[PATCH, 5.x/6.x/7.x] Be more conservative in early inliner if FDO is enabled

["Yuan, Pengfei" <ypf at pku dot edu dot cn>]

Hi,

Previously I have sent a patch on profile based option tuning:
https://gcc.gnu.org/ml/gcc-patches/2014-07/msg01377.html

According to Richard Biener's advice, I try investigating where the code size
reduction comes from. After analyzing the dumped IL, I figure out that it is
related to function inlining. Some cold functions are inlined regardless of
profile feedback, which increases code size.

The problem is with the early inliner. In want_early_inline_function_p, if the
estimated edge growth > 0, want_inline depends on maybe_hot_p, which usually
returns true unless optimize_size, since profile feedback is not available at
this point. Some functions which may be cold according to profile feedback are
inlined regardlessly, resulting in code size increase.

At first, I come up with a solution that preloads some profile info before
pass_early_inline. But it fails with numerous coverage-mismatch errors in
pass_ipa_tree_profile. Therefore, the proposed patch prevents early inlining
with positive code size growth if FDO is enabled.

Experiment results are as follows:

Setup
  Hardware             Core i7-4770, 32GB RAM
  OS                   Debian sid amd64
  Compiler             GCC 5.4.1 20160907
  Firefox source       mozilla-central, cset 91c2b9d5c135
  Training workload    css3test.com, html5test.com, Octane benchmark

Vanilla GCC
  Code size (.text of libxul.so)    48708873
  Octane benchmark (score)          35828   36618   35847
  Kraken benchmark (time)           939.4ms 964.0ms 951.8ms

Patched GCC
  Code size (.text of libxul.so)    44686265
  Octane benchmark (score)          36103   35740   35611
  Kraken benchmark (time)           928.9ms 949.1ms 938.7ms

There is over 8% reduction in code size, while no obvious difference in
performance. The experiment is conducted with GCC 5. There is segmentation
fault when starting Firefox instrumented by GCC 6. GCC 7 encounters ICE when
building Firefox.

Regards,

Yuan, Pengfei


gcc/ChangeLog:
	* ipa-inline.c (want_early_inline_function_p): Be more conservative
	if FDO is enabled.


diff --git a/gcc/ipa-inline.c b/gcc/ipa-inline.c
index 7097cf3..8266f97 100644
--- a/gcc/ipa-inline.c
+++ b/gcc/ipa-inline.c
@@ -628,6 +628,20 @@ want_early_inline_function_p (struct cgraph_edge *e)
 
       if (growth <= 0)
 	;
+      /* Profile feedback is not available at this point.
+	 Be more conservative if FDO is enabled.  */
+      else if ((profile_arc_flag && !flag_test_coverage)
+	       || (flag_branch_probabilities && !flag_auto_profile))
+	{
+	  if (dump_file)
+	    fprintf (dump_file, "  will not early inline: %s/%i->%s/%i, "
+		     "FDO is enabled and code would grow by %i\n",
+		     xstrdup_for_dump (e->caller->name ()),
+		     e->caller->order,
+		     xstrdup_for_dump (callee->name ()), callee->order,
+		     growth);
+	  want_inline = false;
+	}
       else if (!e->maybe_hot_p ()
 	       && growth > 0)
 	{