[OpenWrt-Devel] mfloat-abi=soft

[Christian Lamparter]

On Saturday, April 13, 2019 11:08:38 PM CEST Rosen Penev wrote:
> On Sat, Apr 13, 2019 at 7:07 AM Christian Lamparter <chunkeey at gmail.com> wrote:
> >
> > Cc'd: BangLang Huang <banglang.huang at foxmail.com>
> >
> > On Saturday, April 6, 2019 10:35:48 PM CEST Rosen Penev wrote:
> > > According to rules.mk, -mfloat-abi=soft is used. This is causing build
> > > failures with the botan library:
> > >
> > > https://downloads.openwrt.org/snapshots/faillogs/armeb_xscale/packages/botan/compile.txt
> > >
> > > Is it sensical to change the default to softfp, as the error message suggests?
> > That's the error:
> >
> > #error "NEON intrinsics not available with the soft-float ABI.  Please use -mfloat-abi=softp or -mfloat-abi=hard"
> >
> > I think the best option would be to tell the package to disable neon in this
> > case, just like ffmpeg is doing already. If you can test:
> >
> > ifeq ($(CONFIG_SOFT_FLOAT),y)
> > CONFIGURE_ARGS += --disable-neon
> > endif
> That could work. I find it interesting that it's only the old ARM
> architectures that are getting compilation errors.
Ah, that's because ARM's toolchain was recently changed in order to fix
the gcc 8.x error due to conflicting march vs mcpu compiler flags.

See PR 1913 for the gory details: https://github.com/openwrt/openwrt/pull/1913
Before that commit, botan would probably compile, but the binaries
may not work on the target system (didn't test so take this with a
load of salt).

But if you look on the botan's github issue tracker, you'll find that
someone reported a similar problem with powerpc + altivec vs. SPE:
<https://github.com/randombit/botan/issues/1820>

Which when you think about it might become obvious. The ahead-of-time
autodetection employed by botan does not really work when everything
is cross-compiled. But this has been the bane of OpenWrt and several
other Buildroot and non-buildroot projects from the beginning, so there
are ways to mitigate these problems efficiently.

Cheers,
Christian



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