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

[David Laight <David.Laight@ACULAB.COM>]

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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