linux-kernel - Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC

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Date:   Tue, 19 Jan 2021 22:34:45 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Adrian Ratiu <adrian.ratiu@...labora.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Collabora Kernel ML <kernel@...labora.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

On Tue, Jan 19, 2021 at 10:18 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu <adrian.ratiu@...labora.com> wrote:
> > diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> > index b99dd8e1c93f..f9f3601cc2d1 100644
> > --- a/arch/arm/lib/xor-neon.c
> > +++ b/arch/arm/lib/xor-neon.c
> > @@ -14,20 +14,22 @@ MODULE_LICENSE("GPL");
> >  #error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
> >  #endif
> >
> > +/*
> > + * TODO: Even though -ftree-vectorize is enabled by default in Clang, the
> > + * compiler does not produce vectorized code due to its cost model.
> > + * See: https://github.com/ClangBuiltLinux/linux/issues/503
> > + */
> > +#ifdef CONFIG_CC_IS_CLANG
> > +#warning Clang does not vectorize code in this file.
> > +#endif
>
> Arnd, remind me again why it's a bug that the compiler's cost model
> says it's faster to not produce a vectorized version of these loops?
> I stand by my previous comment: https://bugs.llvm.org/show_bug.cgi?id=40976#c8

The point is that without vectorizing the code, there is no point in building
both the default xor code and a "neon" version that has to save/restore
the neon registers but doesn't actually use them.

          Arnd