[gcc r14-4918] rtl-ssa: Use frequency-weighted insn costs

[gcc r14-4918] rtl-ssa: Use frequency-weighted insn costs

[Richard Sandiford]

https://gcc.gnu.org/g:d7266f655e532ba72c6884edf4bff1d46adb200f

commit r14-4918-gd7266f655e532ba72c6884edf4bff1d46adb200f
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Wed Oct 25 10:39:52 2023 +0100

    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:
---
 gcc/rtl-ssa/changes.cc | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index 3e14069421c..aab532b9f26 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");
     }