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Message-ID: <20250222002459.GE1827@brightrain.aerifal.cx>
Date: Fri, 21 Feb 2025 19:25:00 -0500
From: Rich Felker <dalias@...c.org>
To: Jesse DeGuire <jesse.a.deguire@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] Add additional __ARM_FP checks to ARM FPU math
functions
On Wed, Nov 20, 2024 at 08:26:23PM -0600, Jesse DeGuire wrote:
> Hi everyone!
>
> I found that I was getting compiler errors when I try to build Musl
> for an ARMv8.1M Mainline target that does not have floating-point
> support but does have the M-Profile Vector Extensions (MVE). The
> errors were that Musl wanted to use unsupported floating-point
> instructions for fabsf() and sqrtf().
>
> I was able to correct this by adding checks for (__ARM_FP & 4) to the
> ARM "fabsf.c" and "sqrtf.c" files, which is all this tiny patch does.
>
> The relevant options I used with Clang were "-march=armv8.1m.main+mve
> -mfpu=none -mfloat-abi=hard". MVE uses the FP register file, but
> treats them as 8 128-bit registers instead of 16 64-bit registers, so
> presumably that's why the hard float ABI is used even when
> floating-point operations are not enabled. In this case, an
> integer-only subset of MVE is used.
>
> Here is a Godbolt link that shows that you can make this happen on GCC, too.
> https://www.godbolt.org/z/Mf4h489s8
>
> I'm not sure if this is totally necessary since I suspect this would
> affect only ARM M-Profile devices, but maybe it at least wouldn't hurt
> to have.
I'm looking over this now and based on the ARM docs it sounds like
it's correct. Can anyone else confirm?
It also looks like it matches how we check for hardware double support
in other files.
Rich
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