[Bug middle-end/51766] New: [4.7 regression] sync_fetch_and_xxx atomicity

["dje at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51766

             Bug #: 51766
           Summary: [4.7 regression] sync_fetch_and_xxx atomicity
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dje@gcc.gnu.org


During the C++11 memory model conversion, expand_builtin_sync_operation was
updated.  __sync_fetch_and_xxx and __sync_xxx_and_fetch now invoke
expand_atomic_fetch_op with MEMMODEL_SEQUENTIAL_CST.

  return expand_atomic_fetch_op (target, mem, val, code, MEMMODEL_SEQ_CST,
                                 after);

This has changed the memory model semantics of the builtins and causes the
rs6000/POWER architecture to emit a heavy-weight "sync" instruction instead of
a light-weight "lwsync" instruction, which is a regression.