFORM__ arg,
                         threadblock_dim_y);
   }
 
+  // Store result data from RF to GMEM
 #pragma GCC unroll TM
   for (uint32_t res_idx_m = 0; res_idx_m < TM; res_idx_m++) {
 #pragma GCC unroll TN