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

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Date:   Mon, 24 Feb 2020 10:54:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        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>
Subject: Re: [PATCH v4 1/2] x86: fix bitops.h warning with a moved cast

On Sat, Feb 22, 2020 at 11:39:57AM +0200, Andy Shevchenko wrote:
> On Sat, Feb 22, 2020 at 2:04 AM Jesse Brandeburg

> > -#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))

Note how this is not equivalent, the old code actually handed in a u8
while the new code hands int. By moving the (u8) cast into the parens,
you casl 1 to u8, which then instantly gets promoted to 'int' due to the
'<<' operator.

> >                         : "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.

I'm thinking it's all nonsense anyway :-), the result of either << or ^
is always promoted to int anyway.

The sparse complaint was that ~CONST_MASK(nr) had high bits set which
were lost, which is true, but a copmletely stupid warning IMO.

By using 0xff ^ CONST_MASK(nr), those bits will not be set and will not
be lost.

None of that has anything to do with where we place a pointless cast
more or less.