http://llvm.org/bugs/show_bug.cgi?id=19355
Bug ID: 19355
Summary: __GCC_ATOMIC_LLONG_LOCK_FREE only set to 1 on X86
Product: clang
Version: 3.4
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Hello,
when compiling the Boost Atomic library I noticed a test case failure which
expects that boost::atomic<long long> is always lock-free on X86 [1].
This is caused by the fact that clang sets the __GCC_ATOMIC_LLONG_LOCK_FREE to
1.
In comparison gcc (4.8) sets this preprocessor symbol to 2:
$ gcc --version
gcc (Debian 4.8.2-19) 4.8.2
$ gcc -dM -E - -m32 < /dev/null|grep __GCC_ATOMIC_LLONG_LOCK_FREE
#define __GCC_ATOMIC_LLONG_LOCK_FREE 2
In contrast:
$ clang --version
Debian clang version 3.4-2 (tags/RELEASE_34/final) (based on LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ clang -dM -E - -m32 < /dev/null|grep __GCC_ATOMIC_LLONG_LOCK_FREE
#define __GCC_ATOMIC_LLONG_LOCK_FREE 1
Is this difference a bug? Andrey Semashev stated in the Boost Atomic bug report
that Core2 CPUs all should support cmpxchg8b and thus be always completely lock
free.
Thanks,
Gregor
[1] See https://svn.boost.org/trac/boost/ticket/9842
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