I want to shift SSE/AVX registers multiples of 32 bits left or right while shifting in zeros.
Let me be more precise on the shifts I'm interested in. For SSE I want to do the following shifts of four 32bit floats:
shift1_SSE: [1, 2, 3, 4] -> [0, 1, 2, 3]
shift2_SSE: [1, 2, 3, 4] -> [0, 0, 1, 2]
For AVX I want to shift do the following shifts:
shift1_AVX: [1, 2, 3, 4, 5, 6, 7, 8] -> [0, 1, 2, 3, 4, 5, 6, 7]
shift2_AVX: [1, 2, 3, 4, 5, 6, 7, 8] -> [0, 0, 1, 2, 3, 4, 5, 6]
shift3_AVX: [1, 2, 3, 4 ,5 ,6, 7, 8] -> [0, 0, 0, 0, 1, 2, 3, 4]
For SSE I have come up with the following code
shift1_SSE = _mm_castsi128_ps(_mm_slli_si128(_mm_castps_si128(x), 4));
shift2_SSE = _mm_shuffle_ps(_mm_setzero_ps(), x, 0x40);
//shift2_SSE = _mm_castsi128_ps(_mm_slli_si128(_mm_castps_si128(x), 8));
Is there a better way to do this with SSE?
For AVX I have come up with the following code which needs AVX2 (and it's untested). Edit (as explained by Paul R this code won't work).
shift1_AVX2 =_mm256_castsi256_ps(_mm256_slli_si256(_mm256_castps_si256(x), 4)));
shift2_AVX2 =_mm256_castsi256_ps(_mm256_slli_si256(_mm256_castps_si256(x), 8)));
shift3_AVX2 =_mm256_castsi256_ps(_mm256_slli_si256(_mm256_castps_si256(x), 12)));
How can I do this best with AVX not AVX2 (for example with _mm256_permute
or _mm256_shuffle`)? Is there a better way to do this with AVX2?
Edit:
Paul R has informed me that my AVX2 code won't work and that AVX code is probably not worth it. Instead for AVX2 I should use _mm256_permutevar8x32_ps
along with _mm256_and_ps
. I don't have a system with AVX2 (Haswell) so this is hard to test.
Edit:
Based on Felix Wyss's answer I came up with some solutions for AVX which only needs 3 intrisnics for shift1_AVX and shift2_AVX and only one intrinsic for shift3_AVX. This is due to the fact that _mm256_permutef128Ps
has a zeroing feature.
shift1_AVX
__m256 t0 = _mm256_permute_ps(x, _MM_SHUFFLE(2, 1, 0, 3));
__m256 t1 = _mm256_permute2f128_ps(t0, t0, 41);
__m256 y = _mm256_blend_ps(t0, t1, 0x11);
shift2_AVX
__m256 t0 = _mm256_permute_ps(x, _MM_SHUFFLE(1, 0, 3, 2));
__m256 t1 = _mm256_permute2f128_ps(t0, t0, 41);
__m256 y = _mm256_blend_ps(t0, t1, 0x33);
shift3_AVX
x = _mm256_permute2f128_ps(x, x, 41);
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