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[Bug target/70814] New: atomic store of __int128 is not lock free on aarch64


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70814

            Bug ID: 70814
           Summary: atomic store of __int128 is not lock free on aarch64
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yyc1992 at gmail dot com
  Target Milestone: ---

`std::atomic<__int128>::store` on aarch64 is not lock free and generates a
function call `__atomic_store_16`. However, required atomic instructions
(`stlxp`) exists in armv8 and is used by clang. Should gcc use the same
instruction as well?

Ref https://llvm.org/bugs/show_bug.cgi?id=27081, which fixes the clang codegen
for this case.

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