restrict.md

restrict keyword

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__restrict__ is to mention to the complier that for the scope of the pointer the target will only be accessed through that pointer

int mul(void* src, void* dst)

// will be invoked like mul(a, b)
// and never like mul(a, a+10)

which means that the data will not overlap and it can we vectorized

Use Case

void xor512(void *__restrict__ src, void *__restrict__ dst) {
  unsigned char* srcP = (unsigned char*)src;
  unsigned char* dstP = (unsigned char*)dst;

  for (int i=0; i< 64; ++i) {
    dstP[i] ^= srcP[i];
  }
}

Output

ASM

xor512(void*, void*):                          # @xor512(void*, void*)
        xor     eax, eax
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        movzx   ecx, byte ptr [rdi + rax]
        xor     byte ptr [rsi + rax], cl
        add     rax, 1
        cmp     rax, 64
        jne     .LBB0_1
        ret

With Restrict

Note : In clang 12 with -O3 for the above example is vectorized but it's auto vectorization, but without __restrict__ the compiler will generate two function a scalar and vector and use the corresponding one after checking for overlap. For further details read Clang runtime checks of pointers

xor512(void*, void*):                          # @xor512(void*, void*)
        movups  xmm0, xmmword ptr [rdi]
        movups  xmm1, xmmword ptr [rsi]
        xorps   xmm1, xmm0
        movups  xmm0, xmmword ptr [rsi + 16]
        movups  xmm2, xmmword ptr [rsi + 32]
        movups  xmm3, xmmword ptr [rsi + 48]
        movups  xmmword ptr [rsi], xmm1
        movups  xmm1, xmmword ptr [rdi + 16]
        xorps   xmm1, xmm0
        movups  xmmword ptr [rsi + 16], xmm1
        movups  xmm0, xmmword ptr [rdi + 32]
        xorps   xmm0, xmm2
        movups  xmmword ptr [rsi + 32], xmm0
        movups  xmm0, xmmword ptr [rdi + 48]
        xorps   xmm0, xmm3
        movups  xmmword ptr [rsi + 48], xmm0
        ret

AVX 512

Passing -mavx512bw will xor this with a single AVX 512 vector instruction

xor512(void*, void*):                          # @xor512(void*, void*)
        vmovdqu64       zmm0, zmmword ptr [rsi]
        vpxorq  zmm0, zmm0, zmmword ptr [rdi]
        vmovdqu64       zmmword ptr [rsi], zmm0
        vzeroupper
        ret

References:

• Chunking Optimizations: Let the Knife Do the Work