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Re: [Qemu-devel] [PATCH 00/18] target-arm cleanup

From: Paul Brook
Subject: Re: [Qemu-devel] [PATCH 00/18] target-arm cleanup
Date: Tue, 10 Nov 2009 23:38:49 +0000
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> > Some of the generated tcg code is not very optimal, for example a
> > single vld4.8 instruction can generate over 250 tcg ops. I did some
> > optimizations and got it under 200 but do you think it could be an
> > issue that a single instruction can expand to so many tcg ops? I mean
> > besides the fact that it causes TBs for only one or two guest
> > instructions to be generated.
> Fabrice wrote this (tcg/README):
>   Don't hesitate to use helpers for complicated or seldom used target
>   intructions. There is little performance advantage in using TCG to
>   implement target instructions taking more than about twenty TCG
>   instructions.
> How applicable is it, I can't say.  It'd probably be a good thing
> to benchmark the two versions, TCG vs helper.

The problem is that you can not do memory accesses from within a helper 


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