[Mesa-dev] [PATCH] disk cache: Link with -latomic if necessary

[Dylan Baker]

Quoting Thierry Reding (2018-02-27 02:29:22)
> On Mon, Feb 26, 2018 at 12:05:51PM -0800, Dylan Baker wrote:
> > Quoting Thierry Reding (2018-02-23 05:18:28)
> > > From: Thierry Reding <treding at nvidia.com>
> > > 
> > > The disk cache implementation uses 64-bit atomic operations. For some
> > > architectures, such as 32-bit ARM, GCC will not be able to translate
> > > these operations into lock-free instructions and will instead rely on
> > > the external atomics library to provide these operations.
> > > 
> > > Check at configuration time whether or not linking against libatomic
> > > is necessary and if so, create a dependency that can be used while
> > > linking the mesautil library.
> > > 
> > > This is the meson equivalent of 2ef7f23820a6 ("configure: check if
> > > -latomic is needed for __atomic_*").
> > > 
> > > Signed-off-by: Thierry Reding <treding at nvidia.com>
> > > ---
> > >  meson.build          | 15 +++++++++++++++
> > >  src/util/meson.build |  2 +-
> > >  2 files changed, 16 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/meson.build b/meson.build
> > > index 121341a950c4..7c6f40573421 100644
> > > --- a/meson.build
> > > +++ b/meson.build
> > > @@ -800,6 +800,21 @@ endif
> > >  if cc.compiles('int main() { int n; return __atomic_load_n(&n, __ATOMIC_ACQUIRE); }',
> > >                 name : 'GCC atomic builtins')
> > >    pre_args += '-DUSE_GCC_ATOMIC_BUILTINS'
> > > +
> > > +  # Not all atomic calls can be turned into lock-free instructions, in which
> > > +  # GCC will make calls into the libatomic library. Check whether we need to
> > > +  # link with -latomic.
> > > +  #
> > > +  # This can happen for 64-bit atomic operations on 32-bit architectures such
> > > +  # as ARM.
> > > +  if not cc.links('''#include <stdint.h>
> > > +                     int main() {
> > > +                       uint64_t n;
> > > +                       return (int)__atomic_load_n(&n, __ATOMIC_ACQUIRE);
> > > +                     }''',
> > > +                  name : 'GCC atomic builtins required -latomic')
> > > +    dep_atomic = cc.find_library('atomic')
> > 
> > dep_atomic is undefined if the cc.links() succeeds right?
> 
> Yeah, I noticed that too when building on 64-bit ARM and then forgot to
> send out v2. I'll do that shortly. The fix, though I'm not sure if it's
> the correct way to do it in Meson, is to declare dep_atomic as an empty
> dependency:
> 
>         dep_atomic = declare_dependency()
> 
> unconditionally and then overwrite it with the proper dependency after
> libatomic was detected.
> 
> Any ideas if this can be done more idiomatically?
> 
> Thierry

We've mostly used an empty list for this (which is the old way),
declare_dependency is the way people seem to be doing this now in meson, so
either way. You can overwrite it or put it in the else block, doesn't matter to
me.

Dylan
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