linux-kernel - RE: [PATCH] x86: bitops: fix build regression

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Date:   Thu, 7 May 2020 15:09:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Brian Gerst' <brgerst@...il.com>, "H. Peter Anvin" <hpa@...or.com>
CC:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        stable <stable@...r.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Ilie Halip <ilie.halip@...il.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH] x86: bitops: fix build regression

From: Brian Gerst
> Sent: 07 May 2020 14:32
...
> I think the bug this worked around was that the compiler didn't detect
> that CONST_MASK(nr) was also constant and doesn't need to be put into
> a register.  The question is does that bug still exist on compiler
> versions we care about?

Hmmm...
That ought to have been fixed instead of worrying about the fact
that an invalid register was used.

Alternatively is there any reason not to use the bts/btc instructions?
Yes, I know they'll do wider accesses, but variable bit numbers do.
It is also possible that the assembler will support constant bit
numbers >= 32 by adding to the address offset.

	David

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