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From: hpa@zytor.com
To: David Laight <David.Laight@ACULAB.COM>,
	"'Brian Gerst'" <brgerst@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	stable <stable@vger.kernel.org>,
	Jesse Brandeburg <jesse.brandeburg@intel.com>,
	"kernelci . org bot" <bot@kernelci.org>,
	Andy Shevchenko <andriy.shevchenko@intel.com>,
	Ilie Halip <ilie.halip@gmail.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Marco Elver <elver@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Daniel Axtens <dja@axtens.net>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"clang-built-linux@googlegroups.com" 
	<clang-built-linux@googlegroups.com>
Subject: RE: [PATCH] x86: bitops: fix build regression
Date: Thu, 07 May 2020 12:31:19 -0700	[thread overview]
Message-ID: <A6FD91D2-8D3A-4767-B6AD-D35B056C58C4@zytor.com> (raw)
In-Reply-To: <9c701ca55bc442c1899a70896f3ea73e@AcuMS.aculab.com>

On May 7, 2020 8:09:35 AM PDT, David Laight <David.Laight@ACULAB.COM> wrote:
>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
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>MK1 1PT, UK
>Registration No: 1397386 (Wales)

They're slower, and for unaligned locked fields can be severely so.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

  reply	other threads:[~2020-05-07 19:31 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 17:44 [PATCH] x86: bitops: fix build regression Nick Desaulniers
2020-05-05 18:07 ` hpa
2020-05-05 18:22   ` Nick Desaulniers
2020-05-07 11:34   ` Peter Zijlstra
2020-05-07 14:00     ` Brian Gerst
2020-05-07 19:19       ` Nick Desaulniers
2020-05-07 22:29         ` Nick Desaulniers
2020-05-08  1:57           ` Brian Gerst
2020-05-08 17:21             ` Nick Desaulniers
2020-05-08 17:31               ` H. Peter Anvin
2020-05-10 11:59                 ` David Laight
2020-05-10 12:33                   ` hpa
2020-05-07 19:29     ` hpa
2020-05-06  4:30 ` Nathan Chancellor
2020-05-06  9:22   ` Sedat Dilek
2020-05-06 15:41   ` Nathan Chancellor
2020-05-06 16:37   ` Nick Desaulniers
2020-05-06 16:55     ` Ilie Halip
2020-05-07  6:18 ` Brian Gerst
2020-05-07  7:02   ` hpa
2020-05-07 13:32     ` Brian Gerst
2020-05-07 15:09       ` David Laight
2020-05-07 19:31         ` hpa [this message]
2020-05-07 19:29       ` hpa
2020-05-07  7:44   ` David Laight
2020-05-07  7:59     ` hpa
2020-05-07  8:35       ` David Laight
2020-05-07  8:38         ` hpa
2020-05-07  9:17     ` Andy Shevchenko
2020-05-07 19:22   ` Nick Desaulniers

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