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

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Message-ID: <4AC5875F-4CCD-44D0-9DF6-76A975EC480B@zytor.com>
Date:   Thu, 07 May 2020 12:29:59 -0700
From:   hpa@...or.com
To:     Brian Gerst <brgerst@...il.com>
CC:     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>,
        "Peter Zijlstra (Intel)" <peterz@...radead.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 May 7, 2020 6:32:24 AM PDT, Brian Gerst <brgerst@...il.com> wrote:
>On Thu, May 7, 2020 at 3:02 AM <hpa@...or.com> wrote:
>>
>> On May 6, 2020 11:18:09 PM PDT, Brian Gerst <brgerst@...il.com>
>wrote:
>> >On Tue, May 5, 2020 at 1:47 PM Nick Desaulniers
>> ><ndesaulniers@...gle.com> wrote:
>> >>
>> >> From: Sedat Dilek <sedat.dilek@...il.com>
>> >>
>> >> It turns out that if your config tickles __builtin_constant_p via
>> >> differences in choices to inline or not, this now produces invalid
>> >> assembly:
>> >>
>> >> $ cat foo.c
>> >> long a(long b, long c) {
>> >>   asm("orb\t%1, %0" : "+q"(c): "r"(b));
>> >>   return c;
>> >> }
>> >> $ gcc foo.c
>> >> foo.c: Assembler messages:
>> >> foo.c:2: Error: `%rax' not allowed with `orb'
>> >>
>> >> The "q" constraint only has meanting on -m32 otherwise is treated
>as
>> >> "r".
>> >>
>> >> This is easily reproducible via
>> >Clang+CONFIG_STAGING=y+CONFIG_VT6656=m,
>> >> or Clang+allyesconfig.
>> >>
>> >> Keep the masking operation to appease sparse (`make C=1`), add
>back
>> >the
>> >> cast in order to properly select the proper 8b register alias.
>> >>
>> >>  [Nick: reworded]
>> >>
>> >> Cc: stable@...r.kernel.org
>> >> Cc: Jesse Brandeburg <jesse.brandeburg@...el.com>
>> >> Link: https://github.com/ClangBuiltLinux/linux/issues/961
>> >> Link:
>> >https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
>> >> Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved
>cast")
>> >> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
>> >> Reported-by: kernelci.org bot <bot@...nelci.org>
>> >> Suggested-by: Andy Shevchenko <andriy.shevchenko@...el.com>
>> >> Suggested-by: Ilie Halip <ilie.halip@...il.com>
>> >> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
>> >> Signed-off-by: Sedat Dilek <sedat.dilek@...il.com>
>> >> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> >> ---
>> >>  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 b392571c1f1d..139122e5b25b 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" (CONST_MASK(nr) & 0xff)
>> >> +                       : "iq" ((u8)(CONST_MASK(nr) & 0xff))
>> >
>> >I think a better fix would be to make CONST_MASK() return a u8 value
>> >rather than have to cast on every use.
>> >
>> >Also I question the need for the "q" constraint.  It was added in
>> >commit 437a0a54 as a workaround for GCC 3.4.4.  Now that the minimum
>> >GCC version is 4.6, is this still necessary?
>> >
>> >--
>> >Brian Gerst
>>
>> Yes, "q" is needed on i386.
>
>I think the bug this worked around was that the compiler didn't detect
>that CONST_MASK(nr) was also constant and doesn't need to be put into
>a register.  The question is does that bug still exist on compiler
>versions we care about?
>
>--
>Brian Gerst

The compiler is free to do that, including for legit reasons (common subexpression elimination, especially.) So yes.
-- 
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