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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>,
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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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