[llvm-dev] NEON FP flags

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Renato Golin" <renato.golin at linaro.org>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "LLVM Dev" <llvm-dev at lists.llvm.org>, "James Molloy" <James.Molloy at arm.com>
> Sent: Friday, March 25, 2016 8:23:03 AM
> Subject: Re: NEON FP flags
> 
> On 25 March 2016 at 04:11, Hal Finkel <hfinkel at anl.gov> wrote:
> > As I understand it, the fundamental property being addresses here
> > is: Are the semantics of scalar FP math the same as vector FP
> > math? TTI seems like a good place to expose that information. If
> > the semantics are indeed different, then the vectorizer would
> > require fast-math flags in order to vectorize FP operations
> > (similarly, gcc's man page says it requires
> > -funsafe-math-optimizations for vectorization unless -mfpu=neon or
> > similar is specified). In this context, this different-semantics
> > query would return true if:
> 
> The semantics is indeed different, VFP is IEEE-754 compliant while
> NEON is not. We don't want to stop the compiler from using VFP for FP
> math, but we want to be cautious when using NEON in the same way..
> 
> 
> >   !(isDarwin OR ARMISA >= v8 OR fpMath == NEON)
> >
> > and then we need to teach people to use -mfpu=neon ;)
> 
> So, there's the catch. In GCC, -mfpu=neon means to use NEON, which is
> not enabled by default, so the compiler assumes that the user is
> aware
> that NEON FP is not IEEE compliant. I don't think that's a safe
> assumption, but I also don't want to have a slightly different
> behaviour than GCC gratuitously.
> 
> Clang defaults to -mfpu=neon when we choose -mcpu=cortex-a* or
> -march=armv7a, so our current behaviour is on par with GCC. But I
> think that's a dangerous assumption.
> 
> Furthermore, the only alternatives we have at the moment is to either
> use NEON for everything or nothing. It would be good to have an
> option
> to use NEON for integer arithmetic and VFP for FP if the user
> requires
> IEEE compliance..
> 
> 
> 
> > P.S. Looking at gcc's man page, gcc seems to use -mfpu for ARM and
> > -mfpmath for x86. Do we use -mfpmath for both?
> 
> We already support -mfpmath=vfp/neon in Clang, but it's bogus. My
> proposal is to make it count.
> 
> The best way I can think of is to let -mfpmath=vfp *disable* only FP
> NEON and -mfpmath=neon *enable* only FP NEON, both orthogonal from
> integer math.
> 
> Examples:
> 
> Works today:
> -mfpu=soft -> Int (ALU), FP (LIB), no VFP/NEON instructions
> -mfpu=softfp -> Int (ALU), FP (LIB), VFP/NEON instructions allowed
> -mfpu=vfp -> Int (ALU), FP (VFP)
> -mfpu=neon -> Int (NEON), FP (NEON)
> 
> Change proposed:
> -mfpmath=neon -mfpu=vfp -> Int (ALU), FP (NEON)
> -mfpmath=vfp -mfpu=neon -> Int (NEON), FP (VFP)
> 
> This would be similar enough to GCC, and would allow the small number
> of users that care about IEEE-754 compliance to disable FP NEON on
> demand.

I think this seems reasonable, although it is somewhat unfortunate, in terms of naming, that "-mfpu" affects non-FP operations too. However, I think we're stuck because of what GCC decided to do.

Thanks again,
Hal

> 
> cheers,
> --renato
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory