From: Richard Sandiford <richard.sandiford@arm.com>
Date: Wed, 25 Oct 2023 09:39:52 +0000 (+0100)
Subject: rtl-ssa: Use frequency-weighted insn costs
X-Git-Tag: basepoints/gcc-15~5221
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d7266f655e532ba72c6884edf4bff1d46adb200f;p=thirdparty%2Fgcc.git

rtl-ssa: Use frequency-weighted insn costs

rtl_ssa::changes_are_worthwhile used the standard approach
of summing up the individual costs of the old and new sequences
to see which one is better overall.  But when optimising for
speed and changing instructions in multiple blocks, it seems
better to weight the cost of each instruction by its execution
frequency.  (We already do something similar for SLP layouts.)

gcc/
	* rtl-ssa/changes.cc: Include sreal.h.
	(rtl_ssa::changes_are_worthwhile): When optimizing for speed,
	scale the cost of each instruction by its execution frequency.
---

diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index 3e14069421c8..aab532b9f260 100644
--- a/gcc/rtl-ssa/changes.cc
+++ b/gcc/rtl-ssa/changes.cc
@@ -34,6 +34,7 @@
 #include "emit-rtl.h"
 #include "cfghooks.h"
 #include "cfgrtl.h"
+#include "sreal.h"
 
 using namespace rtl_ssa;
 
@@ -171,18 +172,33 @@ rtl_ssa::changes_are_worthwhile (array_slice<insn_change *const> changes,
 {
   unsigned int old_cost = 0;
   unsigned int new_cost = 0;
+  sreal weighted_old_cost = 0;
+  sreal weighted_new_cost = 0;
+  auto entry_count = ENTRY_BLOCK_PTR_FOR_FN (cfun)->count;
   for (insn_change *change : changes)
     {
       old_cost += change->old_cost ();
+      basic_block cfg_bb = change->bb ()->cfg_bb ();
+      bool for_speed = optimize_bb_for_speed_p (cfg_bb);
+      if (for_speed)
+	weighted_old_cost += (cfg_bb->count.to_sreal_scale (entry_count)
+			      * change->old_cost ());
       if (!change->is_deletion ())
 	{
-	  basic_block cfg_bb = change->bb ()->cfg_bb ();
-	  change->new_cost = insn_cost (change->rtl (),
-					optimize_bb_for_speed_p (cfg_bb));
+	  change->new_cost = insn_cost (change->rtl (), for_speed);
 	  new_cost += change->new_cost;
+	  if (for_speed)
+	    weighted_new_cost += (cfg_bb->count.to_sreal_scale (entry_count)
+				  * change->new_cost);
 	}
     }
-  bool ok_p = (strict_p ? new_cost < old_cost : new_cost <= old_cost);
+  bool ok_p;
+  if (weighted_new_cost != weighted_old_cost)
+    ok_p = weighted_new_cost < weighted_old_cost;
+  else if (strict_p)
+    ok_p = new_cost < old_cost;
+  else
+    ok_p = new_cost <= old_cost;
   if (dump_file && (dump_flags & TDF_DETAILS))
     {
       fprintf (dump_file, "original cost");
@@ -192,6 +208,8 @@ rtl_ssa::changes_are_worthwhile (array_slice<insn_change *const> changes,
 	  fprintf (dump_file, " %c %d", sep, change->old_cost ());
 	  sep = '+';
 	}
+      if (weighted_old_cost != 0)
+	fprintf (dump_file, " (weighted: %f)", weighted_old_cost.to_double ());
       fprintf (dump_file, ", replacement cost");
       sep = '=';
       for (const insn_change *change : changes)
@@ -200,6 +218,8 @@ rtl_ssa::changes_are_worthwhile (array_slice<insn_change *const> changes,
 	    fprintf (dump_file, " %c %d", sep, change->new_cost);
 	    sep = '+';
 	  }
+      if (weighted_new_cost != 0)
+	fprintf (dump_file, " (weighted: %f)", weighted_new_cost.to_double ());
       fprintf (dump_file, "; %s\n",
 	       ok_p ? "keeping replacement" : "rejecting replacement");
     }