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References: <20230718064819.2549052-1-chenhuacai@loongson.cn> <e06ac133-2fd7-85f0-2ce7-d1209c6bb2ac@xen0n.name>
In-Reply-To: <e06ac133-2fd7-85f0-2ce7-d1209c6bb2ac@xen0n.name>
From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Tue, 18 Jul 2023 16:18:30 +0800
Message-ID: <CAAhV-H6NnvSEtb81q5-TZLvK+cR+m8CxzadWmbYjNCTNSDOcRw@mail.gmail.com>
Subject: Re: [PATCH] LoongArch: Cleanup __builtin_constant_p() checking for cpu_has_*
To:     WANG Xuerui <kernel@xen0n.name>
Cc:     Huacai Chen <chenhuacai@loongson.cn>, loongarch@lists.linux.dev,
        linux-arch@vger.kernel.org, Xuefeng Li <lixuefeng@loongson.cn>,
        Guo Ren <guoren@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-kernel@vger.kernel.org, loongson-kernel@lists.loongnix.cn
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On Tue, Jul 18, 2023 at 3:35=E2=80=AFPM WANG Xuerui <kernel@xen0n.name> wro=
te:
>
> 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@loongson.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 add=
ing
> > -      * 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_s=
truct *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@xen0n.name>
>
> --
> WANG "xen0n" Xuerui
>
> Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
>