Re: [Qemu-devel] [PATCH 01/13] target-arm: A64: add support for conditional select

[Richard Henderson <rth@twiddle.net>]

On 12/07/2013 01:45 AM, Peter Maydell wrote:
> On 5 December 2013 22:26, Richard Henderson <rth@twiddle.net> wrote:
>> On 12/06/2013 10:51 AM, Peter Maydell wrote:
>>> +    if (cond >= 0x0e) { /* condition "always" */
>>> +        tcg_src = read_cpu_reg(s, rn, sf);
>>> +        tcg_gen_mov_i64(tcg_rd, tcg_src);
>>
>> I wonder if it's worth adding that 0x0[ef] case to the generic condition
>> processing rather than keep replicating it everywhere.
> 
> I think "always true" is a special case anyway because you don't
> want to emit any kind of branching/label logic at all.

Sure, but unlike unconditional branches, which are useful to special-case, one
sort of expects never to see an unconditional conditional move.  Given
TCG_COND_ALWAYS, we can re-use generic logic and have things fall out
relatively easily.

> I had a think about this and I couldn't really come up with a particularly
> nice looking API for it, given the way that movcondi/setcondi/brcondi work.
> The best I could come up with was something like:

The s390 target has an example with DisasCompare and disas_jcc.
The i386 target has another example with CCPrepare and gen_prepare_cc.

The i386 port uses similar flags to ARM, but represents them differently.  I
suppose good ideas could be taken from either port.


> So I definitely think I'd rather postpone this for now, unless you have
> a neat idea that I've missed for making it look nice.

Let's just postpone for now.


r~