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From: "steven at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/56151] [4.8 Regression] Performance degradation after r194054 on x86 Atom.
Date: Sun, 03 Feb 2013 01:05:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56151

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |steven at gcc dot gnu.org

--- Comment #4 from Steven Bosscher <steven at gcc dot gnu.org> 2013-02-03 01:05:34 UTC ---
Patch to help explain the problem:

Index: optabs.c
===================================================================
--- optabs.c    (revision 195687)
+++ optabs.c    (working copy)
@@ -1452,8 +1452,13 @@ expand_binop_directly (enum machine_mode
     {
       /* If PAT is composed of more than one insn, try to add an appropriate
         REG_EQUAL note to it.  If we can't because TEMP conflicts with an
-        operand, call expand_binop again, this time without a target.  */
+        operand, call expand_binop again, this time without a target.
+
+        However, if target is a MEM, just accept the lossage of not having
+        a REG_EQUAL note.  This avoids splitting up insns of the form
+        "MEM=MEM op X", a form we may not be able to reconstruct later.  */
       if (INSN_P (pat) && NEXT_INSN (pat) != NULL_RTX
+         && ! (target && MEM_P (target))
          && ! add_equal_note (pat, ops[0].value, optab_to_code (binoptab),
                               ops[1].value, ops[2].value))
        {