optimize gemv forwarding on ARM64 systems

CONTRIBUTORS.md

@@ -226,3 +226,6 @@ In chronological order:
 
 * Dirreke <https://github.com/mseminatore>
   * [2024-01-16] Add basic support for the CSKY architecture
+
+* Christopher Daley <https://github.com/cdaley>
+  * [2024-01-24] Optimize GEMV forwarding on ARM64 systems

interface/gemm.c

@@ -39,6 +39,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdbool.h>
 #include "common.h"
 #ifdef FUNCTION_PROFILE
 #include "functable.h"
@@ -499,6 +500,15 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE TransA, enum CBLAS_TRANS
 #endif
 
 #if defined(GEMM_GEMV_FORWARD) && !defined(GEMM3M) && !defined(COMPLEX) && (!defined(BFLOAT16) || defined(GEMM_GEMV_FORWARD_BF16))
+#if defined(ARCH_ARM64)
+  // The gemv kernels in arm64/{gemv_n.S,gemv_n_sve.c,gemv_t.S,gemv_t_sve.c}
+  // perform poorly in certain circumstances. We use the following boolean
+  // variable along with the gemv argument values to avoid these inefficient
+  // gemv cases, see github issue#4951.
+  bool have_tuned_gemv = false;
+#else
+  bool have_tuned_gemv = true;
+#endif
   // Check if we can convert GEMM -> GEMV
   if (args.k != 0) {
     if (args.n == 1) {
@@ -518,8 +528,11 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE TransA, enum CBLAS_TRANS
       if (transb & 1) {
         inc_x = args.ldb;
       }
-      GEMV(&NT, &m, &n, args.alpha, args.a, &lda, args.b, &inc_x, args.beta, args.c, &inc_y);
-      return;
+      bool is_efficient_gemv = have_tuned_gemv || ((NT == 'N') || (NT == 'T' && inc_x == 1));
+      if (is_efficient_gemv) {
+        GEMV(&NT, &m, &n, args.alpha, args.a, &lda, args.b, &inc_x, args.beta, args.c, &inc_y);
+        return;
+      }
     }
     if (args.m == 1) {
       blasint inc_x = args.lda;
@@ -538,8 +551,11 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_TRANSPOSE TransA, enum CBLAS_TRANS
         m = args.n;
         n = args.k;
       }
-      GEMV(&NT, &m, &n, args.alpha, args.b, &ldb, args.a, &inc_x, args.beta, args.c, &inc_y);
-      return;
+      bool is_efficient_gemv = have_tuned_gemv || ((NT == 'N' && inc_y == 1) || (NT == 'T' && inc_x == 1));
+      if (is_efficient_gemv) {
+        GEMV(&NT, &m, &n, args.alpha, args.b, &ldb, args.a, &inc_x, args.beta, args.c, &inc_y);
+        return;
+      }
     }
   }
 #endif