[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: About hardfloat in ppc

From: BALATON Zoltan
Subject: Re: About hardfloat in ppc
Date: Thu, 30 Apr 2020 17:16:23 +0200 (CEST)
User-agent: Alpine 2.22 (BSF 395 2020-01-19)

On Tue, 28 Apr 2020, Alex Bennée wrote:
罗勇刚(Yonggang Luo) <address@hidden> writes:
I am confusing why only  inexact  are set then we can use hard-float.

The inexact behaviour of the host hardware may be different from the
guest architecture we are trying to emulate and the host hardware may
not be configurable to emulate the guest mode.

Have a look in softfloat.c and see all the places where
float_flag_inexact is set. Can you convince yourself that the host
hardware will do the same?

Can you convince me that it won't? This all seems to be guessing without evidence so I think what we need first is some tests to prove it either way. Such tests could then also be used at runtime to decide if the host and guest FPU are compatible enough to enable hardfloat. Are such tests available somewhere or what would need to be done to implement them?

This may not solve the problem with PPC target with non-cumulative status bits but could improve hardfloat performance at least for some host-guest combinations. To see if it worth the effort we should run such test on common combinations (say x86_64. ARM and PPC hosts with at least these guests).

And PPC always clearing inexact  flag before calling to soft-float
funcitons. so we can not
optimize it with hard-float.
I need some resouces about ineact flag and why always clearing inexcat in
PPC FP simualtion.

Because that is the behaviour of the PPC floating point unit. The
inexact flag will represent the last operation done.

More precisely additional to the usual cumulative (or sticky) bits there are two non-sticky bits for inexact and rounded (latter of which is not emulated) that currently need clearing FP status before every FP op. I wonder if we can know when the guest reads these and rerun the last FP op in softfloat to compute them only if these are read, then it's enough to remember the last FP op. This could be relatively simple and may be used even if we don't detect accessing the bits within FPSCR just accessing the FPSCR as likely most guest code does not check that and any cross-platform code won't check PPC specific non-sticky bits so I'd exepect most guest code to be fine with hardfloat. Although what about FP exceptions? We also need to revert to softfloat it FP exceptions are enabled so maybe using host FP exception for managing status bits could be the way to go to let hardware manage this and we don't need to implement everything in software.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]