Re: [PATCH rs6000]: Refine RTL unroll hook for small loops

[Segher Boessenkool]

Hi Jiu Fu,

On Mon, Jul 13, 2020 at 07:50:28PM +0800, guojiufu wrote:
> For very small loops (< 6 insns), it would be fine to unroll 4
> times to run fast with less latency and better cache usage.

> -  /* 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: Using hardcodes here, for tunable, PARAM(s) maybe refined.  */

  /* TODO: These are hardcoded values right now.  We probably should use
 a PARAM here.  */

Okay for trunk with that.  Thanks!


Segher

[PATCH rs6000]: Refine RTL unroll hook for small loops

[guojiufu via Gcc-patches]

Hi,

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).

Bootstrap and regtest pass on powerpc64le. Ok for trunk?

BR,
Jiufu Guo

2020-07-13  Jiufu Guo   

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

---
 gcc/config/rs6000/rs6000.c | 12 ++--
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 58f5d780603..06844fdba57 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -5135,16 +5135,15 @@ rs6000_destroy_cost_data (void *data)
 static unsigned
 rs6000_loop_unroll_adjust (unsigned nunroll, struct loop *loop)
 {
-   if (unroll_only_small_loops)
+  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: Using hardcodes here, for tunable, PARAM(s) maybe refined.  */
+  if (loop->ninsns <= 6)
+   return MIN (4, nunroll);
   if (loop->ninsns <= 10)
return MIN (2, nunroll);
-  else
-   return 0;
+
+  return 0;
 }
 
   return nunroll;
-- 
2.25.1