linux-kernel - Re: [PATCH v4 1/2] x86: fix bitops.h warning with a moved cast

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Date:   Sat, 22 Feb 2020 11:39:57 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andriy Shevchenko <andriy.shevchenko@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH v4 1/2] x86: fix bitops.h warning with a moved cast

On Sat, Feb 22, 2020 at 2:04 AM Jesse Brandeburg
<jesse.brandeburg@...el.com> 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.
>
> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
> ---
> 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 | 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..fed152434ed0 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" (CONST_MASK(nr) ^ 0xff));

I'm wondering if the original, by Peter Z, order allows us to drop
(u8) casting in the CONST_MASK completely.

>         } 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