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Subject: cmov isn't necessarily good

Author: Tom Kerrigan

Date: 12:16:27 07/18/03

On July 18, 2003 at 04:05:52, Walter Faxon wrote:

>>; 326  :     if (bbHalf) bb0 = bb1;              // will code as cmov (ideally)
>>
>>	test	ecx, ecx
>>	je	SHORT $L806
>>	mov	eax, DWORD PTR _bb$[esp]
>>$L806:
>>
>
>
>Stupid compiler, not only no cmov

IIRC, on the P6 (Pentium Pro, Pentium II, Pentium III), the cmov instruction
gets translated into a string of uOps that's equivalent to testing, branching,
and copying.

In other words, there is no performance benefit (I believe there may actually be
a performance penalty) to using cmov on a P6, and it breaks compatibility with
pre-P6 processors, so it's little wonder the P6-era MS compiler doesn't generate
cmovs.

-Tom

Re: cmov isn't necessarily good Robert Hyatt 22:11:31 07/18/03
- Re: cmov isn't necessarily good Tom Kerrigan 23:11:34 07/18/03
  - Re: cmov isn't necessarily good Matt Taylor 15:06:29 07/20/03
    - Re: cmov isn't necessarily good Robert Hyatt 07:13:44 07/21/03
  - Re: cmov isn't necessarily good Robert Hyatt 11:48:09 07/20/03
    - Re: cmov isn't necessarily good Tom Kerrigan 16:15:26 07/21/03
      - Re: cmov isn't necessarily good Robert Hyatt 11:18:23 07/22/03
        
        Re: cmov isn't necessarily good Tom Kerrigan 12:02:46 07/22/03
        
        Re: cmov isn't necessarily good Eugene Nalimov 12:48:08 07/22/03
        
        Re: cmov isn't necessarily good Tom Kerrigan 14:29:50 07/22/03
        
        Re: cmov isn't necessarily good Robert Hyatt 12:52:55 07/22/03
        
        Re: cmov isn't necessarily good (more) Robert Hyatt 12:58:47 07/22/03
        
        Re: cmov isn't necessarily good Robert Hyatt 12:27:10 07/22/03
        
        Re: cmov isn't necessarily good Tom Kerrigan 14:41:55 07/22/03
        
        Re: cmov isn't necessarily good Robert Hyatt 15:31:51 07/22/03
        
        Re: cmov isn't necessarily good Tom Kerrigan 17:01:32 07/22/03
        
        Re: cmov isn't necessarily good Robert Hyatt 17:12:16 07/22/03
Re: cmov isn't necessarily good Gerd Isenberg 13:15:01 07/18/03

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