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

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Date:   Tue, 18 Jul 2023 16:18:30 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     WANG Xuerui <kernel@...0n.name>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>, 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 Tue, Jul 18, 2023 at 3:35 PM WANG Xuerui <kernel@...0n.name> wrote:
>
> 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.
OK, thanks.

Huacai
>
> > 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/
>