Here are some codes I used in an old project (there is a research paper about it). The function popcnt8 below computes the number of bits set in each byte. SSE2-only version (based on Algorithm 3 in Hacker’s Delight book): static const __m128i popcount_mask1 = _mm_set1_epi8(0x77); static const __m128i popcount_mask2 = _mm_set1_epi8(0x0F); static inline __m128i … Read more
You can abuse PSADBW to calculate horizontal sums of bytes without overflow. For example: pxor xmm0, xmm0 psadbw xmm0, [a + 0] ; sum in 2x 64-bit chunks pxor xmm1, xmm1 psadbw xmm1, [a + 16] paddw xmm0, xmm1 ; accumulate vertically pshufd xmm1, xmm0, 2 ; bring down the high half paddw xmm0, xmm1 … Read more
Remember that SSE1 orps came first. (Well actually MMX por mm, mm/mem came even before SSE1.) Having the same opcode with a new prefix be the SSE2 orpd instruction makes sense for hardware decoder logic, I guess, just like movapd vs. movaps. Several instructions like this are redundant between between ps and pd versions, but … Read more
From different inputs, I gathered these solutions. The key to crossing the inter-lane barrier is the align instruction, _mm256_alignr_epi8. _mm256_slli_si256(A, N) 0 < N < 16 _mm256_alignr_epi8(A, _mm256_permute2x128_si256(A, A, _MM_SHUFFLE(0, 0, 2, 0)), 16 – N) N = 16 _mm256_permute2x128_si256(A, A, _MM_SHUFFLE(0, 0, 2, 0)) 16 < N < 32 _mm256_slli_si256(_mm256_permute2x128_si256(A, A, _MM_SHUFFLE(0, 0, 2, … Read more
The biggest problem with x87 is basically that all register operations are done in 80 bits, whereas most of the time people only use 64 bit floats (i.e. double-precision floats). What happens is, you load a 64 bit float into the x87 stack, and it gets converted to 80 bits. You do some operations on … Read more
Is there any difference between using one or another intrinsic (with appropriate type casting). Won’t there be any hidden costs like longer execution in some specific situation? Yes, there can be performance reasons to choose one vs. the other. 1: Sometimes there is an extra cycle or two of latency (forwarding delay) if the output … Read more
SIMD is meant to work on multiple small values at the same time, hence there won’t be any carry over to the higher unit and you must do that manually. In SSE2 there’s no carry flag but you can easily calculate the carry as carry = sum < a or carry = sum < b … Read more