From: hpa@zytor.com
To: Brian Gerst <brgerst@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Sedat Dilek <sedat.dilek@gmail.com>,
stable <stable@vger.kernel.org>,
Jesse Brandeburg <jesse.brandeburg@intel.com>,
"kernelci . org bot" <bot@kernelci.org>,
Andy Shevchenko <andriy.shevchenko@intel.com>,
Ilie Halip <ilie.halip@gmail.com>,
the arch/x86 maintainers <x86@kernel.org>,
Marco Elver <elver@google.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Daniel Axtens <dja@axtens.net>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
clang-built-linux@googlegroups.com
Subject: Re: [PATCH] x86: bitops: fix build regression
Date: Thu, 07 May 2020 12:29:59 -0700 [thread overview]
Message-ID: <4AC5875F-4CCD-44D0-9DF6-76A975EC480B@zytor.com> (raw)
In-Reply-To: <CAMzpN2iCgr0rb=MCYPGMx8tcfLq2qdzv0h7YnX5hkzBB+O7JJQ@mail.gmail.com>
On May 7, 2020 6:32:24 AM PDT, Brian Gerst <brgerst@gmail.com> wrote:
>On Thu, May 7, 2020 at 3:02 AM <hpa@zytor.com> wrote:
>>
>> On May 6, 2020 11:18:09 PM PDT, Brian Gerst <brgerst@gmail.com>
>wrote:
>> >On Tue, May 5, 2020 at 1:47 PM Nick Desaulniers
>> ><ndesaulniers@google.com> wrote:
>> >>
>> >> From: Sedat Dilek <sedat.dilek@gmail.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@vger.kernel.org
>> >> Cc: Jesse Brandeburg <jesse.brandeburg@intel.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@gmail.com>
>> >> Reported-by: kernelci.org bot <bot@kernelci.org>
>> >> Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
>> >> Suggested-by: Ilie Halip <ilie.halip@gmail.com>
>> >> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
>> >> Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
>> >> Signed-off-by: Nick Desaulniers <ndesaulniers@google.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.
--
Sent from my Android device with K-9 Mail. Please excuse my brevity.
next prev parent reply other threads:[~2020-05-07 19:30 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-05 17:44 [PATCH] x86: bitops: fix build regression Nick Desaulniers
2020-05-05 18:07 ` hpa
2020-05-05 18:22 ` Nick Desaulniers
2020-05-07 11:34 ` Peter Zijlstra
2020-05-07 14:00 ` Brian Gerst
2020-05-07 19:19 ` Nick Desaulniers
2020-05-07 22:29 ` Nick Desaulniers
2020-05-08 1:57 ` Brian Gerst
2020-05-08 17:21 ` Nick Desaulniers
2020-05-08 17:31 ` H. Peter Anvin
2020-05-10 11:59 ` David Laight
2020-05-10 12:33 ` hpa
2020-05-07 19:29 ` hpa
2020-05-06 4:30 ` Nathan Chancellor
2020-05-06 9:22 ` Sedat Dilek
2020-05-06 15:41 ` Nathan Chancellor
2020-05-06 16:37 ` Nick Desaulniers
2020-05-06 16:55 ` Ilie Halip
2020-05-07 6:18 ` Brian Gerst
2020-05-07 7:02 ` hpa
2020-05-07 13:32 ` Brian Gerst
2020-05-07 15:09 ` David Laight
2020-05-07 19:31 ` hpa
2020-05-07 19:29 ` hpa [this message]
2020-05-07 7:44 ` David Laight
2020-05-07 7:59 ` hpa
2020-05-07 8:35 ` David Laight
2020-05-07 8:38 ` hpa
2020-05-07 9:17 ` Andy Shevchenko
2020-05-07 19:22 ` Nick Desaulniers
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