[llvm-dev] NEON FP flags

Renato Golin via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 1 06:56:53 PDT 2016

On 29 March 2016 at 11:09, James Greenhalgh <james.greenhalgh at arm.com> wrote:
> That is to say, GCC will only auto-vectorize floationg-point arithmetic
> if both -mfpu=neon AND -funsafe-math-optimizations are given. -mfpu=neon
> by itself does not imply that it is OK for GCC to generate non-IEEE
> compliant code. The default is safe until explicitly told otherwise.

Right, that was what I originally though from Hal's bug report, but
recent emails on the thread confused me.

I think this is the right behaviour, and I'm glad GCC does it, so we
can follow the correct approach from start.

>> 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.
> In GCC, this is -mfpu=neon.

This makes my life *so* much easier! :)

> In GCC today:
>   -mfpu=vfp is the minimum floating-point instruction set supported, the
>     choice of which ABI you use (-mfloat-abi) is independent from the choice
>     of floating-point hardware that exists. -mfpu=soft and -mfpu=softfp are
>     rejected by GCC.

Yes, I mixed mfpu with mfloat-abi, my bad.

> For your set of use cases:
>   Int (ALU), FP (LIB), no VFP/NEON instructions
>     -mfloat-abi=soft
>   Int (ALU), FP (LIB), VFP/NEON instructions allowed
>     Impossible

I mentioned this as -mfloat-abi=floatfp. Now I see my representation
of int/fp mixed concepts. Ignore this.

>   Int (ALU), FP (VFP)
>     -mfloat-abi=hard or -mfloat-abi=softfp
>    + -mfpu=vfp (or other non-neon FPU)
>   Int (NEON), FP (VFP)
>     -float-abi=hard or -mfloat-abi=softfp
>    + -mfpu=neon (or greater)

Excellent! This means I can only make -fsubnormal flags count, and all
will be the same.

This was my first approach, but Hal convinced me that we may want a
specific flag that is included by fast/unsafe maths flags. See below.

>   Int (NEON), FP (NEON)
>     -float-abi=hard or -mfloat-abi=softfp
>    + -mfpu=neon (or greater)
>    + -funsafe-math-optimizations (or equivalent)

Do you have one specifically for subnormals? -funsafe-math is a bit of
a big hammer and will enable other (potentially unwanted) behaviour
from the vectorizer.

However, -ffast-math / unsafe-math should include subnormal support.

>   Int (ALU), FP (NEON)
>     Impossible (as far as I know).

Irrelevant, as far as I care. :)


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