Question

Let’s say you have a uint64_t and care only about the high order bit for each byte in your uint64_t. Like so:

uint32_t:
0000 … 1000 0000 1000 0000 1000 0000 1000 0000 —> 0000 1111

Is there a faster way than:

   return
   (
     ((x >> 56) & 128)+
     ((x >> 49) &  64)+
     ((x >> 42) &  32)+
     ((x >> 35) &  16)+
     ((x >> 28) &   8)+
     ((x >> 21) &   4)+
     ((x >> 14) &   2)+
     ((x >>  7) &   1)
   )

Aka shifting x, masking, and adding the correct bit for each byte? This will compile to a lot of assembly and I’m looking for a quicker way… The machine I’m using only has up to SSE2 instructions and I failed to find helpful SIMD ops.

Thanks for the help.

Answer 1

As I mentioned in a comment, pmovmskb does what you want. Here’s how you could use it:

MMX + SSE1:

movq mm0, input ; input can be r/m
pmovmskb output, mm0 ; output must be r

SSE2:

movq xmm0, input
pmovmskb output, xmm0

And I looked up the new way

BMI2:

mov rax, 0x8080808080808080
pext output, input, rax ; input must be r