linux-kernel - Re: [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast

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Message-ID: <20200225115219.GI10400@smile.fi.intel.com>
Date:   Tue, 25 Feb 2020 13:52:19 +0200
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux@...musvillemoes.dk,
        dan.j.williams@...el.com, peterz@...radead.org
Subject: Re: [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast

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