[RFA] [PR tree-optimization/68599] Avoid over-zealous optimization with -funsafe-loop-optimizations

[Jeff Law]

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?

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;