Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression

From: Arnd Bergmann <arnd@arndb.de>
To: Marco Elver <elver@google.com>
Cc: Will Deacon <will@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Dmitry Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression
Date: Tue, 26 May 2020 21:00:20 +0200	[thread overview]
Message-ID: <CAK8P3a3ZawPnzmzx4q58--M1h=v4X-1GtQLiwL1=G6rDK8=Wpg@mail.gmail.com> (raw)
In-Reply-To: <20200526173312.GA30240@google.com>

On Tue, May 26, 2020 at 7:33 PM 'Marco Elver' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
> On Tue, 26 May 2020, Marco Elver wrote:
> > On Tue, 26 May 2020 at 14:19, Arnd Bergmann <arnd@arndb.de> wrote:
> > Note that an 'allyesconfig' selects KASAN and not KCSAN by default.
> > But I think that's not relevant, since KCSAN-specific code was removed
> > from ONCEs. In general though, it is entirely expected that we have a
> > bit longer compile times when we have the instrumentation passes
> > enabled.
> >
> > But as you pointed out, that's irrelevant, and the significant
> > overhead is from parsing and pre-processing. FWIW, we can probably
> > optimize Clang itself a bit:
> > https://github.com/ClangBuiltLinux/linux/issues/1032#issuecomment-633712667
>
> Found that optimizing __unqual_scalar_typeof makes a noticeable
> difference. We could use C11's _Generic if the compiler supports it (and
> all supported versions of Clang certainly do).
>
> Could you verify if the below patch improves compile-times for you? E.g.
> on fs/ocfs2/journal.c I was able to get ~40% compile-time speedup.

Yes, that brings both the preprocessed size and the time to preprocess it
with clang-11 back to where it is in mainline, and close to the speed with
gcc-10 for this particular file.

I also cross-checked with gcc-4.9 and gcc-10 and found that they do see
the same increase in the preprocessor output, but it makes little difference
for preprocessing performance on gcc.

       Arnd