linux-kernel - Re: [PATCH v2 1/2] x86: fix bitops.h warning with a moved cast

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Date:   Thu, 20 Feb 2020 19:12:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
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,
        andriy.shevchenko@...el.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 1/2] x86: fix bitops.h warning with a moved cast

On Thu, Feb 20, 2020 at 09:37:21AM -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)
> 

> @@ -72,9 +74,11 @@ static __always_inline void
>  arch_clear_bit(long nr, volatile unsigned long *addr)
>  {
>  	if (__builtin_constant_p(nr)) {
> +		u8 cmaski = ~CONST_MASK(nr);
> +
>  		asm volatile(LOCK_PREFIX "andb %1,%0"
>  			: CONST_MASK_ADDR(nr, addr)
> -			: "iq" ((u8)~CONST_MASK(nr)));
> +			: "iq" (cmaski));
>  	} else {
>  		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
>  			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");

Urgh, that's sad. So why doesn't this still generate a warning, ~ should
promote your u8 to int, and then you down-cast to u8 on assignment
again.

So now you have more lines, more ugly casts and exactly the same
generated code; where the win?

Perhaps you should write it like:

		: "iq" (0xFF ^ CONST_MASK(nr))

hmm?