Using __builtin_ia32_shufps to shift a vector by 32 bits? - 【StackMirror】|c|vectorization|simd|shift

This is the prototype for the function:

v4si __builtin_ia32_shufps (v4si, v4si, int)

On some websites I found they had but hex in the int field, and it looked liked it separated high and low bits, but what I want is a logical 32 bit shift.

X3 X2 X1 X0 shifted by 32 bits to get X2 X1 X0 0

Another example using 2 v4si vectors:

X7 X6 X5 X4 | X3 X2 X1 X0, where each X is a 32 bit and what I want for a shift is the 
same a logical shift, but with each vector element. So:
X7 X6 X5 X4 | X3 X2 X1 X0 << 2 = X5 X4 X3 X2 | X1 X0  0  0

Is shufps the right command to do this?

2012-04-04 23:11
by Jim

I think you mean "bits" not "bytes" - Mysticial 2012-04-04 23:13

What is 'v4sf' defined as - Richard J. Ross III 2012-04-04 23:17

Correct I did mean bits, v4sf is a 128 bit vector, split into logical 4 32 bit vectors. I think - Jim 2012-04-04 23:18

Are you sure you want to bitshift floating point numbers? Because the v4sf type is for 4 32bit floating point numbers - C2H5OH 2012-04-04 23:29

Yes, but if it makes it simpler to think about as ints, then I can redefine the question - Jim 2012-04-04 23:35

Looking at your example with two vectors I think what you're probably looking for is _mm_alignr_epi8 (PALIGNR). This works for any shift of a pair of vectors by an arbitrary number of bytes, so you would need to multiply the shift parameter by sizeof(int), e.g.

v = _mm_alignr_epi8(v0, v1, 2 * sizeof(int));

Note that this instruction is only available in SSSE3 and later, which means pretty much any Intel CPU since ~ 2005.

2012-04-05 05:50
by Paul R