Best way to XOR two 128 bit values in MSVC?

Question

I'm trying to XOR the 128 bit Initialization Vector with the Plaintext as seen here

In linux x86-64 gcc 12.2, there's a one liner

*(unsigned __int128 *)( plaintext ) ^= *(unsigned __int128 *)( ivvectext );

For example, https://godbolt.org/z/sc8e66qeo

#include <stdio.h>
#include <stdint.h>

int main()
{
    uint8_t plaintext[16] = {'t','h','e','q','u','i','c','k','b','r','o','w','n','f','o','x'};
    uint8_t ivvectext[16] = {'w','a','1','2','o','b','s','q','m','v','c','s','s','q','u','w'};

    *(unsigned __int128 *)( plaintext ) ^= *(unsigned __int128 *)( ivvectext );

    for (int i = 0; i < sizeof(plaintext); i++) { printf("%02X ", (unsigned char)plaintext[i]); }

    return 0;
}

Question

In MSVC, what's the preferred method to XOR these 128 bit values?

Update

As noted in one of the answers, use the compiler intrinsic _mm_xor_si128

#include <stdint.h>
#include <immintrin.h>
#include <iostream>
#include <ios>
#include <iomanip>

int main() {

    uint8_t plaintext[16] = { 't','h','e','q','u','i','c','k','b','r','o','w','n','f','o','x' };
    uint8_t ivvectext[16] = { 'w','a','1','2','o','b','s','q','m','v','c','s','s','q','u','w' };

    __m128i plain = _mm_loadu_si128((__m128i*)plaintext);
    __m128i ivvec = _mm_loadu_si128((__m128i*)ivvectext);

    __m128i xored = _mm_xor_si128(plain, ivvec);

    uint8_t* xored_array = (uint8_t*)&xored;
    for (int i = 0; i < 16; i++) {
        std::cout << std::uppercase << std::setw(2) << std::setfill('0') << std::hex << (int)xored_array[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

The output matches linux

03 09 54 43 1A 0B 10 1A 0F 04 0C 04 1D 17 1A 0F 

However, other answers suggest more readable code

for (int i = 0; i < sizeof(plaintext); i++) 
{
    plaintext[i] ^= ivvectext[i];
}

and let the compiler optimizations figure out the internal assembly code. :)

Answer 1

If your goal is to optimize your code, then leave this task to the compiler. (Of course you might have to enable optimization.)

You can write a simple loop like

for (int i = 0; i < sizeof(plaintext); i++) 
{
    plaintext[i] ^= ivvectext[i];
}

and let the compiler optimize this.

For example, x86 msvc v19.latest with option -O2 creates SSE2 instructions from this loop resulting in a single 128-bit operation.

_main   PROC                                      ; COMDAT
        sub     esp, 36                             ; 00000024H
        mov     eax, DWORD PTR ___security_cookie
        xor     eax, esp
        mov     DWORD PTR __$ArrayPad$[esp+36], eax
        mov     DWORD PTR _plaintext$[esp+36], 1902471284 ; 71656874H
        mov     DWORD PTR _plaintext$[esp+40], 1801677173 ; 6b636975H
        mov     DWORD PTR _plaintext$[esp+44], 2003792482 ; 776f7262H
        mov     DWORD PTR _plaintext$[esp+48], 2020566638 ; 786f666eH
        movups  xmm1, XMMWORD PTR _plaintext$[esp+36]
        mov     DWORD PTR _ivvectext$[esp+36], 842097015 ; 32316177H
        mov     DWORD PTR _ivvectext$[esp+40], 1903387247 ; 7173626fH
        mov     DWORD PTR _ivvectext$[esp+44], 1935898221 ; 7363766dH
        mov     DWORD PTR _ivvectext$[esp+48], 2004185459 ; 77757173H
        movups  xmm0, XMMWORD PTR _ivvectext$[esp+36]
        push    esi
        xor     esi, esi
        pxor    xmm1, xmm0
        movups  XMMWORD PTR _plaintext$[esp+40], xmm1
...

See https://godbolt.org/z/afTPK5von

Additional hints from comments:

Even if you determine that you need to hand-optimize the code and use the intrinsic functions explicitly (e.g., the optimizer doesn't use them for some reason, sad panda), I recommend also keeping the straightforward implementation as a reference implementation for development & debugging purposes. (Eljay's comment)

Sometimes the MS compiler won't optimize what looks like a simple loop, in this case you can enable Vectorizer and parallelizer messages which can give you hints as to why it didn't. (user20716902's comment)

Answer 2

As mentioned in my comment I would use intrincis: Here is how to do it in MSVC:

#include <immintrin.h>
int main() {

    uint8_t plaintext[16] = { 't','h','e','q','u','i','c','k','b','r','o','w','n','f','o','x' };
    uint8_t ivvectext[16] = { 'w','a','1','2','o','b','s','q','m','v','c','s','s','q','u','w' };

    __m128i plain = _mm_loadu_si128((__m128i*)plaintext);
    __m128i ivvec = _mm_loadu_si128((__m128i*)ivvectext);

    __m128i xored = _mm_xor_si128(plain, ivvec);
    
    return 0;
}