Re: ARM32 configury changes, with no FPU as a default

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

On 17/09/2021 11:23, Florian Weimer via Gcc wrote:
> * Matthias Klose:
>
> > Starting with GCC 8, the configury allows to encode extra features into the
> > architecture string. Debian and Ubuntu's armhf (hard float) architecture is
> > configured with
> >
> >    --with-arch=armv7-a --with-fpu=vfpv3-d16
> >
> > and now should be configured with
> >
> >    --with-arch=armv7-a+fp
> >
> > The --with-fpu configure option is deprecated.  The problem with this approach
> > is that there is no default for the fpu setting, while old compilers silently
> > pick up the -mfpu from the configured compiler.
>
> FWIW, Fedora uses:
>
>          --with-tune=generic-armv7-a --with-arch=armv7-a \
> 	--with-float=hard --with-fpu=vfpv3-d16 --with-abi=aapcs-linux \
>
> Not sure how it is impacted by this change.
>
> > This breaks software which explicitly configures things like
> > -march=armv7-a, or where the architecture string is embedded in the
> > source as an attribute.  So going from one place in the compiler about
> > configuring the ABI for a distro arch, this config now moves to some
> > dozen places in different packages.  Not the thing I would expect.
>
> I don't know if we have seen such problems in Fedora.  I don't remember
> any reports.

That's still using the now-deprecated --with-fpu option.  I want to 
remove that from GCC eventually in favour of the new way of adding the 
FP configuration as part of the architecture.  Recent versions of the 
Arm architecture do not document separate FPU versions, but just add 
features to the main architecture, so we need to move away from the old 
approach.

R.

> Thanks,
> Florian