Re: [PATCH] x86: bitops: fix build regression

From: hpa@zytor.com
To: Brian Gerst <brgerst@gmail.com>,
	Nick Desaulniers <ndesaulniers@google.com>
Cc: 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 00:02:32 -0700	[thread overview]
Message-ID: <6A99766A-59FB-42DF-9350-80EA671A42B0@zytor.com> (raw)
In-Reply-To: <CAMzpN2idWF2_4wtPebM2B2HVyksknr9hAqK8HJi_vjQ06bgu2g@mail.gmail.com>

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.
-- 
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