Re: [Buildroot] [PATCH 1/2] package/libgit2: depends on native 64bit atomics

[Thomas Petazzoni via buildroot <buildroot@buildroot.org>]

Hello Nicolas,

On Tue, 9 Aug 2022 17:57:16 +0200
Nicolas Cavallari <nicolas.cavallari@green-communications.fr> wrote:

> It's a bit more complicated than that and i probably mixed things up.
> 
> libgit2 can use sync builtins or atomic builtins depending on the gcc 
> version.
> 
> But if we assume that the new gcc minimum version is 4.9, then I can 
> probably just ignore the sync builtins.

Looking at src/thread.h in libgit2, we can see:

# if (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1))
#  error Atomic primitives do not exist on this version of gcc; configure libgit2 with -DUSE_THREADS=OFF
# elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
#  define GIT_BUILTIN_ATOMIC
# else
#  define GIT_BUILTIN_SYNC
# endif

So basically, with gcc >= 4.7, libgit2 uses atomic built-ins, with gcc
< 4.7, it uses sync built-ins. However neither are guaranteed to be
present on all architectures.

So if we want to be fully correct, we would probably need something
like this:

	depends on (BR2_TOOLCHAIN_HAS_ATOMIC && BR2_TOOLCHAIN_GCC_AT_LEAST_4_7) || \
		(BR2_TOOLCHAIN_HAS_SYNC_4 && BR2_TOOLCHAIN_HAS_SYNC_8 && !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7)

Best regards,

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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