Comparing 2 vectors in AVX/AVX2 (c)

Question

I have two __m256i vectors (each containing chars), and I want to find out if they are completely identical or not. All I need is true if all bits are equal, and 0 otherwise.

What's the most efficient way of doing that? Here's the code loading the arrays:

char * a1 = "abcdefhgabcdefhgabcdefhgabcdefhg";
__m256i r1 = _mm256_load_si256((__m256i *) a1);

char * a2 = "abcdefhgabcdefhgabcdefhgabcdefhg";
__m256i r2 = _mm256_load_si256((__m256i *) a2);

Answer 1

The most efficient way on current Intel and AMD CPUs is an element-wise comparison for equality, and then check that the comparison was true for all elements.

This compiles to multiple instructions, but they're all cheap and (if you branch on the result) the compare+branch even macro-fuses into a single uop.

#include <immintrin.h>
#include <stdbool.h>

bool vec_equal(__m256i a, __m256i b) {
    __m256i pcmp = _mm256_cmpeq_epi32(a, b);  // epi8 is fine too
    unsigned bitmask = _mm256_movemask_epi8(pcmp);
    return (bitmask == 0xffffffffU);
}

The resulting asm should be vpcmpeqd / vpmovmskb / cmp 0xffffffff / je, which is only 3 uops on Intel CPUs.

vptest is 2 uops and doesn't macro-fuse with jcc, so equal or worse than movmsk / cmp for testing the result of a packed-compare. (See http://agner.org/optimize/