From patchwork Thu Jun 24 03:47:21 2010
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Subject: [PR44610] don't delegitimize MEM from base without offset
Date: Wed, 23 Jun 2010 17:47:21 -0000
From: Alexandre Oliva <aoliva@redhat.com>
X-Patchwork-Id: 56741
Message-Id: <orhbktccs6.fsf@livre.localdomain>
To: gcc-patches@gcc.gnu.org

Interesting bug...  All words of a multi-word variable ended up mapped
to the same VALUE during cselib in var-tracking, and then their location
was set to the same REG, that held only the first word.

Problem was that we were initializing the variable within a loop, from
an array element that lived in memory and varied per iteration of the
loop.  This means the MEM_OFFSETs couldn't be set: they weren't
constant.

Turns out that some recently-added code that attempted to simplify MEMs
to something more palatable than complex expressions involving frame,
stack or GOT pointers, mistook the absence of an offset for a zero
offset.  So, because of this delegitimization, each of the words loaded
into part of the variable was regarded by cselib as a MEM referencing
the first word of the array.  Oops.

This patch prevents the delegitimization when the offset is unknown, so
that we don't get bogus equivalences.

Regstrapped on x86_64-linux-gnu, i686-linux-gnu and ia64-linux-gnu.

Ok for trunk?


for  gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	PR debug/44610
	* simplify-rtx.c (delegitimize_mem_from_attrs): Don't use a base
	address if the offset is unknown.

Index: gcc/simplify-rtx.c
===================================================================
--- gcc/simplify-rtx.c.orig	2010-06-23 01:15:14.000000000 -0300
+++ gcc/simplify-rtx.c	2010-06-23 01:20:21.000000000 -0300
@@ -208,10 +208,11 @@ avoid_constant_pool_reference (rtx x)
 rtx
 delegitimize_mem_from_attrs (rtx x)
 {
+  /* MEMs without MEM_OFFSETs may have been offset, so we can't just
+     use their base addresses as equivalent.  */
   if (MEM_P (x)
       && MEM_EXPR (x)
-      && (!MEM_OFFSET (x)
-	  || GET_CODE (MEM_OFFSET (x)) == CONST_INT))
+      && MEM_OFFSET (x))
     {
       tree decl = MEM_EXPR (x);
       enum machine_mode mode = GET_MODE (x);
@@ -264,8 +265,7 @@ delegitimize_mem_from_attrs (rtx x)
 	{
 	  rtx newx;
 
-	  if (MEM_OFFSET (x))
-	    offset += INTVAL (MEM_OFFSET (x));
+	  offset += INTVAL (MEM_OFFSET (x));
 
 	  newx = DECL_RTL (decl);