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[PATCH] Fix if-conversion (PR tree-optimization/70916)
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Richard Biener <rguenther at suse dot de>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Tue, 3 May 2016 20:16:18 +0200
- Subject: [PATCH] Fix if-conversion (PR tree-optimization/70916)
- Authentication-results: sourceware.org; auth=none
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
Hi!
As mentioned in the PR and on IRC, the initial problem on the testcase
from this PR is that if-conv mistakenly assumes that some bb predicates
are true predicates, when they really should not be.
The problem is if we have a loop (perhaps finite) inside of some infinite
loop, when computing post dominators we pick quite randomly some bb and
add a fake edge to exit to that for the computation. In the testcase we
had in the inner loop:
bb7:
...
if (something)
{
bb14:
...
}
bb8:;
and we were unfortunate enough that we pretended to add fake edge to exit
to exactly bb14. Immediate dominator of bb14 is (correctly) bb7, but
due to the fake edge to exit immediate post-dominator of bb7 used to be
bb14, rather than bb8, so we thought that bb14 is a join block and used
true predicate for it, when it should use something instead.
The following patch fixes this by making the fake edges (still quite
randomly chosen) explicit, so if we are unlucky enough we just don't
if-convert, but don't miscompile stuff.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2016-05-03 Jakub Jelinek <jakub@redhat.com>
Richard Biener <rguenther@suse.de>
PR tree-optimization/70916
* tree-if-conv.c: Include cfganal.h.
(pass_if_conversion::execute): Call connect_infinite_loops_to_exit
and remove_fake_exit_edges around the optimization pass.
--- gcc/tree-if-conv.c.jj 2016-05-03 13:42:48.000000000 +0200
+++ gcc/tree-if-conv.c 2016-05-03 14:06:59.375341147 +0200
@@ -113,6 +113,7 @@ along with GCC; see the file COPYING3.
#include "varasm.h"
#include "builtins.h"
#include "params.h"
+#include "cfganal.h"
/* Only handle PHIs with no more arguments unless we are asked to by
simd pragma. */
@@ -2812,6 +2813,14 @@ pass_if_conversion::execute (function *f
if (number_of_loops (fun) <= 1)
return 0;
+ /* If there are infinite loops, during CDI_POST_DOMINATORS computation
+ we can pick pretty much random bb inside of the infinite loop that
+ has the fake edge. If we are unlucky enough, this can confuse the
+ add_to_predicate_list post-dominator check to optimize as if that
+ bb or some other one is a join block when it actually is not.
+ See PR70916. */
+ connect_infinite_loops_to_exit ();
+
FOR_EACH_LOOP (loop, 0)
if (flag_tree_loop_if_convert == 1
|| flag_tree_loop_if_convert_stores == 1
@@ -2819,6 +2828,8 @@ pass_if_conversion::execute (function *f
&& !loop->dont_vectorize))
todo |= tree_if_conversion (loop);
+ remove_fake_exit_edges ();
+
if (flag_checking)
{
basic_block bb;
Jakub