[LLVMbugs] [Bug 24191] New: Possibly inefficient std::atomic<int> codegen on x86 for simple arithmetic

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=24191

            Bug ID: 24191
           Summary: Possibly inefficient std::atomic<int> codegen on x86
                    for simple arithmetic
           Product: clang
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tkoeppe at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

[I also reported this issue to GCC:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66881]

Consider these two simple versions of addition:

  #include <atomic>

  std::atomic<int> x;
  int y;

  void f(int a) {
    x.store(x.load(std::memory_order_relaxed) + a, std::memory_order_relaxed);
  }

  void g(int a) {
    y += a;
  }

Clang generates the following assembly (https://goo.gl/IWtwkr):

  f(int):                                  # @f(int)
    mov    eax, dword ptr [rip + x]
    add    eax, edi
    mov    dword ptr [rip + x], eax
    ret

  g(int):                                  # @g(int)
    add    dword ptr [rip + y], edi
    ret

Now, it is clear to me that the correct atomic codegen for store() and load()
is "mov", as it appears here, but why aren't the two consecutive operations not
folded into a single add? Aren't the semantics and the memory ordering the
same? x86 says that (most) "reads" and "writes" are strongly ordered; doesn't
that apply to the read and write produced by "add", too?

(My original motivation came from a variant of this with floats, where the
non-atomic code executed noticeably faster, even though I would have expected
the two to produce the same machine code.)

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