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[Bug target/42869] GOMP_critical_start wrong on Itanium due to __sync miscompilation
- From: "sje at cup dot hp dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 9 Mar 2010 23:25:23 -0000
- Subject: [Bug target/42869] GOMP_critical_start wrong on Itanium due to __sync miscompilation
- References: <bug-42869-6957@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #2 from sje at cup dot hp dot com 2010-03-09 23:25 -------
I think the code is wrong. Looking at ToT, config/ia64/sync.md will generate
the code shown where the memory fence is in front of the cmpxchg4.rel
instruction.
cmpxchg4.rel has release semantics which are defined in the Itanium manual as:
"Release instructions guarentee that all previous orderable instructions are
made visible prior to being made visible themselves." This would imply to me
that the memory fence before the cmpxchg4.rel is not needed but we do need one
after it in order to prevent things being moved up ahead of the cmpxchg4.rel.
Alternatively, we could change to code to use cmpxchg4.acq instead of
cmpxchg4.rel and keep the memory fence where it is. cmpxchg4.acq has acquire
semantics instead of release semantics.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42869