linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Date:   Wed, 1 Jul 2020 09:03:38 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Wed, Jul 01, 2020 at 05:05:12PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 01, 2020 at 07:06:54AM -0700, Paul E. McKenney wrote:
> 
> > The current state in the C++ committee is that marking variables
> > carrying dependencies is the way forward.  This is of course not what
> > the Linux kernel community does, but it should not be hard to have a
> > -fall-variables-dependent or some such that causes all variables to be
> > treated as if they were marked.  Though I was hoping for only pointers.
> > Are they -sure- that they -absolutely- need to carry dependencies
> > through integers???
> 
> What's 'need'? :-)

Turning off all dependency-killing optimizations on all pointers is
likely a non-event.  Turning off all dependency-killing optimizations
on all integers is not the road to happiness.

So whatever "need" might be, it would need to be rather earthshaking.  ;-)
It is probably not -that- hard to convert to pointers, even if they
are indexing multiple arrays.

> I'm thinking __ktime_get_fast_ns() is better off with a dependent load
> than it is with an extra smp_rmb().
> 
> Yes we can stick an smp_rmb() in there, but I don't like it. Like I
> wrote earlier, if I wanted a control dependency, I'd have written one.

No argument here.

But it looks like we are going to have to tell the compiler.

							Thanx, Paul