Messages in this thread | | | Date | Wed, 16 Jan 2002 11:16:55 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: [Q] Looking for an emulation for CMOV* instructions. |
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On Wed, 16 Jan 2002, Jamie Lokier wrote:
> Alan Cox wrote: > > > What's the point of optimizing an IF to a cmov if I have > > > to insert another IF to see if I can use cmov? > > > > I've always wondered. Intel made the instruction optional yet there isnt > > an obvious way to do runtime fixups on it > > Yes there is -- emulation! :-) >
It's just as bad, probably worse! You trap on an invalid op-code. The trap-handler checks the op-code and if it's emulated, it emulates it and returns to the executing task. This takes many instruction cycles, certainly more than `if(cmov) doit; else do_something_else;` --which, itself, takes many more instruction cycles than cmov is supposed to reduce. It's a no-win situation. The only way to win is a compile-time choice. This means customizing for your CPU IFF it has the cmov instruction.
Cheers, Dick Johnson
Penguin : Linux version 2.4.1 on an i686 machine (797.90 BogoMips).
I was going to compile a list of innovations that could be attributed to Microsoft. Once I realized that Ctrl-Alt-Del was handled in the BIOS, I found that there aren't any.
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