On Wed, Jan 03, 2007 at 03:12:13AM +0100, Mikael Pettersson wrote:
> On Tue, 2 Jan 2007 17:43:00 -0800 (PST), Linus Torvalds wrote:
> > > The suggestions I've had so far which I have not yet tried:
> > >
> > > - Select a different x86 CPU in the config.
> > > - Unfortunately the C3-2 flags seem to simply tell GCC
> > > to schedule for ppro (like i686) and enabled MMX and SSE
> > > - Probably useless
> >
> > Actually, try this one. Try using something that doesn't like "cmov".
> > Maybe the C3-2 simply has some internal cmov bugginess.
>
> That's a good suggestion. Earlier C3s didn't have cmov so it's
> not entirely unlikely that cmov in C3-2 is broken in some cases.
Agreed! When I developped the cmov emulator, I used an early C3 for the
tests (well, a "Samuel2" to be precise), because it did not report "cmov"
in its flags. I first thought "wow, my emulator is amazingly fast!" because
it took something like 50 cycles to do cmovne %eax,%ebx.
Then I realized that this processor performed cmov itself between
registers, and only triggered the invalid opcode when one of the operand
was a memory reference. And this time, for a hard-coded instruction, it
was really slow...
For this reason, I would not be surprized at all that there would be some
buggy behaviour in the cmov right there. Maybe a bug in the decoder unit
making it skip a byte when the next instruction in the prefetch queue is
a cmov affecting same registers... When vendors can do dirty things such
as executing unsupported instructions, we can expect anything from them.
> Configuring for P5MMX or 486 should be good safe alternatives.
I generally use the P5MMX target for such processors.
> /Mikael
Regards,
Willy
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