[llvm-bugs] [Bug 38515] New: 64-bit atomic_load

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38515

            Bug ID: 38515
           Summary: 64-bit atomic_load
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: nruslan_devel at yahoo.com
                CC: llvm-bugs at lists.llvm.org

I have noticed that clang/llvm generates atomic_load (64-bit) for i686+ as
cmpxchg8b, whereas gcc avoids it by using FPU instructions.

#include <stdatomic.h>


unsigned long long func(_Atomic(unsigned long long) *a)
{
        return atomic_load(a);
}

clang -Wall -O2 -m32 -S test.c


clang/llvm:

func:                                   # @func
# %bb.0:
        pushl   %ebx
        pushl   %esi
        movl    12(%esp), %esi
        xorl    %eax, %eax
        xorl    %edx, %edx
        xorl    %ecx, %ecx
        xorl    %ebx, %ebx
        lock            cmpxchg8b       (%esi)
        popl    %esi
        popl    %ebx
        retl
.Lfunc_end0:


whereas gcc generates:

func:
.LFB0:
        .cfi_startproc
        subl    $12, %esp
        .cfi_def_cfa_offset 16
        movl    16(%esp), %eax
        fildq   (%eax)
        fistpq  (%esp)
        movl    (%esp), %eax
        movl    4(%esp), %edx
        addl    $12, %esp
        .cfi_def_cfa_offset 4
        ret

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