On Wed, Dec 2, 2015 at 5:27 PM, Jeff Law <l...@redhat.com> wrote:
>
>
> I strongly recommend reading the analysis in pr45122 since pr68599 uses the
> same testcase and just triggers the same bug in the RTL optimizers instead
> of the tree optimziers.
>
> As noted in 45122, with -funsafe-loop-optimizations, we may exit the loop an
> iteration too early. The loop in question is finite and the counter does
> not overflow. Yet -funsafe-loop-optimizations munges it badly.
>
> As is noted in c#6 and patched in c#8, when there's more than one exit from
> the loop, simply discarding the assumptions for the trip count is "a bit too
> unsafe". Richi & Zdenek agreed that disabling the optimization when the
> loop has > 1 exit was the preferred approach. Alex's patch did just that,
> but only for the tree optimizers.
>
> This patch does essentially the same thing for the RTL loop optimizer. If
> the candidate loop has > 1 exit, then we don't allow
> -funsafe-loop-optimizations to drop the assumptions/infinite notes for the
> RTL loop.
>
> This required ensuring that LOOPS_HAVE_RECORDED_EXITS when initializing the
> loop optimizer.
>
> Bootstrapped and regression tested on x86_64-linux-gnu and
> powerpc64-linux-gnu. For the latter, pr45122.c flips to a pass. Given this
> is covered by the pr45122 testcase, I didn't add a new one.
>
> OK for the trunk?
Ok.
Note that I believe we should dump -funsafe-loop-optimizations in
favor of a per-loop
#pragma now that we can properly track such. Globally it's known to miscompile
SPEC at least.
Thanks,
Richard.
> Jeff
>
> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> index 9a78b7a..ed677ec 100644
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,12 @@
> +2015-12-02 Jeff Law <l...@redhat.com>
> +
> + PR tree-optimization/68599
> + * loop-init.c (rtl_loop_init): Set LOOPS_HAVE_RECORDED_EXITS
> + in call to loop_optimizer_init.
> + * loop-iv.c (get_simple_loop_desc): Only allow unsafe loop
> + optimization to drop the assumptions/infinite notations if
> + the loop has a single exit.
> +
> 2015-12-01 Andreas Tobler <andre...@gcc.gnu.org>
>
> * config/rs6000/freebsd64.h (ELFv2_ABI_CHECK): Add new macro.
> diff --git a/gcc/loop-init.c b/gcc/loop-init.c
> index e32c94a..120316d 100644
> --- a/gcc/loop-init.c
> +++ b/gcc/loop-init.c
> @@ -395,7 +395,7 @@ rtl_loop_init (void)
> dump_flow_info (dump_file, dump_flags);
> }
>
> - loop_optimizer_init (LOOPS_NORMAL);
> + loop_optimizer_init (LOOPS_NORMAL | LOOPS_HAVE_RECORDED_EXITS);
> return 0;
> }
>
> diff --git a/gcc/loop-iv.c b/gcc/loop-iv.c
> index c7d5164..dfa3ca3 100644
> --- a/gcc/loop-iv.c
> +++ b/gcc/loop-iv.c
> @@ -3054,7 +3054,7 @@ get_simple_loop_desc (struct loop *loop)
> }
> }
>
> - if (flag_unsafe_loop_optimizations)
> + if (flag_unsafe_loop_optimizations && single_exit (loop))
> {
> desc->assumptions = NULL_RTX;
> desc->infinite = NULL_RTX;
>
