Author: Matt Taylor
Date: 14:27:07 12/03/02
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On December 03, 2002 at 15:15:27, Russell Reagan wrote: >On December 03, 2002 at 10:09:31, Gerd Isenberg wrote: > >>Let's see how fast Hammer's bsf reg64,reg64 will be. >>If it sucks like Athlon, there are a lot of good alternatives. >>I guess that with 64-bit arithmetic, to do a fast single bit isolation the trick >>with 64int to 32bit float conversion is promising. > >I remember Eugene Nalimov giving a C routine for Crafty's FirstOne() that would >run in ~12 cycles on the Itanium, after optimizations. bsf might not be needed >at all. > >Which is faster on for finding the lsb of a 32-bit value on 32-bit hardware? bsf >or a C-routine? If one is faster than the other on 32-bit hardware for 32-bit >values, would the same be faster for a 64-bit value on 64-bit hardware? Even between existing 32-bit hardware (e.g. Athlon, P3, P4) it varies widely. The bsf instruction in particular takes variable time because it's really a CISC instruction that codes for a slightly complex loop in microcode. Another thing to remember is that, though Itanium may do it in 12 cycles, Itanium's paradigm of computing is the VLIW idea of a heavily parrallel machine executing larger numbers of instructions per clock. A single Itanium cycle does much more work than a single cycle on your average P4 2 GHz or AthlonXP 2000 system, which is why Itanium can afford to lag behind 1 GHz without sacrificing its performance crown. Existing IA-32 processors are very different, and observations made about Itanium will not necessarily hold true for Athlon and P4. Elsewhere in this discussion I posted some timings recorded from my AthlonMP for several bit-scan algorithms that had been mentioned, including the version using the bsf instruction. The bsf instruction was faster in one case where it had the advantage of an early-out algorithm.
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