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Re: [Qemu-devel] qemu vs gcc4
From: |
Rob Landley |
Subject: |
Re: [Qemu-devel] qemu vs gcc4 |
Date: |
Wed, 1 Nov 2006 11:34:23 -0500 |
User-agent: |
KMail/1.9.1 |
On Tuesday 31 October 2006 10:22 pm, Paul Brook wrote:
> > > and glue them together like we do with dyngen. However once you've done
> > > that you've implemented most of what's needed for fully dynamic qops, so
> > > it doesn't really seem worth it.
> >
> > I missed a curve. What's "fully dynamic qops"? (There's no translation
> > cache?)
>
> I mean all the qop stuff I've implemented.
Still lost. Where does the "fully dynamic" part come in?
> > Any idea where I can get a toolchain that can output a "hello world"
> > program for m68k nommu? (Or perhaps you have a statically linked "hello
> > world" program for the platform lying around?)
>
> Funnily enough I do :-)
> http://www.codesourcery.com/gnu_toolchains/coldfire/
Woot.
This download page was designed by your company's legal department, I take it?
(There's no such thing as GNU/Linux, and you don't have to accept the GPL
because it's based on copyright law, not contract law, so "informed consent"
is not the basis for enforcement.)
> > > Theoretically possible, but not so easy in practice. Especially when you
> > > get things like partial flag clobbers, and lazy flag evaluation. Doing
it
> > > as a target specific hack is much simpler and quicker.
> >
> > I think I know what partial flag clobbers are (although if you're working
> > your way back, in theory you could handle it with a mask of exposed bits),
> > but what's lazy flag evaulation? (I thought that was the point of
> > eliminating the unused flag setting. Are you saying the hardware also
does
> > this and we have to emulate that?)
>
> Lazy flag evaluation is where you don't bother calculating the actual flags
> when executing the flag-setting instruction. Instead you save the
> operands/result and compute the flags when you actually need them.
Such as when the computation's in a loop but you only test after exiting the
loop for other reasons? I'd have to see examples to figure out how it would
make sense to optimize that...
> > The exponential complexity is if you have to write different code for each
> > combination of host and target. If every target disassembles to the same
> > set of target QOPs, then you could have a hand-written assembly version of
> > each QOP for each host platform, and still have N rather than N^2 of them.
>
> Right, but by the time you've got everything to use the same set of ops you
> may as well teach qemu how to generate code instead of using potted
> fragments.
I'm thinking of "here's the code for performing this QOP on this processor".
I'm not sure what distinction you're making.
> Using hand-written assembly fragments probably doesn't make qemu any faster,
> it just removes the gcc dependency.
Which seems like a good thing to me. (Or at least the gcc _version_
dependency.)
> Using qops also allows qemu to generate better (faster) translated code.
Currently, I'm interested in building qemu under compiler versions I actually
have installed, without having to apply patches that the patch authors
consider too disgusting to integrate.
> Paul
Rob
--
"Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away." - Antoine de Saint-Exupery
- Re: [Qemu-devel] qemu vs gcc4,
Rob Landley <=