Author: Dan Honeycutt
Date: 07:34:01 02/18/04
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On February 17, 2004 at 20:58:28, Robert Hyatt wrote: >On February 17, 2004 at 17:38:13, Dan Honeycutt wrote: > >>I realize I'm adding where you are subtracting due to different numbering. What >>I'm saying is that 3/4 of the time I still have to make "an adjustment" to the >>value bsf/bsr gives. Do you not have to make an adjustment 4/4 of the time, or >>do you have to make 2 adjustments? >> >>Dan H. > > >I do it at the end, one time. IE for the Opteron, it looks like this: > >movq %rax,63 >bsfq %rbx, %rcx >subq %rcx, %rax ; done, result in rax (64 bit reg). > >for X86, I do it in three steps: > >first check one half, then add 32, >second check other half >subtract from 63 > >I can post the x86 if you want... I'm with you. on x86 I'm check one half, if there is a bit I'm done check other half add 32 I get a slight advantage on 32 bit but more on 64 bit where I do nothing but you still have to subtract. Thanks for the help. Dan H.
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