[LLVMbugs] [Bug 13675] New: [optimization] C++11 std::atomic<>.fetch_or() could be optimized to use 'bts'

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13675

             Bug #: 13675
           Summary: [optimization] C++11 std::atomic<>.fetch_or() could be
                    optimized to use 'bts'
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: mgorny at gentoo.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


$ clang --version
clang version 3.2 (trunk) (04622b05755fb9dbb062735a53e779e1deb29a97)
Target: x86_64-pc-linux-gnu
Thread model: posix

The minimal test case:

#include <atomic>

int main()
{
    std::atomic_int foo;

    return foo.fetch_or(1);
}

Results in the following assembly (-O3 -S):

# BB#0:
    movl    $1, %ecx
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
    movl    -8(%rsp), %eax
    movl    %eax, %edx
    orl %ecx, %edx
    lock
    cmpxchgl    %edx, -8(%rsp)
    jne .LBB0_1
# BB#2:
    ret

I believe that such an operation (atomic bit-setting) could be done much easier
and faster using the 'bts' (bit test & set) mnemonic.

I'm not sure if this can be enhanced in clang, llvm or whether it needs changes
to libstdc++.

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