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Date: Thu, 2 Jul 2020 11:00:42 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: David Laight <David.Laight@...LAB.COM>
Cc: 'Peter Zijlstra' <peterz@...radead.org>, 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" <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 Thu, Jul 02, 2020 at 09:37:26AM +0000, David Laight wrote:
> From: Paul E. McKenney
> > Sent: 01 July 2020 17:06
> ...
> > > Would an asm statement that uses the same 'register' for input and
> > > output but doesn't actually do anything help?
> > > It won't generate any code, but the compiler ought to assume that
> > > it might change the value - so can't do optimisations that track
> > > the value across the call.
> >
> > It might replace the volatile load, but there are optimizations that
> > apply to the downstream code as well.
> >
> > Or are you suggesting periodically pushing the dependent variable
> > through this asm? That might work, but it would be easier and
> > more maintainable to just mark the variable.
>
> Marking the variable requires compiler support.
> Although what 'volatile register int foo;' means might be interesting.
>
> So I was thinking that in the case mentioned earlier you do:
> ptr += LAUNDER(offset & 1);
> to ensure the compiler didn't convert to:
> if (offset & 1) ptr++;
> (Which is probably a pessimisation - the reverse is likely better.)
Indeed, Akshat's prototype follows the "volatile" qualifier in many
ways. https://github.com/AKG001/gcc/
Thanx, Paul
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