Re: [PATCH 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

From: Nathan Chancellor <natechancellor@gmail.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Adrian Ratiu <adrian.ratiu@collabora.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Russell King <linux@armlinux.org.uk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Collabora kernel ML <kernel@collabora.com>
Subject: Re: [PATCH 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
Date: Fri, 6 Nov 2020 11:03:11 -0700	[thread overview]
Message-ID: <20201106180311.GC2959494@ubuntu-m3-large-x86> (raw)
In-Reply-To: <CAK8P3a0kXkoeAbNiPH5-7-73V2nAsnCMkQSrPZq4ysVYNyE-WQ@mail.gmail.com>

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@collabora.com> wrote:
> >
> > From: Nathan Chancellor <natechancellor@gmail.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@google.com>
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.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