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Date:   Wed, 1 Jul 2020 11:19:23 +0100
From:   Will Deacon <will@kernel.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Matt Turner <mattst88@gmail.com>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Richard Henderson <rth@twiddle.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
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        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when
 CLANG_LTO=y
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On Tue, Jun 30, 2020 at 09:25:03PM +0200, Arnd Bergmann wrote:
> On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will@kernel.org> wrote:
> > +#define __READ_ONCE(x)                                                 \
> > +({                                                                     \
> > +       int atomic = 1;                                                 \
> > +       union { __unqual_scalar_typeof(x) __val; char __c[1]; } __u;    \
> > +       typeof(&(x)) __x = &(x);                                        \
> > +       switch (sizeof(x)) {                                            \
> ...
> > +       atomic ? (typeof(x))__u.__val : (*(volatile typeof(x) *)__x);   \
> > +})
> 
> This expands (x) nine times (five in __unqual_scala_typeof()), which can
> lead to significant code bloat after preprocessing if something passes a
> compound expression into READ_ONCE().
> The compiler works it out eventually, but we've seen an actual slowdown
> in compile speed from this recently, especially on clang.
> 
> I think if you move the
> 
>         typeof(&(x)) __x = &(x);
> 
> line first, all other instances can use typeof(*__x) instead of typeof(x)
> and avoid this problem.

Cheers, I was only thinking about side-effects when I wrote this, but
bloating built time is very unpopular, so I'll go with your suggestion.

> Once we make gcc-4.9 the minimum version,
> this could be further improved to
> 
>        __auto_type __x = &(x);

Is anybody working on moving to 4.9? I've seen the mails from Linus
championing it, but I thought there was a RHEL in support that people
might care about?

Will