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

[Aurelien Jarno]

On Tue, Nov 10, 2009 at 11:38:49PM +0000, Paul Brook wrote:
> > > 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 
> function.
> 

It is something possible, it's done for example for unaligned memory
access on MIPS (see for example helper_ldr).

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net