[Buildroot] [PATCH 1/2] toolchain: introduce BR2_TOOLCHAIN_HAS_LIBATOMIC
Yann E. MORIN
yann.morin.1998 at free.fr
Mon Mar 7 20:41:11 UTC 2016
Thomas, All,
On 2016-03-06 21:47 +0100, Thomas Petazzoni spake thusly:
> Until now, we were assuming that whenever you have gcc 4.8, libatomic
> is available. It turns out that this is not correct, since libatomic
> will not be available if thread support is disabled in the toolchain.
>
> Therefore, __atomic_*() intrinsics may not be available even if the
> toolchain uses gcc 4.8.
>
> To solve this problem, we introduce a BR2_TOOLCHAIN_HAS_LIBATOMIC
> boolean, which indicates whether the toolchain has libatomic. It is
> the case when you are using gcc >= 4.8 *and* thread support is
> enabled. We then use this new BR2_TOOLCHAIN_HAS_LIBATOMIC to define
> BR2_TOOLCHAIN_HAS_ATOMIC.
>
> As explained in the comment, on certain architectures, libatomic is
> technically not needed to provide the __atomic_*() intrinsics since
> they might be all built-in. However, since libatomic is only absent in
> non-thread capable toolchains, it is not worth making things more
> complex for such seldomly used configuration.
>
> Note that we are introducing the intermediate
> BR2_TOOLCHAIN_HAS_LIBATOMIC option because it will be useful on its
> own for certain packages.
>
> Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
> ---
> toolchain/toolchain-common.in | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/toolchain/toolchain-common.in b/toolchain/toolchain-common.in
> index 1b7b416..596d1eb 100644
> --- a/toolchain/toolchain-common.in
> +++ b/toolchain/toolchain-common.in
> @@ -373,15 +373,28 @@ config BR2_TOOLCHAIN_HAS_SYNC_8
> default y if BR2_TOOLCHAIN_ARM_HAS_SYNC_8
> default y if BR2_TOOLCHAIN_X86_HAS_SYNC_8
>
> +# libatomic is available since gcc 4.8, when thread support is
> +# enabled.
> +config BR2_TOOLCHAIN_HAS_LIBATOMIC
> + bool
> + default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 && \
> + BR2_TOOLCHAIN_HAS_THREADS
> +
> # __atomic intrinsics are available:
> # - with gcc 4.8, either through built-ins or libatomic, on all
> -# architectures
> +# architectures. Since we don't want to separate the cases where
> +# libatomic is needed vs. not needed, we simplify thing and only
> +# support situations where libatomic is available, even if on some
> +# architectures libatomic is not strictly needed as all __atomic
> +# intrinsics might be built-in. But the only case where libatomic is
> +# not available is when thread support is disabled, which is pretty
> +# unlikely.
That last sentence got me confused for a while, and your explanations on
IRC were really necessary to me to understand it.
What about:
The only case where libatomic is missing entirely is when the
toolchain does nto have support for threads. However, a package
that does not need threads but still uses atomics is quite a
corner case, which does not warrant the added complexity.
Otherwise, I'm fine with this:
Reviewed-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
Regards,
Yann E. MORIN.
> # - with gcc 4.7, libatomic did not exist, so only built-ins are
> # available. This means that __atomic can only be used in a subset
> # of the architectures
> config BR2_TOOLCHAIN_HAS_ATOMIC
> bool
> - default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
> + default y if BR2_TOOLCHAIN_HAS_LIBATOMIC
> default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_arm
> default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_armeb
> default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_xtensa
> --
> 2.6.4
>
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