Re: R: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC

[Aleksandar Markovic]

On Wed, Feb 26, 2020 at 6:04 PM G 3 <address@hidden> wrote:
>
> Accuracy is an important part of the IEEE 754 floating point standard. The 
> whole purpose of this standard is to ensure floating point calculations are 
> consistent across multiple CPUs. I believe referring to this patch as 
> inaccurate is itself inaccurate. That gives the impression that this patch 
> produces calculations that are not inline with established standards. This is 
> not true. The only part of this patch that will produce incorrect values are 
> the flags. There *may* be a program or two out there that depend on these 
> flags, but for the majority of programs that only care about basic floating 
> point arithmetic this patch will produce correct values. Currently the 
> emulated PowerPC's FPU already produces wrong values for the flags. This 
> patch does set the Inexact flag (which I don't like), but since I have never 
> encountered any source code that cares for this flag, I can let it go. I 
> think giving the user the ability to decide which option to use is the best 
> thing to do.
>

>From the experiments described above, the patch in question changes the 
>behavior
of applications (for example, sound is different with and without the
patch), which is
in contradiction with your claim that you "never encountered any
source code that
cares for this flag" and that "the only part of this patch that will
produce incorrect
values are the flags".

In other words, and playing further with them:

The claim that "referring to this patch as inaccurate is itself
inaccurate" is itself inaccurate.

Best regards,
Aleksandar


> On Wed, Feb 26, 2020 at 10:51 AM Aleksandar Markovic <address@hidden> wrote:
>>
>>
>>
>> On Wed, Feb 26, 2020 at 3:29 PM Alex Bennée <address@hidden> wrote:
>> >
>> >
>> > Dino Papararo <address@hidden> writes:
>> >
>> > > Please let's go with hardfloat pps support, it's really a good feature 
>> > > to implement.
>> > > Even if in a first step it could lead to inaccuracy results, later it
>> > > could solved with other patches.
>> >
>> > That's the wrong way around. We have regression tests for a reason.
>>
>> I tend to agree with Alex here, and additionally want to expand more on
>> this topic.
>>
>> In my view: (that I think is at least very close to the community consensus)
>>
>> This is *not* a ppc-specific issue. There exist a principle across all 
>> targets
>> that QEMU FPU calculation must be accurate - exactly as specified in any
>> applicable particular ISA document. Any discrepancy is an outright bug.
>>
>> We even recently had several patches for FPU in ppc target that handled
>> some fairly obscure cases of inaccuracies, I believe they were authored
>> by Paul Clarke, so there are people in ppc community that care about
>> FPU accuracy (as I guess is the case for any target).
>>
>> There shouldn't be a target that decides by itself and within itself
>> "ok, we don't need accuracy, let's trade it for speed". This violates
>> the architecture of QEMU. Please allow that for any given software
>> project, there is an architecture that should be respected.
>>
>> This doesn't mean that anybody's experimentation is discouraged. No-one
>> can stop anybody from forking from QEMU upstream tree and do whatever
>> is wanted.
>>
>> But, this doesn't mean such experimentation will be upstreamed. QEMU
>> upstream should be collecting place for the best ideas and implementations,
>> not for arbitrary experimentations.
>>
>> Best regards,
>> Aleksandar
>>
>>
>> > I'll happily accept patches to turn on hardfloat for PPC if:
>> >
>> >  a) they don't cause regressions in our fairly extensive floating point
>> >  tests
>> >  b) the PPC maintainers are happy with the new performance profile
>> >
>> > The way forward would be to:
>> >
>> >  1. patch to drop #if defined(TARGET_PPC) || defined(__FAST_MATH__)
>> >  2. audit target/ppc/fpu_helper.c w.r.t chip manual and fix any unneeded
>> >  splatting of flags (if any)
>> >  3. measure the before/after performance effect and decide if on balance
>> >  it's worth keeping
>> >
>> > > I think it's important for qemu to as global as possible and don't
>> > > target only recent hardware.
>> >
>> > Are you referring to guests or hosts? For guests we will always favour
>> > accuracy of speed of emulation. For hosts we need to have IEEE compliant
>> > FPU HW to even stand a chance of using hardfloat.
>> >
>> > --
>> > Alex Bennée
>> >