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

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Message-ID: <20200225103050.GD10400@smile.fi.intel.com>
Date:   Tue, 25 Feb 2020 12:30:50 +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 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.
The problem is integer promotion of negation operation for any bit basically.

For example, if we have bit 3 (nr = 3) and got it in the clear bit we will
get (according to what I see in the warnings) 0xfffffff7. Which is simple
~(1 << 3) promoted to integer.

I think it is a C standard which dictates this, compiler just follows.

> Verified with a test module (see next patch) and assembly inspection
> that the patch doesn't introduce any change in generated code.
> 
> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
> ---
> v5: changed code to use simple AND and XOR, updated commit message
> v4: reverse argument order as suggested by David Laight, added reviewed-by
> v3: Clean up the header file changes as per peterz.
> v2: use correct CC: list
> ---
>  arch/x86/include/asm/bitops.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index 062cdecb2f24..53f246e9df5a 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -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) & 0xff)
>  			: "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" (CONST_MASK(nr) ^ 0xff));
>  	} 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