Re: [Qemu-devel] [PATCH 00/14] VSX Stage 4

[Tom Musta]

On 11/7/2013 6:23 PM, Richard Henderson wrote:
> Modulo my comments wrt the actual computation of fma, the patches all look 
> fine.
> 
> But I'd like to also mention a pre-existing flaw/niggle in the ppc port.
> 
> The conversions to/from in-register representation for the single-precision
> values should never raise exceptions.  Yet we always use
> 
>     d.d = float32_to_float64(f.f, &env->fp_status);
>     f.f = float64_to_float32(d.d, &env->fp_status);
> 
> The use of env->fp_status is either wrong or extremely misleading.  It sure
> looks like the operation affects global cpu state.  It may be that that state
> is never copied back to the "real" fpscr and so doesn't actually affect cpu
> state, but how can I see that for sure?
> 
> I think it would be better to implement ConvertSPtoDP_NP and ConvertSP64toSP
> exactly as written in the spec.
> 
> Or at minimum use a dummy fp_status that's not associated with env.  It should
> not matter what the "real" rounding mode is in either case, since values that
> are not exactly representable as single-precision values give undefined 
> results.

I've looked more closely at the code and have performed some experiments.  There
are several status flags that are being set by the float32_to_float64 call. And
they are copied back near the end of these routines via the 
helper_float_check_status.
So I think this is all necessary.

That said, rather than repeating the float32_to_float64() / float64_to_float32()
pattern everywhere, I should have reused the existing helper_frsp() routine.

So I will be publishing a V2.