Re: [Qemu-devel] [PATCH] target/mips: Fix minor bug in FPU

[Aleksandar Markovic]

> From: Peter Maydell <address@hidden>
> Subject: Re: [PATCH] target/mips: Fix minor bug in FPU
> 
> On Mon, 11 Mar 2019 at 11:52, Aleksandar Markovic
> <address@hidden> wrote:
> >
> > > From: Mateja Marjanovic <address@hidden>
> > > Subject: [PATCH] target/mips: Fix minor bug in FPU
> > >
> > > Wrong type of NaN was generated by maddf and msubf insturctions
> > > when the arguments were inf, zero, nan or zero, inf, nan
> > > respectively.
> >
> > I did the applicable tests on both pre-NaN2008 and NaN2008 MIPS hardware,
> > and compared results with QEMu emulations. The underlying reason for this
> > patch is correct, but, as Alex also pointed out, it needs some improvements.
> > However, the softfreeze being so close, I am going to amend the patch while
> > creating the pull request. No respin needed. All in all:
> 
> Since this is a bug fix, there is no requirement that it goes in
> before softfreeze, FWIW -- pretty much any bug fix is OK for
> rc1, and a focused bugfix like this one that doesn't affect
> other guest architectures would be ok in rc2 as well.
> 

Thanks, Peter, for the clarification!

In this case, I think, the most natural would be to wait for the
submitter to submit the v2.

The reference to the documentation that Alex rightfully asked for
is this:

"MIPS Architecture for Programmers Volume IV-j: The MIPS32
SIMD Architecture Module", Revision 1.12, February 3, 2016

The key sentence is on page 53:

   "If the destination format is floating-point, all NaN propagating
   operations with one NaN operand produce a NaN with the
   payload of the input NaN."

There are other details in surrounding paragraphs. The document
is for SIMD (MSA) ASE, but both the SIMD and FPU NaN2008
floating point instruction should follow the same rules. I could't
find a separate document on FPU instructions.

The problem with the current implementation of 0*inf+nan is that
it returns the default NaN (all zeroes in the payload segment),
whereas the documentation says the payload should be from
the input NaN. The behavior specified in the documentation is
confirmed also with tests on hardware.

Sincerely,
Aleksandar