[store-merging] Use store order as tie-breaker in sort_by_bitpos

As Alexander pointed out in the thread starting at [1] the sort_by_bitpos sorting function
was behaving badly when we had multiple stores at the same position.  He fixed that (thanks!)
but we can do better by not returning zero when the bitpositions are equal but by falling back
to comparing the order the stores appear in, which is guaranteed to be unique (barring other
bugs elsewhere).

This patch does that.

Bootstrapped and tested on aarch64-none-linux-gnu.

[1] https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00895.html

	* gimple-ssa-store-merging.c (sort_by_bitpos): Compare store order
	when bitposition is the same.

From-SVN: r252084

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index ef793e4d8ccedd5f8df56a1378c3b11af927e600..0e0257d9675d9f8468b4a1a8765cbade4642ed1a 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2017-09-13  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+       * gimple-ssa-store-merging.c (sort_by_bitpos): Compare store order
+       when bitposition is the same.
+
 2017-09-13  Richard Biener  <rguenther@suse.de>
 
        * dwarf2out.c (output_die_symbol): Remove.

diff --git a/gcc/gimple-ssa-store-merging.c b/gcc/gimple-ssa-store-merging.c
index a4a38b104bfcc454ed49a38946706b00d997aeba..cb764032205a8233f826178e799a26ee3ae41083 100644 (file)
--- a/gcc/gimple-ssa-store-merging.c
+++ b/gcc/gimple-ssa-store-merging.c
@@ -521,7 +521,9 @@ sort_by_bitpos (const void *x, const void *y)
   else if ((*tmp)->bitpos > (*tmp2)->bitpos)
     return 1;
   else
-    return 0;
+    /* If they are the same let's use the order which is guaranteed to
+       be different.  */
+    return (*tmp)->order - (*tmp2)->order;
 }
 
 /* Sorting function for store_immediate_info objects.