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Date: Thu, 2 Jul 2020 09:37:26 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "''" <>
CC: 'Peter Zijlstra' <>, Marco Elver <>,
	Nick Desaulniers <>, Sami Tolvanen
	<>, Masahiro Yamada <>, "Will
 Deacon" <>, Greg Kroah-Hartman <>,
	Kees Cook <>, clang-built-linux
	<>, Kernel Hardening
	<>, linux-arch
	<>, Linux ARM
	<>, Linux Kbuild mailing list
	<>, LKML <>,
	"" <>, "maintainer:X86
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.)


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