re PR rtl-optimization/71976 (insn-combiner deletes a live 64-bit shift)

gcc/
	PR rtl-optimization/71976
	* combine.c (get_last_value): Return 0 if the argument for which
	the function is called has a wider mode than the recorded value.

Co-Authored-By: Georg-Johann Lay <avr@gjlay.de>

From-SVN: r238863

gcc/ChangeLog

+2016-07-29  Segher Boessenkool  <segher@kernel.crashing.org>
+	    Georg-Johann Lay  <avr@gjlay.de>
+
+	PR rtl-optimization/71976
+	* combine.c (get_last_value): Return 0 if the argument for which
+	the function is called has a wider mode than the recorded value.
+
 2016-07-29  Marek Polacek  <polacek@redhat.com>
 
 	PR c/7652

gcc/combine.c

@@ -13210,6 +13210,12 @@ get_last_value (const_rtx x)
       && DF_INSN_LUID (rsp->last_set) >= subst_low_luid)
     return 0;
 
+  /* If fewer bits were set than what we are asked for now, we cannot use
+     the value.  */
+  if (GET_MODE_PRECISION (rsp->last_set_mode)
+      < GET_MODE_PRECISION (GET_MODE (x)))
+    return 0;
+
   /* If the value has all its registers valid, return it.  */
   if (get_last_value_validate (&value, rsp->last_set, rsp->last_set_label, 0))
     return value;