Is there an 8-bit atomic CAS (cmpxchg) intrinsic for X64 in Visual C++?

Question

The following code is possible in 32-bit Visual Studio C++. Is there a 64-bit equivalent using intrinsics since inline ASM isn't supported in the 64-bit version of Visual Studio C++?

FORCEINLINE bool bAtomicCAS8(volatile UINT8 *dest, UINT8 oldval, UINT8 newval)
{
    bool result=false;
    __asm
    {
        mov     al,oldval
        mov     edx,dest
        mov     cl,newval
        lock cmpxchg    byte ptr [edx],cl
        setz    result
    }
    return(result);
}

The following instrinsics compile under Visual Studio C++

_InterlockedCompareExchange16
_InterlockedCompareExchange
_InterlockedCompareExchange64
_InterlockedCompareExchange128

What I am looking for is something along the lines of

_InterlockedCompareExchange8

But that doesn't seem to exist.

score 3 · Answer 1 · answered Jan 05 '14 at 13:07

3

Verified for Visual Studio 2012 that this intrinsic does exist :

char _InterlockedCompareExchange8(volatile char*, char NewValue, char Expected)

However, it appears nowhere in the documentation.

answered Jan 05 '14 at 13:07

harrymc

1,069
8
33

At the time I was using 2010 which doesn't have this intrinsic. I'm using 2012 now though so all is better :-) – Adisak Feb 14 '14 at 01:05

score 3 · Accepted Answer · answered Apr 27 '11 at 20:44

3

No, that doesn't exist. You can implement it out-of-line though, if needed.

atomic_msvc_x64.asm

_text   SEGMENT

; char _InterlockedCompareExchange8(volatile char*, char NewValue, char Expected) 
;      - RCX, RDX, R8

_InterlockedCompareExchange8  PROC

    mov    eax,r8d
    lock cmpxchg [rcx],dl
    ret

_InterlockedCompareExchange8  ENDP

_text  ENDS

       END

answered Apr 27 '11 at 20:44

Bo Persson

90,663
31
146
203

What register is the return variable for a 64-bit asm function ? – Adisak Apr 28 '11 at 06:49
1

The return value is in AL, AX, EAX, RAX depending on the size. Here it is put there by the cmpxchg instruction. – Bo Persson Apr 28 '11 at 07:19
At least since Visual Studio 2012, this is no longer required, although not documented. See my answer. – harrymc Jan 05 '14 at 13:08

Is there an 8-bit atomic CAS (cmpxchg) intrinsic for X64 in Visual C++?

2 Answers2

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