[PATCH] Fix PR77937

[Bill Schmidt]

The previous patch for
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77937 is necessary, but not
sufficient in all cases.  It allows -1 to be used with a pointer
increment, which we really do not want given that this is generally not
profitable.  Disable this case for now.  We can add logic later to
estimate the cost for the rare case where it can be useful.

Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
regressions, committed.

Thanks,
Bill


2016-10-13  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	PR tree-optimization/77937
	* gimple-ssa-strength-reduction.c (analyze_increments): Set cost
	to infinite when we have a pointer with an increment of -1.


Index: gcc/gimple-ssa-strength-reduction.c
===================================================================
--- gcc/gimple-ssa-strength-reduction.c	(revision 241120)
+++ gcc/gimple-ssa-strength-reduction.c	(working copy)
@@ -2818,6 +2818,11 @@ analyze_increments (slsr_cand_t first_dep, machine
 	       || (incr == -1
 		   && !POINTER_TYPE_P (first_dep->cand_type)))
 	incr_vec[i].cost = COST_NEUTRAL;
+
+      /* FIXME: We don't handle pointers with a -1 increment yet.
+         They are usually unprofitable anyway.  */
+      else if (incr == -1 && POINTER_TYPE_P (first_dep->cand_type))
+	incr_vec[i].cost = COST_INFINITE;
       
       /* FORNOW: If we need to add an initializer, give up if a cast from
 	 the candidate's type to its stride's type can lose precision.