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Message-ID: <e06ac133-2fd7-85f0-2ce7-d1209c6bb2ac@xen0n.name>
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
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