Commits on Oct 17, 2023

1. optimize popcount implementation …

   In the current implementation, the gcc backend of rustc currently emits the
   following for a function that implements popcount for a u32 (x86_64 targeting
   AVX2, using standard unix calling convention):
   
       popcount:
           mov     eax, edi
           and     edi, 1431655765
           shr     eax
           and     eax, 1431655765
           add     edi, eax
           mov     edx, edi
           and     edi, 858993459
           shr     edx, 2
           and     edx, 858993459
           add     edx, edi
           mov     eax, edx
           and     edx, 252645135
           shr     eax, 4
           and     eax, 252645135
           add     eax, edx
           mov     edx, eax
           and     eax, 16711935
           shr     edx, 8
           and     edx, 16711935
           add     edx, eax
           movzx   eax, dx
           shr     edx, 16
           add     eax, edx
           ret
   
   Rather than using this implementation, gcc could be told to use Wenger's
   algorithm.  This would give the same function the following implementation:
   
       popcount:
           xor eax, eax
           xor edx, edx
           popcnt eax, edi
           test edi, edi
           cmove eax, edx
           ret
   
   This patch implements the popcount operation in terms of Wenger's algorithm in
   all cases.
   
   Signed-off-by: Andy Sadler <[email protected]>

   

   sadlerap committed Oct 17, 2023
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