[llvm-bugs] [Bug 45651] New: target attribute "cx16" is ignored

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45651

            Bug ID: 45651
           Summary: target attribute "cx16" is ignored
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: myriachan at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Compile the following with -O2 on x86_64:

__attribute__((__target__("cx16")))
void write128(_Atomic(__int128)* p, __int128 v)
{
    __c11_atomic_store(p, v, __ATOMIC_SEQ_CST);
}

That generates:

<source>:4:5: warning: large atomic operation may incur significant performance
penalty [-Watomic-alignment]
    __c11_atomic_store(p, v, __ATOMIC_SEQ_CST);
    ^
1 warning generated.

_Z8write128Pnn: # @_Z8write128Pnn
  sub rsp, 24
  mov rax, rdi
  mov qword ptr [rsp + 8], rdx
  mov qword ptr [rsp], rsi
  mov rdx, rsp
  mov edi, 16
  mov rsi, rax
  mov ecx, 5
  call __atomic_store
  add rsp, 24
  ret

If you add -mcx16 to the command line, Clang generates the intended code:

_Z8write128Pnn: # @_Z8write128Pnn
  push rbx
  mov rcx, rdx
  mov rbx, rsi
  mov rax, qword ptr [rdi]
  mov rdx, qword ptr [rdi + 8]
.LBB0_1: # =>This Inner Loop Header: Depth=1
  lock cmpxchg16b xmmword ptr [rdi]
  jne .LBB0_1
  pop rbx
  ret

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