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Re: [Qemu-devel] tcg conditional set/move, round 3
From: |
Aurelien Jarno |
Subject: |
Re: [Qemu-devel] tcg conditional set/move, round 3 |
Date: |
Sun, 20 Dec 2009 00:02:33 +0100 |
User-agent: |
Mutt/1.5.20 (2009-06-14) |
On Sat, Dec 19, 2009 at 08:19:32AM -0800, Richard Henderson wrote:
> On 12/19/2009 05:03 AM, Aurelien Jarno wrote:
> >- For the movcond instruction, is there a real use for vtrue and vfalse
> > values? Most CPU actually implement a version with one value.
> > Implementing it with two values moves complexity within the arch
> > specific tcg code.
>
> The reason I added both is that rather than force TCG to insert
> extra moves in order to always match VFALSE with DEST, I could have
> the backend invert the condition if VTRUE happened to match DEST
> already.
>
> I suppose it would be possible to tweek the TCG register allocator
> to understand such commutative operations. That seemed harder, but
> perhaps it really isn't considering that inversion code has to be
> replicated across all the targets. It's certainly something to
> think about.
>
My main concerns here is that we are introducing a complex code here
that has to be multiplied by the number of TCG targets we have. On the
other hand I am not sure there are a lot of use cases for the different
architectures we support.
In addition I am reserved to make such bug changes given I am not sure
it will result in a speed gain. All the previous setcond implementations
have been stopped because there was no measurable gain.
--
Aurelien Jarno GPG: 1024D/F1BCDB73
address@hidden http://www.aurel32.net
- [Qemu-devel] Re: [PATCH 5/6] tcg-i386: Simplify brcond2., (continued)
- [Qemu-devel] Re: [PATCH 0/6] tcg conditional set/move, round 2, Laurent Desnogues, 2009/12/18
- [Qemu-devel] tcg conditional set/move, round 3, Richard Henderson, 2009/12/18
- Re: [Qemu-devel] tcg conditional set/move, round 3, Andreas Färber, 2009/12/19
- Re: [Qemu-devel] tcg conditional set/move, round 3, Aurelien Jarno, 2009/12/19
- Re: [Qemu-devel] tcg conditional set/move, round 3, Aurelien Jarno, 2009/12/19
- Re: [Qemu-devel] tcg conditional set/move, round 3, Richard Henderson, 2009/12/19
- Re: [Qemu-devel] tcg conditional set/move, round 3,
Aurelien Jarno <=