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

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Date:   Fri, 6 Nov 2020 11:03:11 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Adrian Ratiu <adrian.ratiu@...labora.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Collabora kernel ML <kernel@...labora.com>
Subject: Re: [PATCH 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6
 warning

On Fri, Nov 06, 2020 at 03:46:36PM +0100, Arnd Bergmann wrote:
> On Fri, Nov 6, 2020 at 6:14 AM 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>
> > ---
> >  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
> 
> I think we still need the #else path, otherwise we don't warn about
> clang being broken here.
> 
> If it is intentional that we now silently build this code with clang
> without it working as intended, that should be mentioned in the
> changelog.
> 
>       Arnd

Maybe patch 2/2 should come before this one? With that patch, we are not
even going to build this code so this patch purely becomes a "we do not
support this GCC version anymore" cleanup patch.

Cheers,
Nathan