popcountsi2.c 1.2 KB

123456789101112131415161718192021222324252627282930313233
  1. /* ===-- popcountsi2.c - Implement __popcountsi2 ---------------------------===
  2. *
  3. * The LLVM Compiler Infrastructure
  4. *
  5. * This file is dual licensed under the MIT and the University of Illinois Open
  6. * Source Licenses. See LICENSE.TXT for details.
  7. *
  8. * ===----------------------------------------------------------------------===
  9. *
  10. * This file implements __popcountsi2 for the compiler_rt library.
  11. *
  12. * ===----------------------------------------------------------------------===
  13. */
  14. #include "int_lib.h"
  15. /* Returns: count of 1 bits */
  16. COMPILER_RT_ABI si_int
  17. __popcountsi2(si_int a)
  18. {
  19. su_int x = (su_int)a;
  20. x = x - ((x >> 1) & 0x55555555);
  21. /* Every 2 bits holds the sum of every pair of bits */
  22. x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
  23. /* Every 4 bits holds the sum of every 4-set of bits (3 significant bits) */
  24. x = (x + (x >> 4)) & 0x0F0F0F0F;
  25. /* Every 8 bits holds the sum of every 8-set of bits (4 significant bits) */
  26. x = (x + (x >> 16));
  27. /* The lower 16 bits hold two 8 bit sums (5 significant bits).*/
  28. /* Upper 16 bits are garbage */
  29. return (x + (x >> 8)) & 0x0000003F; /* (6 significant bits) */
  30. }