C: is it possible to cast a uint64_t to const __m256i_u?

Question

I'm playing by writing with some AVX-optimised functions. I want to use a standard unsigned integer argument type like uint64_t instead of the 256-bit unsigned integer the AVX instructions require ( __m256i_u ). Is it possible to do the cast?

Answer 1

No, Intel's intrinsics API doesn't allow actual C casts between integer and vector types, I think not even between uint64_t and __m64 (a 64-bit MMX vector).

Use _mm256_set... and _mm_cvtsi128_si64 . (And _mm256_castsi256_si128 when necessary) to get value(s) into or the low value out of a vector, with a broadcast or a list of operands. See Intel's intrinsics guide for cvt and _mm256_set intrinsics; Google the intrinsic name for examples of using it, especially with site:stackoverflow.com . You might want to limit your intrinsics guide searches to SSE4, not AVX2, to limit the number of intrinsics to wade through. And so the parameter list is shorter; it's more immediately visible that _mm_set_epi32() takes 4 int args, for a total of 128 bits.

See also What are the names and meanings of the intrinsic vector element types, like epi64x or pi32? re: the existence of epi64x vs. epi64 (MMX to XMM vs. 64-bit integer)

Also, use __m256i , not GCC's internal __m256i_u unaligned type. Use __m256i v = __mm256_loadu_si256((const __m256i*) ptr); to do an unaligned load.