On May 3, 2016 8:16:18 PM GMT+02:00, Jakub Jelinek <ja...@redhat.com> wrote:
>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?
OK.
Thanks,
Richard.
>2016-05-03 Jakub Jelinek <ja...@redhat.com>
> Richard Biener <rguent...@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
