From: Andy Shevchenko <andriy.shevchenko@intel.com>
To: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
x86@kernel.org, linux-kernel@vger.kernel.org,
linux@rasmusvillemoes.dk, dan.j.williams@intel.com,
peterz@infradead.org
Subject: Re: [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast
Date: Tue, 25 Feb 2020 13:52:19 +0200 [thread overview]
Message-ID: <20200225115219.GI10400@smile.fi.intel.com> (raw)
In-Reply-To: <20200225103050.GD10400@smile.fi.intel.com>
On Tue, Feb 25, 2020 at 12:30:50PM +0200, Andy Shevchenko wrote:
> On Mon, Feb 24, 2020 at 02:50:19PM -0800, Jesse Brandeburg wrote:
> > Fix many sparse warnings when building with C=1.
> >
> > When the kernel is compiled with C=1, there are lots of messages like:
> > arch/x86/include/asm/bitops.h:77:37: warning: cast truncates bits from constant value (ffffff7f becomes 7f)
> >
> > CONST_MASK() is using a signed integer "1" to create the mask which
> > is later cast to (u8) when used, in order to yield an 8-bit value
> > for the assembly instructions to use. Simplify the expressions used to
> > clearly indicate they are working on 8-bit values only, which still
> > keeps sparse happy without an accidental promotion to a 32 bit integer.
> >
>
> > The reason the warning was occurring is because certain bitmasks that
> > end with a mask next to a natural boundary like 7, 15, 23, 31, end up
> > with a mask like 0x7f, which then results in sign extension when doing
> > an invert (but I'm not a compiler expert). It was really only
> > "clear_bit" that was having problems, and it was only on bit checks next
> > to a byte boundary (top bit).
>
> I guess this describes it incorrectly.
Forget about it, I looked at the warning again and there is the 0x7f byte at
the end. Sorry for noise.
> I think it is a C standard which dictates this, compiler just follows.
--
With Best Regards,
Andy Shevchenko
next prev parent reply other threads:[~2020-02-25 11:52 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-24 22:50 [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast Jesse Brandeburg
2020-02-24 22:50 ` [PATCH v5 2/2] lib: make a test module with set/clear bit Jesse Brandeburg
2020-02-25 10:30 ` [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast Andy Shevchenko
2020-02-25 11:52 ` Andy Shevchenko [this message]
2020-02-25 12:50 ` Peter Zijlstra
2020-03-10 22:22 ` Jesse Brandeburg
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