Re: [Qemu-devel] [PULL 5/7] tcg: Add mmu helpers that take a return address argument

[Richard Henderson <rth@twiddle.net>]

On 08/26/2013 03:26 PM, Paolo Bonzini wrote:
> Something that can be done on top of this patch: what about moving the
> "-1" to helper_ret_*?  It is common to pretty much all the targets
> (except ARM has -2), and it would allow some simplifications.

I suppose so, yes.

>     li   rN, retaddr
>     mtlr rN
>     b    st_trampoline[i]
> 
> sequence instead of one of
> 
>     li   rN, retaddr
>     mtlr rN
>     bl   st_trampoline[i]
>     b    retaddr

This sort of thing is very difficult to evaluate, because of the
cpu's return address prediction stack.  I have so far avoided it.

The only cpus that I believe can make good use of tail calls into
the memory helpers are those with predicated stores and calls, i.e.
arm and ia64.


r~