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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/57448] GCSE generates incorrect code with acquire barrier
Date: Wed, 29 May 2013 09:34:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57448

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-05-29
                 CC|                            |stevenb.gcc at gmail dot com
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
;; _7 = __atomic_load_4 (&seq, 2);

(insn 13 12 0 (set (reg:SI 61 [ D.2227 ])
        (mem/v:SI (symbol_ref:DI ("seq") [flags 0x40]  <var_decl 0x7ffff6e67558
seq>) [-1 S4 A32])) t.c:8 -1
     (nil))

the MEM alias-set of -1 (ALIAS_SET_MEMORY_BARRIER) is supposed to prevent this,
but PRE does not know about this special property (it should "kill" all refs
crossing it).