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

[Paul Brook <paul@codesourcery.com>]

> > 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.

Paul