[Mesa-dev] [PATCH v2] disk cache: Link with -latomic if necessary

[Dylan Baker]

Quoting Thierry Reding (2018-03-01 05:28:07)
> 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 atomic, 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_*").
> 
> For some background information on this, see:
> 
>         https://gcc.gnu.org/wiki/Atomic/GCCMM
> 
> Changes in v2:
> - clarify meaning of lock-free in commit message
> - fix build if -latomic is not necessary
> 
> Acked-by: Matt Turner <mattst88 at gmail.com>
> Signed-off-by: Thierry Reding <treding at nvidia.com>
> ---
>  meson.build          | 17 +++++++++++++++++
>  src/util/meson.build |  2 +-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/meson.build b/meson.build
> index e9928a379313..bb6a835084fe 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -790,9 +790,26 @@ else
>  endif
>  
>  # Check for GCC style atomics
> +dep_atomic = declare_dependency()
> +
>  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')
> +  endif
>  endif
>  if not cc.links('''#include <stdint.h>
>                     uint64_t v;
> diff --git a/src/util/meson.build b/src/util/meson.build
> index b23dba3a9851..eece1cefef6a 100644
> --- a/src/util/meson.build
> +++ b/src/util/meson.build
> @@ -102,7 +102,7 @@ libmesa_util = static_library(
>    'mesa_util',
>    [files_mesa_util, format_srgb],
>    include_directories : inc_common,
> -  dependencies : [dep_zlib, dep_clock, dep_thread],
> +  dependencies : [dep_zlib, dep_clock, dep_thread, dep_atomic],
>    c_args : [c_msvc_compat_args, c_vis_args],
>    build_by_default : false
>  )
> -- 
> 2.16.2
> 

Reviewed-by: Dylan Baker <dylan at pnwbakers.com>
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