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Date:   Wed, 20 Jan 2021 15:15:40 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>, Ard Biesheuvel <ardb@...nel.org>
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>,
        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 Wed, Jan 20, 2021 at 3:09 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Jan 19, 2021 at 1:35 PM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > 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.
>
> Doesn't that already happen today with GCC when the pointer arguments
> are overlapping?  The loop is "versioned" and may not actually use the
> NEON registers at all at runtime for such arguments.
> https://godbolt.org/z/q48q8v  See also:
> https://bugs.llvm.org/show_bug.cgi?id=40976#c11.  Or am I missing
> something?
>
> So I'm thinking if we extend out this pattern to the rest of the
> functions, we can actually avoid calls to
> kernel_neon_begin()/kernel_neon_end() for cases in which pointers
> would be too close to use the vectorized loop version; meaning for GCC
> this would be an optimization (don't save neon registers when you're
> not going to use them).  I would probably consider moving
> include/asm-generic/xor.h somewhere under arch/arm/
> perhaps...err...something for the other users of <asm-generic/xor.h>.
>
> diff --git a/arch/arm/include/asm/xor.h b/arch/arm/include/asm/xor.h
> index aefddec79286..abd748d317e8 100644
> --- a/arch/arm/include/asm/xor.h
> +++ b/arch/arm/include/asm/xor.h
> @@ -148,12 +148,12 @@ extern struct xor_block_template const
> xor_block_neon_inner;
>  static void
>  xor_neon_2(unsigned long bytes, unsigned long *p1, unsigned long *p2)
>  {
> -       if (in_interrupt()) {
> -               xor_arm4regs_2(bytes, p1, p2);
> -       } else {
> +       if (!in_interrupt() && abs((uintptr_t)p1, (uintptr_t)p2) >= 8) {
>                 kernel_neon_begin();
>                 xor_block_neon_inner.do_2(bytes, p1, p2);
>                 kernel_neon_end();
> +       } else {
> +               xor_arm4regs_2(bytes, p1, p2);
>         }
>  }
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index b99dd8e1c93f..0e8e474c0523 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -14,22 +14,6 @@ MODULE_LICENSE("GPL");
>  #error You should compile this file with '-march=armv7-a
> -mfloat-abi=softfp -mfpu=neon'
>  #endif
>
> -/*
> - * Pull in the reference implementations while instructing GCC (through
> - * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
> - * NEON instructions.
> - */
> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
> -#pragma GCC optimize "tree-vectorize"

Err...we need to keep this but use the -f flag with __restrict for GCC
to vectorize:
https://godbolt.org/z/9acnEv

-- 
Thanks,
~Nick Desaulniers

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