rs6000: Refine RTL unroll hook for small loops

For very small loops (< 6 insns), it would be fine to unroll 4
times to run fast with less latency and better cache usage.  Like
below loops:
 while (i) a[--i] = NULL;   while (p < e)  *d++ = *p++;

With this patch enhances, we could see some performance improvement
for some workloads(e.g. SPEC2017).

2020-07-13  Jiufu Guo   <guojiufu@cn.ibm.com>

	* config/rs6000/rs6000.c (rs6000_loop_unroll_adjust): Refine hook.

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index dda51d5877a2a366b5937853f48a6995eadbf5c8..c5c3300c59c23e91eb9009d24a97b54003368a7a 100644 (file)
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -5212,13 +5212,14 @@ rs6000_loop_unroll_adjust (unsigned nunroll, struct loop *loop)
 {
    if (unroll_only_small_loops)
     {
-      /* TODO: This is hardcoded to 10 right now.  It can be refined, for
-        example we may want to unroll very small loops more times (4 perhaps).
-        We also should use a PARAM for this.  */
+      /* TODO: These are hardcoded values right now.  We probably should use
+        a PARAM here.  */
+      if (loop->ninsns <= 6)
+       return MIN (4, nunroll);
       if (loop->ninsns <= 10)
        return MIN (2, nunroll);
-      else
-       return 0;
+
+      return 0;
     }
 
   return nunroll;