http://d.puremagic.com/issues/show_bug.cgi?id=4717



--- Comment #10 from Don <clugdbug@yahoo.com.au> 2010-09-22 13:10:44 PDT ---
(In reply to comment #9)
> See also:
> http://www.strchr.com/crc32_popcnt
> http://wm.ite.pl/articles/sse-popcount.html

Yes, I saw those.
I made a simple 256-entry table lookup implementation (below, not optimised for
size) which runs at 5 cycles for 4 bytes. It'd be painful to beat that for
general-purpose 32 bit code (because AMD 32bit processors don't support SSE2).
Cache misses will kill you, though, unless the array is quite long.
I include my code here anyway, for future reference.

For 64 bits, SWAR on SSE2 is a clear winner.

----------
const(uint[256]) makepopcountlookup(){

    uint [256] result;
    for (int i = 0; i<= 0xFF; ++i)
    {
        result[i] = (i&1) + ((i&2)>>1) + ((i&4)>>2) + ((i&8)>>3) +
            ((i&16)>>4) + ((i&32)>>5) + ((i&64)>>6) + ((i&128)>>7);
    }
    return result;
}

__gshared uint[256] POPCOUNT_LOOKUP_TABLE = makepopcountlookup();

/*
 A lookup table is normally a bad way to do popcount since it risks a cache
miss.
 But 1K table is not so terrible, and we're dealing with a large source array.

 The address of the lookup table is passed as a parameter to avoid PIC
problems.
*/
int popcountArray(uint[] src, uint *lookuptable = &POPCOUNT_LOOKUP_TABLE[0])
{
    enum { LASTPARAM = 4*4 } // 3* pushes + return address.

   // TIMING: Core2: 12uops, 5.0 cycles/uint
   // It's entirely limited by the 5 loads.

    asm {
        naked;

        push ESI;
        push EDI;
        push EBX;

        mov EDI, EAX; // EDI = lookup table.

        mov ECX, [ESP + LASTPARAM + 0*4]; // src.length;
        mov ESI, [ESP + LASTPARAM + 1*4]; // src.ptr
        xor EAX, EAX;

        lea ESI, [ESI + 4*ECX]; // ESI = end of src
        neg ECX; // count UP to zero.
        mov EBX, [ESI + 4*ECX];
        xor EDX, EDX;
        add ECX, 1;
        jz onlyone;
    L1:
        add EAX, [EDI + EDX * 4];
        movzx EDX, BL;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BH;
        shr EBX, 16;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BH;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BL;

        mov EBX, [ESI + 4*ECX];
        add ECX, 1;
        jnz L1;

onlyone:
        add EAX, [EDI + EDX * 4];
        movzx EDX, BL;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BH;
        shr EBX, 16;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BH;

        add EAX, [EDI + EDX * 4];
        movzx EDX, BL;

        add EAX, [EDI + EDX * 4];

        pop EBX;
        pop EDI;
        pop ESI;
        ret 2*4;
    }
}

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