c++ - Looking for sse 128 bit shift operation for non-immediate shift value

Question

The intrinsic _mm_slli_si128 will do a logical shift left of a 128 bit register, but is restricted to immediate shift values, and shifts by bytes not bits.

I can use an intrinsic like _mm_sll_epi64 or _mm_sll_epi32 to shift left a set of values within the __m128i register, but these don't carry the "overflow" bits.

For a shift by N bits imagine that I could do a something like:

• _mm_sll_epi64
• _mm_srr_epi64 (for the bits I want to carry: move them into the low order )
• shuffle the srr result
• or these together.

(but probably also have to include checks of N relative to 64).

Is there a better way?

Answer 1

Not your ideal solution, but if you want to rotate or shift an SSE register by a number of bits that is a multiple of 8, then the PSHUFB instruction (and the _mm_shuffle_epi8() intrinsic) can help. It takes a second SSE register as an input; each byte in the register holds a value that is used to index the bytes in the first input register.