[Buildroot] [PATCH 1/2] toolchain: introduce BR2_TOOLCHAIN_HAS_LIBATOMIC

[Yann E. MORIN <yann.morin.1998@free.fr>]

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@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@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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