Author: Matt Taylor
Date: 11:16:47 12/31/02
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On December 31, 2002 at 04:44:27, Matt Taylor wrote: >On December 30, 2002 at 18:12:29, Walter Faxon wrote: > >>On December 24, 2002 at 23:28:27, Matt Taylor wrote: >> >>>http://65.186.75.165/~mtaylor/bitscan/bitscan.html >>> >><snip> >>> >>>-Matt >> >> >>Hi, Matt. >> >>Well, here is Yet Another Bit Scanner (yabs), this time based on my 32-bit >>folding code. Logically this one shouldn't require use of more than three >>general purpose registers, and thus it should be about as fast as anything I've >>posted before. Just for fun, I tried to be super-pedantic and thus guide any >>reasonable late-Pentium compiler into producing near-optimal code. >>Consequently, you should be able to hand-translate this into assembler in about >>3 minutes, ignoring register choices and scheduling issues. Eek. VC chose a branch again. It also did something rather funny: ... mov ecx, DWORD PTR [esi] test ecx, ecx sete dl test ecx, ecx mov DWORD PTR _index$[esp+8], 0 push edi mov BYTE PTR _index$[esp+12], dl jne SHORT $L73441 ... Note the duplication of the "test ecx, ecx" sequence. I guess it didn't realize that the flags don't get trashed by setcc. Anyway, you might have been too pedantic, if there is such a thing. The VC 7 compiler with full optimization targetted to Pentium Pro and beyond did not recognize the cmov possibility here. Since cmov is cheap (1 clock latency/throughput just like ALU instructions), I'm going to tweak the code a bit. Oh, and FYI: either because of your rigorous attempt to point out optimization potential or because of the different sequence to mask out the LSB, the 32-bit version of the routine is now faster by 2 clocks than the bsf instruction on my Athlon. I'm not really sure how much of a speed-up it is %-wise because I ran into issues with the timing code. Things were executing in negative time on Intel (P6/P7) chips when I tried to subtract out the timing bias, so I finally gave up and left it in. (Amusingly this was after I replaced my timing code with Intel's timing code.) -Matt
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