linux-kernel - Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC

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Date:   Fri, 13 Nov 2020 12:41:04 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Collabora Kernel ML <kernel@...labora.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

On Fri, 13 Nov 2020 at 12:05, Adrian Ratiu <adrian.ratiu@...labora.com> wrote:
>
> Hi Ard,
>
> On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@...nel.org> wrote:
> > On Thu, 12 Nov 2020 at 22:23, Adrian Ratiu
> > <adrian.ratiu@...labora.com> wrote:
> >>
> >> From: Nathan Chancellor <natechancellor@...il.com>
> >>
> >> Drop warning because kernel now requires GCC >= v4.9 after
> >> commit 6ec4476ac825 ("Raise gcc version requirement to 4.9").
> >>
> >> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> >> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com>
> >
> > Again, this does not do what it says on the tin.
> >
> > If you want to disable the pragma for Clang, call that out in
> > the commit log, and don't hide it under a GCC version change.
>
> I am not doing anything for Clang in this series.
>
> The option to auto-vectorize in Clang is enabled by default but
> doesn't work for some reason (likely to do with how it computes
> the cost model, so maybe not even a bug at all) and if we enable
> it explicitely (eg via a Clang specific pragma) we get some
> warnings we currently do not understand, so I am not changing the
> Clang behaviour at the recommendation of Nick.
>
> So this is only for GCC as the "tin" says :) We can fix clang
> separately as the Clang bug has always been present and is
> unrelated.
>

But you are adding the IS_GCC check here, no? Is that equivalent? IOW,
does Clang today identify as GCC <= 4.6?

> >
> > Without the pragma, the generated code is the same as the
> > generic code, so it makes no sense to build xor-neon.ko at all,
> > right?
> >
>
> Yes that is correct and that is the reason why in v1 I opted to
> not build xor-neon.ko for Clang anymore, but that got NACKed, so
> here I'm fixing the low hanging fruit: the very obvious & clear
> GCC problems.
>
>

Fair enough.

> >> ---
> >>  arch/arm/lib/xor-neon.c | 9 +--------
> >>  1 file changed, 1 insertion(+), 8 deletions(-)
> >>
> >> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> >> index b99dd8e1c93f..e1e76186ec23 100644
> >> --- a/arch/arm/lib/xor-neon.c
> >> +++ b/arch/arm/lib/xor-neon.c
> >> @@ -19,15 +19,8 @@ MODULE_LICENSE("GPL");
> >>   * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
> >>   * NEON instructions.
> >>   */
> >> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
> >> +#ifdef CONFIG_CC_IS_GCC
> >>  #pragma GCC optimize "tree-vectorize"
> >> -#else
> >> -/*
> >> - * While older versions of GCC do not generate incorrect code, they fail to
> >> - * recognize the parallel nature of these functions, and emit plain ARM code,
> >> - * which is known to be slower than the optimized ARM code in asm-arm/xor.h.
> >> - */
> >> -#warning This code requires at least version 4.6 of GCC
> >>  #endif
> >>
> >>  #pragma GCC diagnostic ignored "-Wunused-variable"
> >> --
> >> 2.29.2
> >>