linux-kernel - Re: [PATCH] ARM: xor-neon: Replace __GNUC__ checks with CONFIG_CC_IS

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Date:   Fri, 31 May 2019 13:26:05 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nico@...xnic.net>,
        Stefan Agner <stefan@...er.ch>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: xor-neon: Replace __GNUC__ checks with
 CONFIG_CC_IS_GCC

On Fri, May 31, 2019 at 01:06:13PM -0700, Nick Desaulniers wrote:
> On Fri, May 31, 2019 at 12:21 PM Arnd Bergmann <arnd@...db.de> wrote:
> > clang, I would suggest dropping your patch then, and instead adding
> 
> I disagree.  The minimum version of gcc required to build the kernel
> is 4.6, so the comment about older versions of gcc is irrelevant and
> should be removed.
> 
> Nathan's -Rpass warnings are warning that vectorization was not
> calculated to be profitable **for 1 of the 4 functions** by LLVM.
> Surely we wouldn't disable NEON opts for XOR because 1 of 4 was not
> vectorized?

Well I kept it short but clang warns that all of the loops are not
profitable.

However, the config option for xor-neon.c is CONFIG_XOR_BLOCKS, which
also controls the arm64 implementation. We wouldn't want to disable it
for clang altogether if it works on arm64 fine.

If it turns out to be broken for both, I suppose I would be okay with
disabling CONFIG_XOR_BLOCKS for clang but it should be done in a
separate patch as this one should be applied regardless of clang working
or not (because this warning will appear again when clang is fixed).

Cheers,
Nathan