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

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Message-ID: <20200221102154.GL10400@smile.fi.intel.com>
Date:   Fri, 21 Feb 2020 12:21:54 +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 v3 1/2] x86: fix bitops.h warning with a moved cast

On Thu, Feb 20, 2020 at 03:21:54PM -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. 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.

FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>

Thanks for fixing this, I have experienced tons of such messages.

> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> ---
> v3: Clean up
> the header file changes as per peterz.
> 
> v2: use correct CC: list
> ---
>  arch/x86/include/asm/bitops.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> 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
> @@ -46,7 +46,7 @@
>   * a mask operation on a byte.
>   */
>  #define CONST_MASK_ADDR(nr, addr)	WBYTE_ADDR((void *)(addr) + ((nr)>>3))
> -#define CONST_MASK(nr)			(1 << ((nr) & 7))
> +#define CONST_MASK(nr)			((u8)1 << ((nr) & 7))
>  
>  static __always_inline void
>  arch_set_bit(long nr, volatile unsigned long *addr)
> @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
>  	if (__builtin_constant_p(nr)) {
>  		asm volatile(LOCK_PREFIX "orb %1,%0"
>  			: CONST_MASK_ADDR(nr, addr)
> -			: "iq" ((u8)CONST_MASK(nr))
> +			: "iq" (CONST_MASK(nr))
>  			: "memory");
>  	} else {
>  		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
> @@ -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)));
>  	} else {
>  		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
>  			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");
> 
> base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2
> -- 
> 2.24.1
> 

-- 
With Best Regards,
Andy Shevchenko