[ifcombine] improve reverse checking and operand swapping

Don't reject an ifcombine field-merging opportunity just because the
left-hand operands aren't both reversed, if the second compare needs
to be swapped for operands to match.

Also mention that reversep does NOT affect the turning of range tests
into bit tests.

for  gcc/ChangeLog

	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Document
	reversep's absence of effects on range tests.  Don't reject
	reversep mismatches before trying compare swapping.

diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index 01c4d076af26c16adbf928d15c7c624471dd855a..5f2fe56ea383c4e8ee22b6479c079a5c92a2a630 100644 (file)
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -8095,8 +8095,9 @@ fold_truth_andor_for_ifcombine (enum tree_code code, tree truth_type,
       return 0;
     }
 
-  /* Prepare to turn compares of signed quantities with zero into
-     sign-bit tests.  */
+  /* Prepare to turn compares of signed quantities with zero into sign-bit
+     tests.  We need not worry about *_reversep here for these compare
+     rewrites: loads will have already been reversed before compares.  */
   bool lsignbit = false, rsignbit = false;
   if ((lcode == LT_EXPR || lcode == GE_EXPR)
       && integer_zerop (lr_arg)
@@ -8203,10 +8204,11 @@ fold_truth_andor_for_ifcombine (enum tree_code code, tree truth_type,
      the rhs's.  If one is a load and the other isn't, we have to be
      conservative and avoid the optimization, otherwise we could get
      SRAed fields wrong.  */
-  if (volatilep || ll_reversep != rl_reversep)
+  if (volatilep)
     return 0;
 
-  if (! operand_equal_p (ll_inner, rl_inner, 0))
+  if (ll_reversep != rl_reversep
+      || ! operand_equal_p (ll_inner, rl_inner, 0))
     {
       /* Try swapping the operands.  */
       if (ll_reversep != rr_reversep