linux-kernel - Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN

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Date:   Thu, 25 Feb 2021 13:18:01 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>,
        Marco Elver <elver@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Randy Dunlap <rdunlap@...radead.org>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN_BSWAP*

On Thu, Feb 25, 2021 at 12:14:17PM -0800, Kees Cook wrote:
> On Thu, Feb 25, 2021 at 12:06:37PM -0800, Andrew Morton wrote:
> > On Thu, 25 Feb 2021 12:03:48 -0800 Kees Cook <keescook@...omium.org> wrote:
> > 
> > > On Thu, Feb 25, 2021 at 05:45:09PM +0100, Arnd Bergmann wrote:
> > > > From: Arnd Bergmann <arnd@...db.de>
> > > > 
> > > > Separating compiler-clang.h from compiler-gcc.h inadventently dropped the
> > > > definitions of the three HAVE_BUILTIN_BSWAP macros, which requires falling
> > > > back to the open-coded version and hoping that the compiler detects it.
> > > > 
> > > > Since all versions of clang support the __builtin_bswap interfaces,
> > > > add back the flags and have the headers pick these up automatically.
> > > > 
> > > > This results in a 4% improvement of compilation speed for arm defconfig.
> > > > 
> > > > Fixes: 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h mutually exclusive")
> > > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > > 
> > > Cc: stable@...r.kernel.org
> > 
> > I figured 4% better compile time isn't significant enough to justify a
> > backport.  Thoughts?
> 
> It's a trivial change, so I think it's worth it?

Indeed. Any wins that we can get with compile time, we should take.
Clang is being widely used in production systems now so I feel like with
a trivial change plus user visible impact, it should be backported.

Not to mention that the generated code in theory should be better
because it is the compiler's builtin, rather than a hand rolled one, AND
this is technically a regression, given that it worked before compiler.h
was split.

Cheers,
Nathan

> > 
> > > Reviewed-by: Kees Cook <keescook@...omium.org>
> > 
> 
> -- 
> Kees Cook