linux-kernel - RE: [PATCH v3 1/2] x86: fix bitops.h warning with a moved cast

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Message-ID: <ca95b4325c8c44258672b7822ab4d16b@AcuMS.aculab.com>
Date:   Fri, 21 Feb 2020 15:33:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jesse Brandeburg' <jesse.brandeburg@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
        "andriy.shevchenko@...el.com" <andriy.shevchenko@...el.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: RE: [PATCH v3 1/2] x86: fix bitops.h warning with a moved cast

From: Jesse Brandeburg
> 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. Move the cast to the definition and
> clean up the calling sites to prevent sparse from warning.
> 
> 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).
> 
> Verified with a test module (see next patch) and assembly inspection
> that the patch doesn't introduce any change in generated code.
...
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index 062cdecb2f24..96ef19dcbde6 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
...
> @@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
>  	if (__builtin_constant_p(nr)) {
>  		asm volatile(LOCK_PREFIX "andb %1,%0"
>  			: CONST_MASK_ADDR(nr, addr)
> -			: "iq" ((u8)~CONST_MASK(nr)));
> +			: "iq" (0xff ^ CONST_MASK(nr)));

IMHO (CONST_MASK(nr) ^ 0xff) is better.

	David

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