Re: [Qemu-devel] [PATCH 12/12] target-arm: A64: add support for compare and branch imm

[Richard Henderson <rth@twiddle.net>]

On 12/04/2013 01:32 PM, Peter Maydell wrote:
> You're right that we can just make this function return the TCGv
> temp rather than making the caller pass one in. Are you suggesting
> the 64-bit case should return cpu_X[reg] rather than a copy of it,
> though? I think it would be pretty hard to reason about if you had to
> remember that sometimes you got a trashable copy and sometimes
> the real register.

I think that the normal case is to write to the output in one step, and thus
having an input overlap an output isn't your problem but tcg's.  I would think
the case of multi-step output to be fairly rare, and easily solvable by always
allocating an output temporary.

Am I off-base on the multi-output thing?  Modulo flags setting, of course, but
I think that special case ought to be relatively solvable.

> (I'm still a bit on the fence about that pool of auto-freeing temporaries.
> Manual temp management is certainly a fertile source of decoder bugs,
> but in the longer term we might want to push the functionality down into
> the common code rather than having an ad-hoc thing in one decoder.)

See also Sparc and S390.  ;-)

I'm open to suggestions for doing this generically, but I've no great ideas
off-hand.


r~