Re: [FFmpeg-devel] [PATCH] configure: link to libatomic when it's present

[Anton Khirnov <anton@khirnov.net>]

Quoting James Almer (2022-01-19 13:34:20)
> 
> 
> On 1/19/2022 8:54 AM, Anton Khirnov wrote:
> > C11 atomics in some configurations (e.g. 64bit operations on ppc64 with
> > GCC) require linking to libatomic.
> > ---
> > Testing welcome, especially in configurations where
> > * libatomic is not present
> > * libatomic is actually needed
> > ---
> >   configure | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/configure b/configure
> > index 1413122d87..1ff5dbee5b 100755
> > --- a/configure
> > +++ b/configure
> > @@ -6324,7 +6324,14 @@ check_headers asm/types.h
> >   # it seems there are versions of clang in some distros that try to use the
> >   # gcc headers, which explodes for stdatomic
> >   # so we also check that atomics actually work here
> > -check_builtin stdatomic stdatomic.h "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"
> > +#
> > +# some configurations also require linking to libatomic, so try
> > +# both with -latomic and without
> > +for LATOMIC in "-latomic" ""; do
> 
> Shouldn't you try without it first? On my toolchain libatomic is 
> present, but libraries compile without linking to it just fine. That 
> changes after this patch, where it starts linking to it explicitly.

No, because it may only be needed for some atomic sizes and operations,
which the test in configure doesn't necessarily catch.
And because we pass as-needed to the linker, the built libraries
shouldn't actually require libatomic unless it's really needed.

> 
> > +    check_builtin stdatomic stdatomic.h                                                 \
> > +        "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"  \
> > +        $LATOMIC && add_extralibs $LATOMIC && break
> 
> You should probably add it to the required libraries' extralibs only.
> Just replace the add_extralibs part with setting stdatomic_extralibs to 
> $LATOMIC, and then add stdatomic to all the libraries' _suggest lists, 
> same as we do for libm.

Then we need to actively track which libraries actually use atomics,
which also depends on which features are enabled. Given that this only
fails on less-common arches, this sounds like a recipe for obscure build
failures. Given that it's only really linked when needed, it seems
better to just add it unconditionally.

-- 
Anton Khirnov
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