Re: [Qemu-ppc] PPC hardfloat (was: Re: [Qemu-devel] [Bug 1841442] Re: floating point emulation can fail to set FE_INEXACT)

[Richard Henderson]

On 8/26/19 1:28 PM, BALATON Zoltan wrote:
> On Mon, 26 Aug 2019, Richard Henderson wrote:
>> That said, qemu-system-ppc64 will *never* use hardfloat, because ppc always
>> need the current and correct result of inexact for emulation of the FI bit,
>> which requires that we use the softfloat path.
> 
> That's bad news. I hoped that hardfloat for PPC can be implemented and
> previously it was thought it could be done after some reorganisation to 
> prevent
> always reseting flags by moving them to environment or somewhere else (but I
> don't remember the details and maybe I never fully understood it in the first
> place). Could you please explain why do you think it's not possible?

We only use hardfloat if we do not need to compute whether the current
operation produces an inexact exception.  We need not care when the sticky
inexact bit is already set.

However, for ppc, the FPSCR[FI] bit indicates whether the previous fp operation
was inexact.  Thus we need to compute the inexact exception for every single
operation.


r~