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Message-ID: <aeed740a4d86470d84ae7d5f1cf07951@AcuMS.aculab.com>
Date: Thu, 2 Jul 2020 09:37:26 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'paulmck@...nel.org'" <paulmck@...nel.org>
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

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.)

	David

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