From: Brian Gerst <brgerst@gmail.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
"H. Peter Anvin" <hpa@zytor.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>,
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 <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] x86: bitops: fix build regression
Date: Thu, 7 May 2020 21:57:07 -0400 [thread overview]
Message-ID: <CAMzpN2gu4stkRKTsMTVxyzckO3SMhfA+dmCnSu6-aMg5QAA_JQ@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOd=Ogbp0oVgmF2B9cePjyWnvLLFRSp2EnaonA-ZFN3K7Dg@mail.gmail.com>
On Thu, May 7, 2020 at 6:29 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Thu, May 7, 2020 at 12:19 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Thu, May 7, 2020 at 7:00 AM Brian Gerst <brgerst@gmail.com> wrote:
> > >
> > > This change will make sparse happy and allow these cleanups:
> > > #define CONST_MASK(nr) ((u8)1 << ((nr) & 7))
> >
> > yep, this is more elegant, IMO. Will send a v3 later with this
> > change. Looking at the uses of CONST_MASK, I noticed
> > arch_change_bit() currently has the (u8) cast from commit
> > 838e8bb71dc0c ("x86: Implement change_bit with immediate operand as
> > "lock xorb""), so that instance can get cleaned up with the above
> > suggestion.
>
> Oh, we need the cast to be the final operation. The binary AND and
> XOR in 2 of the 3 uses of CONST_MASK implicitly promote the operands
> of the binary operand to int, so the type of the evaluated
> subexpression is int.
> https://wiki.sei.cmu.edu/confluence/display/c/EXP14-C.+Beware+of+integer+promotion+when+performing+bitwise+operations+on+integer+types+smaller+than+int
> So I think this version (v2) is most precise fix, and would be better
> than defining more macros or (worse) using metaprogramming.
One last suggestion. Add the "b" modifier to the mask operand: "orb
%b1, %0". That forces the compiler to use the 8-bit register name
instead of trying to deduce the width from the input.
--
Brian Gerst
next prev parent reply other threads:[~2020-05-08 1:57 UTC|newest]
Thread overview: 45+ 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 [this message]
2020-05-08 17:21 ` Nick Desaulniers
2020-05-08 17:31 ` H. Peter Anvin
2020-05-08 18:05 ` [PATCH v3] " Nick Desaulniers
2020-05-08 18:08 ` Nick Desaulniers
2020-05-08 18:22 ` Brian Gerst
2020-05-08 18:28 ` [PATCH v4] " Nick Desaulniers
2020-05-08 18:32 ` [PATCH v5] " Nick Desaulniers
2020-05-08 20:28 ` Nathan Chancellor
2020-05-08 23:47 ` Jesse Brandeburg
2020-05-09 4:44 ` Sedat Dilek
2020-05-09 12:20 ` Andy Shevchenko
2020-05-09 15:43 ` Brian Gerst
2020-05-11 17:22 ` Nick Desaulniers
2020-05-10 13:54 ` David Laight
2020-05-11 18:52 ` Brian Gerst
2020-05-14 23:47 ` Nick Desaulniers
2020-05-10 11:59 ` [PATCH] " 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
2020-05-06 17:05 ` [PATCH v2] " Nick Desaulniers
2020-05-07 6:18 ` [PATCH] " 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
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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