linux-kernel - RE: [PATCH v5] x86: bitops: fix build regression

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Date:   Sun, 10 May 2020 13:54:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
CC:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        "Andy Shevchenko" <andriy.shevchenko@...el.com>,
        Brian Gerst <brgerst@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Ilie Halip <ilie.halip@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Daniel Axtens <dja@...ens.net>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH v5] x86: bitops: fix build regression

From: Nick Desaulniers
> Sent: 08 May 2020 19:32
..
> It turns out that if your config tickles __builtin_constant_p via
> differences in choices to inline or not, these statements produce
> invalid assembly:
...
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index b392571c1f1d..35460fef39b8 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -52,9 +52,9 @@ static __always_inline void
>  arch_set_bit(long nr, volatile unsigned long *addr)
>  {
>  	if (__builtin_constant_p(nr)) {
> -		asm volatile(LOCK_PREFIX "orb %1,%0"
> +		asm volatile(LOCK_PREFIX "orb %b1,%0"
>  			: CONST_MASK_ADDR(nr, addr)
> -			: "iq" (CONST_MASK(nr) & 0xff)
> +			: "iq" (CONST_MASK(nr))
>  			: "memory");

What happens if CONST_MASK() is changed to:
#define CONST_MASK_(n) (n == 0 ? 1 : n == 1 ? 2 : n ....)
#define CONST_MASK(n) CONST_MASK_(((n) & 7))

and a separate definition for the inverse mask.

The lack of arithmetic promotion may mean that the only "i"
constraint is needed.

	David

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