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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
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