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

[Peter Maydell]

On 4 December 2013 00:48, Richard Henderson <address@hidden> wrote:
> 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.

Does a copy to temp actually cost us anything, though? I was
under the impression the TCG optimizer would basically throw
it away again.

I suppose you could just treat the semantics of it as "assume you
can't trash this TCGv" in all cases and rely on the auto-free.

> 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.  ;-)

Ah, I hadn't noticed we were already doing this in other frontends.

-- PMM