re PR rtl-optimization/42522 ((zero_extract:SI (mem:QI) ...) misoptimized)

        PR rtl-optimization/42522
        * cse.c (fold_rtx): Try to simplify a ZERO_EXTRACT or
        SIGN_EXTRACT as a whole object rather than simplifying
        its operand.

From-SVN: r222125

gcc/ChangeLog

+2015-04-15  Jeff Law  <law@redhat.com>
+
+	PR rtl-optimization/42522
+	* cse.c (fold_rtx): Try to simplify a ZERO_EXTRACT or
+	SIGN_EXTRACT as a whole object rather than simplifying
+	its operand.
+
 2015-04-15  Jakub Jelinek  <jakub@redhat.com>
 
 	PR ipa/65765

gcc/cse.c

@@ -3150,6 +3150,15 @@ fold_rtx (rtx x, rtx_insn *insn)
     {
     case MEM:
     case SUBREG:
+    /* The first operand of a SIGN/ZERO_EXTRACT has a different meaning
+       than it would in other contexts.  Basically its mode does not
+       signify the size of the object read.  That information is carried
+       by size operand.    If we happen to have a MEM of the appropriate
+       mode in our tables with a constant value we could simplify the
+       extraction incorrectly if we allowed substitution of that value
+       for the MEM.   */
+    case ZERO_EXTRACT:
+    case SIGN_EXTRACT:
       if ((new_rtx = equiv_constant (x)) != NULL_RTX)
         return new_rtx;
       return x;