Redistribute least significant bits from a 4-byte array to a nibble

Question

I wish to move bits 0,8,16,24 of a 32-bit value to bits 0,1,2,3 respectively. All other bits in the input and output will be zero.

Obviously I can do that like this:

c = c>>21 + c>>14 + c>>7 + c;
c &= 0xF;

But is there a faster (fewer instructions) way?

Answer 1

c = (((c&BITS_0_8_16_24) * BITS_0_7_14_21) >> 21) & 0xF;

Or wait for Intel Haswell processor, doing all this in exactly one instruction (pext).

Update

Taking into account clarified constraints and assuming 32-bit unsigned values, the code may be simplified to this:

c = (c * BITS_7_14_21_28) >> 28;

Answer 2

If you don't care about portability, and can use SSE instructions, look at the PMOVMSKB instruction and its compiler intrinsic. [I noticed that your bit positions are most significant (sign) bits of the 4 bytes comprising the 32-bit word.]

Answer 3

Instead of writing some obfuscated one-line goo, the below code is what I would write, for maximum portability and maintainability. I would let the optimizer worry about whether or not it is the most effective code.

#include <stdint.h>
#include <limits.h>
#include <stdio.h>

#define BITS_TO_MOVE  4

static const uint32_t OLD_MASK [BITS_TO_MOVE] =
{
  0x0008u,
  0x0080u,
  0x0800u,
  0x8000u
};

static const uint32_t NEW_MASK [BITS_TO_MOVE] =
{
  0x1000u,
  0x2000u,
  0x4000u,
  0x8000u
};


int main()
{
  uint32_t  c     = 0xAAAAu;
  uint32_t  new_c = 0;
  uint8_t   i;

  printf("%.4X\n", c);


  for(i=0; i<BITS_TO_MOVE; i++)
  {
    if ( (c & OLD_MASK[i]) > 0 )
    {
      new_c |= NEW_MASK[i];
    }
  }


  printf("%.4X\n", new_c);
  getchar();

  return 0;
}