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[patch] Fix PR71976 in combine.c:get_last_value()


The issue with wrong-code bug PR71976 is that combine.c:get_last_value was called for a hard register in a wider mode (DImode) than the stored value had (QImode). The patch introduces a test of the mode precision and only returns the recorded value if the according mode is not smaller than the mode for which get_last_value is called.

Segher already tested the patch on 32 different (sub-)architectures of Linux without any regression, cf.

https://gcc.gnu.org/ml/gcc/2016-07/msg00215.html

Ok for trunk and to backport?


Johann

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.
Index: combine.c
===================================================================
--- combine.c	(revision 238849)
+++ combine.c	(working copy)
@@ -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;

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