Author: Frank Phillips
Date: 03:33:00 12/07/02
Go up one level in this thread
On December 06, 2002 at 00:49:11, Walter Faxon wrote:
>On December 02, 2002 at 20:27:39, Gerd Isenberg wrote:
>
>>Ahh.. i see,
>>
>>a special kind of popcount for consecutive trailing "ones".
>>I love this interactive programming lessions.
>>
>>Gerd
>>
>>Btw.: Impressive assembler listing:
>>
>>00401003 8B 4C 24 0C mov ecx,dword ptr [esp+0Ch]
>>00401007 8B 31 mov esi,dword ptr [ecx]
>>00401009 8B 79 04 mov edi,dword ptr [ecx+4]
>>0040100C 8B D6 mov edx,esi
>>0040100E 8B C7 mov eax,edi
>>00401010 83 C2 FF add edx,0FFFFFFFFh
>>00401013 83 D0 FF adc eax,0FFFFFFFFh
>>00401016 23 F2 and esi,edx
>>00401018 23 F8 and edi,eax
>>0040101A 89 31 mov dword ptr [ecx],esi
>>0040101C 89 79 04 mov dword ptr [ecx+4],edi
>>0040101F 8B CF mov ecx,edi
>>00401021 33 D6 xor edx,esi
>>00401023 33 C1 xor eax,ecx
>>00401025 33 C2 xor eax,edx
>>00401027 5F pop edi
>>00401028 35 81 FC C5 01 xor eax,1C5FC81h
>>0040102D 5E pop esi
>>0040102E 8B D0 mov edx,eax
>>00401030 C1 EA 10 shr edx,10h
>>00401033 03 C2 add eax,edx
>>00401035 33 D2 xor edx,edx
>>00401037 8B C8 mov ecx,eax
>>00401039 C1 E9 08 shr ecx,8
>>0040103C 2A C1 sub al,cl
>>0040103E 25 FF 00 00 00 and eax,0FFh
>>00401043 8A 90 D5 62 40 00 mov dl,byte ptr [eax+4062D5h]
>>00401049 8B C2 mov eax,edx
>>0040104B C3 ret
>
>
>Hi, Gerd.
>
>I have noticed that most compilers aren't too smart where it comes to bitwise
>dataflow analysis. After the "add eax, edx" instruction above, better (well,
>shorter!) code would be:
>
> sub al, ah
> and eax, 0FFh
> mov al, byte ptr [eax+4062D5h]
> ret
>
>Perhaps if we added a little "hint" in the last 2 lines of source:
>
>inline // inline declaration may differ by compiler
>u8 LSB_64( u64* bb )
> {
> u64 t64;
> u32 t32;
> u8 t8;
> t64 = *bb - 1;
> *bb &= t64; // omit this line to retain current LSB
> t64 ^= *bb;
> t32 = (u32)t64 ^ (u32)(t64 >> 32);
> t32 ^= LSB_64_magic;
> t32 += t32 >> 16;
> t8 = (u8)t32 - (u8)(t32 >> 8);
> return LSB_64_table [LSB_64_adj + t8];
> }
>
>Of course, this really only helps because we already know that the x86 class can
>independently address both the first and 2nd bytes of a register. And it's
>wishful thinking that much better asm code will actually be produced! I'd be
>interested to see.
>
>If/when you run your test suite again, please try this version. Thanks.
>
>-- Walter
Has anybody posted Walter's method with the table for 0 == highest order bit?
Frank
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