Messages in this thread | | | Subject | Re: [Q] Looking for an emulation for CMOV* instructions. | From | Ronald Wahl <> | Date | 16 Jan 2002 18:48:49 +0100 |
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On Wed, 16 Jan 2002 11:16:55 -0500 (EST), Richard B Johnson wrote:
> 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.
It is a big win in the situation where you have only a binary for i686 and want or better must execute it e.g. on a i586. This was the reason why I asked initially.
ron
-- /\/\ Dipl.-Inf. Ronald Wahl /\/\ C S N /\/\ \/\/ ronald.wahl@informatik.tu-chemnitz.de \/\/ ------------------ \/\/ /\/\ http://www.tu-chemnitz.de/~row/ /\/\ network and system /\/\ \/\/ GnuPG/PGP key available \/\/ administration \/\/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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