linux-kernel - Re: [PATCH 04/11] ARM: Remove Thumb2 __builtin_thread

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Message-ID: <CAMj1kXFzi=ijUBU0i3gq4Q9rcCvMw-a1wLc1u7fXRRN8QbN9hw@mail.gmail.com>
Date: Fri, 26 Jan 2024 09:44:37 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: akpm@...ux-foundation.org, masahiroy@...nel.org, nicolas@...sle.eu, 
	linux-kbuild@...r.kernel.org, llvm@...ts.linux.dev, patches@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, linux@...linux.org.uk, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 04/11] ARM: Remove Thumb2 __builtin_thread_pointer
 workaround for Clang

On Thu, 25 Jan 2024 at 23:56, Nathan Chancellor <nathan@...nel.org> wrote:
>
> Now that the minimum supported version of LLVM for building the kernel
> has been bumped to 13.0.1, the conditional expression added to
> get_current() by commit c1e42efacb9b ("ARM: 9151/1: Thumb2: avoid
> __builtin_thread_pointer() on Clang") is always true, as the build will
> fail during the configuration stage for older LLVM versions. Remove it,
> effectively reverting the aforementioned change.
>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Reviewed-by: Ard Biesheuvel <ardb@...nel.org>

> ---
> Cc: linux@...linux.org.uk
> Cc: ardb@...nel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
>  arch/arm/include/asm/current.h | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/arch/arm/include/asm/current.h b/arch/arm/include/asm/current.h
> index 1e1178bf176d..5225cb1c803b 100644
> --- a/arch/arm/include/asm/current.h
> +++ b/arch/arm/include/asm/current.h
> @@ -18,18 +18,12 @@ static __always_inline __attribute_const__ struct task_struct *get_current(void)
>  {
>         struct task_struct *cur;
>
> -#if __has_builtin(__builtin_thread_pointer) && \
> -    defined(CONFIG_CURRENT_POINTER_IN_TPIDRURO) && \
> -    !(defined(CONFIG_THUMB2_KERNEL) && \
> -      defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 130001)
> +#if __has_builtin(__builtin_thread_pointer) && defined(CONFIG_CURRENT_POINTER_IN_TPIDRURO)
>         /*
>          * Use the __builtin helper when available - this results in better
>          * code, especially when using GCC in combination with the per-task
>          * stack protector, as the compiler will recognize that it needs to
>          * load the TLS register only once in every function.
> -        *
> -        * Clang < 13.0.1 gets this wrong for Thumb2 builds:
> -        * https://github.com/ClangBuiltLinux/linux/issues/1485
>          */
>         cur = __builtin_thread_pointer();
>  #elif defined(CONFIG_CURRENT_POINTER_IN_TPIDRURO) || defined(CONFIG_SMP)
>
> --
> 2.43.0
>