Re: [Qemu-devel] [PATCH 1/2] tcg/arm: Factor out code to emit immediate or reg-reg op

[Peter Maydell]

On 26 September 2012 20:46, Peter Maydell <address@hidden> wrote:
> On 26 September 2012 20:01, Richard Henderson <address@hidden> wrote:
>> The patch itself is fine.  But as a followup, if movcc is no longer "free",
>> then perhaps the setcond sequence is better as
>>
>>         cmp
>>         mov
>>         movcc
>>
>> i.e. the second move is unconditional?  A register renaming OOO core could
>> then schedule the mov before the cmp.
>
> Well, maybe. But my bar for changing existing code requires more
> proof that it's worth making the change (basically in both cases
> I'm optimising for convenience of development). My suspicion
> (entirely unbenchmarked) is that at the moment both these sequences
> will pan out about the same cost, so we might as well pick the
> one that's easiest to code.

Apparently there is really very little in it but if we did change then
the recommendation would be to try something like:
    MOV dest, #0
    CMP c1, c2
    ADDcc dest, dest, #1

I could tell you why, but then I'd have to kill you :-)

-- PMM