linux-kernel - Re: [PATCH] x86: bitops: fix build regression

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Date:   Thu, 7 May 2020 10:00:16 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        stable <stable@...r.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Ilie Halip <ilie.halip@...il.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: bitops: fix build regression

On Thu, May 7, 2020 at 7:38 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, May 05, 2020 at 11:07:24AM -0700, hpa@...or.com wrote:
> > On May 5, 2020 10:44:22 AM PDT, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> > >@@ -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" (CONST_MASK(nr) & 0xff)
> > >+                    : "iq" ((u8)(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" (CONST_MASK(nr) ^ 0xff));
> > >+                    : "iq" ((u8)(CONST_MASK(nr) ^ 0xff)));
> > >     } else {
> > >             asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
> > >                     : : RLONG_ADDR(addr), "Ir" (nr) : "memory");
> >
> > Drop & 0xff and change ^ 0xff to ~.
>
> But then we're back to sparse being unhappy, no? The thing with ~ is
> that it will set high bits which will be truncated, which makes sparse
> sad.

This change will make sparse happy and allow these cleanups:
#define CONST_MASK(nr)                 ((u8)1 << ((nr) & 7))

Tested with GCC 9.3.1.

--
Brian Gerst