linux-kernel - Re: [PATCH] LoongArch: Cleanup __builtin_constant_p() checking for cpu_has

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Date:   Tue, 18 Jul 2023 15:34:47 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-arch@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH] LoongArch: Cleanup __builtin_constant_p() checking for
 cpu_has_*

On 2023/7/18 14:48, Huacai Chen wrote:
> In current configuration, cpu_has_lsx and cpu_has_lasx are impossible
> constants. So cleanup the __builtin_constant_p() checking to reduce the

"cannot be constants"? "impossible constants" sounds like a compile-time 
error to me.

> complexity.
> 
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
>   arch/loongarch/include/asm/fpu.h | 11 +++--------
>   1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/loongarch/include/asm/fpu.h b/arch/loongarch/include/asm/fpu.h
> index e4193d637f66..f02f4a0d0d64 100644
> --- a/arch/loongarch/include/asm/fpu.h
> +++ b/arch/loongarch/include/asm/fpu.h
> @@ -218,12 +218,7 @@ static inline void restore_lsx(struct task_struct *t)
>   
>   static inline void init_lsx_upper(void)
>   {
> -	/*
> -	 * Check cpu_has_lsx only if it's a constant. This will allow the
> -	 * compiler to optimise out code for CPUs without LSX without adding
> -	 * an extra redundant check for CPUs with LSX.
> -	 */
> -	if (__builtin_constant_p(cpu_has_lsx) && !cpu_has_lsx)
> +	if (!cpu_has_lsx)
>   		return;
>   
>   	_init_lsx_upper();
> @@ -294,7 +289,7 @@ static inline void restore_lasx_upper(struct task_struct *t) {}
>   
>   static inline int thread_lsx_context_live(void)
>   {
> -	if (__builtin_constant_p(cpu_has_lsx) && !cpu_has_lsx)
> +	if (!cpu_has_lsx)
>   		return 0;
>   
>   	return test_thread_flag(TIF_LSX_CTX_LIVE);
> @@ -302,7 +297,7 @@ static inline int thread_lsx_context_live(void)
>   
>   static inline int thread_lasx_context_live(void)
>   {
> -	if (__builtin_constant_p(cpu_has_lasx) && !cpu_has_lasx)
> +	if (!cpu_has_lasx)
>   		return 0;
>   
>   	return test_thread_flag(TIF_LASX_CTX_LIVE);

For the foreseeable future, the changes seem appropriate to me. FWIW the 
LoongArch kernel would stay as generic as possible so I expect the 
various predicates to almost never exist as constants.

Reviewed-by: WANG Xuerui <git@...0n.name>

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/