Re: [Qemu-devel] TCG: AREG0 removal planning

[Avi Kivity <avi@redhat.com>]

On 05/11/2011 08:25 PM, Blue Swirl wrote:
> >
> >  I think a useful, and incremental goal is elimination of global cpu_env state
> >  in C code (i.e eliminate HELPER_CFLAGS and dyngen-exec.h).
> >  We already have much of the infrastructure for this - op_helper v.s. helper.c
> >  and code_gen_prologue for transition in/out of "generated code" state.
> >
> >  In practice generated code probably accesses CPUState often enough that a
> >  dedicated register isn't a bad idea.  My guess is that eliminating it from C
> >  code gets us almost all of the useful benefit.  Removing it from the code
> >  generator (i.e. TCG_AREG0) may be more pain that it's worth.
>
> I don't think moving the helpers from op_helper.c to helper.c will be
> a performance win if AREG0 is not eliminated. The code gets to use one
> register more, but AREG0 needs to be moved to a function argument
> register in most cases and AREG0 has to be restored. I think the
> benefit should come from generated code getting one more available
> register.

If you use a callee-saved register then you don't need to restore it.  
Looks like that's already the case, at least on x86.

-- 
error compiling committee.c: too many arguments to function