What’s the fastest way to pack 32 0/1 values into the bits of a single 32-bit variable?

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The fastest way on recent x86 processors is probably to make use of the MOVMSKB family of instructions which extract the MSBs of a SIMD word and pack them into a normal integer register.

I fear SIMD intrinsics are not really my thing but something along these lines ought to work if you’ve got an AVX2 equipped processor:

uint32_t bitpack(const bool array[32]) {
    __mm256i tmp = _mm256_loadu_si256((const __mm256i *) array);
    tmp = _mm256_cmpgt_epi8(tmp, _mm256_setzero_si256());
    return _mm256_movemask_epi8(tmp);
}

Assuming sizeof(bool) = 1. For older SSE2 systems you will have to string together a pair of 128-bit operations instead. Aligning the array on a 32-byte boundary and should save another cycle or so.

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