R: About hardfloat in ppc

[Dino Papararo]

Hello,
about handling of PPC fpu exceptions and Hard Floats support we could consider 
a different approach for different instructions.
i.e. not all fpu instructions take care about inexact or exceptions bits: if I 
take a simple fadd f0,f1,f2 I'll copy value derived from adding f1+f2 into f1 
register and no one will check about inexact or exception bits raised into 
FPSCR register.
Instead if I'll take fadd. f0,f1,f2 the dot following the add instructions 
means I want take inexact or exceptions bits into account.
So I could use hard floats for first case and softfloats for second case.
Could this be a fast solution to start implement hard floats for PPC??

A little of documentation here: 
http://mirror.informatimago.com/next/developer.apple.com/documentation/mac/PPCNumerics/PPCNumerics-154.html

Regards,
Dino Papararo

-----Messaggio originale-----
Da: Qemu-devel <qemu-devel-bounces+skizzato73=address@hidden> Per conto di Alex 
Bennée
Inviato: martedì 28 aprile 2020 10:37
A: address@hidden
Cc: address@hidden; address@hidden
Oggetto: Re: About hardfloat in ppc


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?

> 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.

> I am looking for two possible solution:
> 1. do not clear inexact flag in PPC simulation 2. even the inexact are 
> cleared, we can still use alternative hard-float.
>
> But now I am the beginner, Have no clue about all the things.

Well you'll need to learn about floating point because these are rather 
fundamental aspects of it's behaviour. In the old days QEMU used to use the 
host floating point processor with it's template based translation.
However this led to lots of weird bugs because the floating point answers under 
qemu where different from the target it was trying to emulate. It was for this 
reason softfloat was introduced. The hardfloat optimisation can only be done 
when we are confident that we will get the exact same answer of the target we 
are trying to emulate - a "faster but incorrect" mode is just going to cause 
confusion as discussed in the previous thread. Have you read that yet?

>
> On Mon, Apr 27, 2020 at 7:10 PM Alex Bennée <address@hidden> wrote:
>
>>
>> BALATON Zoltan <address@hidden> writes:
>>
>> > On Mon, 27 Apr 2020, Alex Bennée wrote:
>> >> 罗勇刚(Yonggang Luo) <address@hidden> writes:
>> >>> Because ppc fpu-helper are always clearing float_flag_inexact, So 
>> >>> is that possible to optimize the performance when
>> float_flag_inexact
>> >>> are cleared?
>> >>
>> >> There was some discussion about this in the last thread about 
>> >> enabling hardfloat for PPC. See the thread:
>> >>
>> >>  Subject: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC
>> >>  Date: Tue, 18 Feb 2020 18:10:16 +0100
>> >>  Message-Id: <address@hidden>
>> >
>> > I've answered this already with link to that thread here:
>> >
>> > On Fri, 10 Apr 2020, BALATON Zoltan wrote:
>> > : Date: Fri, 10 Apr 2020 20:04:53 +0200 (CEST)
>> > : From: BALATON Zoltan <address@hidden>
>> > : To: "罗勇刚(Yonggang Luo)" <address@hidden>
>> > : Cc: address@hidden, Mark Cave-Ayland, John Arbuckle,
>> address@hidden, Paul Clarke, Howard Spoelstra, David Gibson
>> > : Subject: Re: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC
>> > :
>> > : On Fri, 10 Apr 2020, 罗勇刚(Yonggang Luo) wrote:
>> > :> Are this stable now? I'd like to see hard float to be landed:)
>> > :
>> > : If you want to see hardfloat for PPC then you should read the 
>> > replies to : this patch which can be found here:
>> > :
>> > : http://patchwork.ozlabs.org/patch/1240235/
>> > :
>> > : to understand what's needed then try to implement the solution 
>> > with FP : exceptions cached in a global that maybe could work. I 
>> > won't be able to : do that as said here:
>> > :
>> > : 
>> > https://lists.nongnu.org/archive/html/qemu-ppc/2020-03/msg00006.htm
>> > l
>> > :
>> > : because I don't have time to learn all the details needed. I think :
>> > others are in the same situation so unless somebody puts in the :
>> > necessary effort this won't change.
>> >
>> > Which also had a proposed solution to the problem that you could 
>> > try to implement, in particular see this message:
>> >
>> >
>> http://patchwork.ozlabs.org/project/qemu-devel/patch/20200218171702.9
>> address@hidden/#2375124
>> >
>> > amd Richard's reply immediately below that. In short to optimise 
>> > FPU emulation we would either find a way to compute inexact flag 
>> > quickly without reading the FPU status (this may not be possible) 
>> > or somehow get status from the FPU but the obvious way of claring 
>> > the flag and reading them after each operation is too slow. So 
>> > maybe using exceptions and only clearing when actually there's a 
>> > change could be faster.
>> >
>> > As to how to use exceptions see this message in above thread:
>> >
>> > https://lists.nongnu.org/archive/html/qemu-ppc/2020-03/msg00005.htm
>> > l
>> >
>> > But that's only to show how to hook in an exception handler what it 
>> > does needs to be implemented. Then tested and benchmarked.
>> >
>> > I still don't know where are the extensive PPC floating point tests 
>> > to use for checking results though as that was never answered.
>>
>> Specifically for PPC we don't have them. We use the softfloat test 
>> cases to exercise our softfloat/hardfloat code as part of "make 
>> check-softfloat". You can also re-build fp-bench for each guest 
>> target to measure raw throughput.
>>
>> >> However in short the problem is if the float_flag_inexact is clear 
>> >> you must use softfloat so you can properly calculate the inexact 
>> >> status. We can't take advantage of the inexact stickiness without 
>> >> loosing the fidelity of the calculation.
>> >
>> > I still don't get why can't we use hardware via exception handler 
>> > to detect flags for us and why do we only use hardfloat in some 
>> > corner cases. If reading the status is too costly then we could 
>> > mirror it in a global which is set by an FP exception handler. 
>> > Shouldn't that be faster? Is there a reason that can't work?
>>
>> It would work but it would be slow. Almost every FP operation sets 
>> the inexact flag so it would generate an exception and exceptions 
>> take time to process.
>>
>> For the guests where we use hardfloat operations with inexact already 
>> latched is not a corner case - it is the common case which is why it 
>> helps.
>>
>> >
>> > Regards,
>> > BALATON Zoltan
>>
>>
>> --
>> Alex Bennée
>>


--
Alex Bennée