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From: Nick Desaulniers <ndesaulniers@google.com>
To: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: clang-built-linux@googlegroups.com, andriy.shevchenko@intel.com,
	bp@alien8.de, dan.j.williams@intel.com,
	linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk,
	mingo@redhat.com, peterz@infradead.org, tglx@linutronix.de,
	x86@kernel.org, ilie.halip@gmail.com, natechancellor@gmail.com
Subject: Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast
Date: Mon, 4 May 2020 12:37:29 -0700
Message-ID: <20200504193524.GA221287@google.com> (raw)
In-Reply-To: <20200310221747.2848474-1-jesse.brandeburg@intel.com>

On Tue, Mar 10, 2020 at 03:17:46PM -0700, Jesse Brandeburg wrote:
> Fix many sparse warnings when building with C=1. These are useless
> noise from the bitops.h file and getting rid of them helps devs
> make more use of the tools and possibly find real bugs.
> 
> When the kernel is compiled with C=1, there are lots of messages like:
>   arch/x86/include/asm/bitops.h:77:37: warning: cast truncates bits from constant value (ffffff7f becomes 7f)
> 
> CONST_MASK() is using a signed integer "1" to create the mask which is
> later cast to (u8), in order to yield an 8-bit value for the assembly
> instructions to use. Simplify the expressions used to clearly indicate
> they are working on 8-bit values only, which still keeps sparse happy
> without an accidental promotion to a 32 bit integer.
> 
> The warning was occurring because certain bitmasks that end with a bit
> set next to a natural boundary like 7, 15, 23, 31, end up with a mask
> like 0x7f, which then results in sign extension due to the integer
> type promotion rules[1]. It was really only "clear_bit" that was
> having problems, and it was only on some bit checks that resulted in a
> mask like 0xffffff7f being generated after the inversion.
> 
> Verified with a test module (see next patch) and assembly inspection
> that the patch doesn't introduce any change in generated code.
> 
> [1] https://stackoverflow.com/questions/46073295/implicit-type-promotion-rules
> 
> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
> ---
> v6: reworded commit message, enhanced explanation
> v5: changed code to use simple AND and XOR, updated commit message
> v4: reverse argument order as suggested by David Laight, added reviewed-by
> v3: Clean up the header file changes as per peterz.
> v2: use correct CC: list
> ---
>  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 062cdecb2f24..53f246e9df5a 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" ((u8)CONST_MASK(nr))
> +			: "iq" (CONST_MASK(nr) & 0xff)

Sorry for the very late report.  It turns out that if your config
tickles __builtin_constant_p just right, 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".

Since we have the mask (& 0xff), can we drop the `b` suffix from the
instruction? Or is a revert more appropriate? Or maybe another way to
skin this cat?

Link: https://github.com/ClangBuiltLinux/linux/issues/961
This is blowing up our KernelCI reports.

>  			: "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" ((u8)~CONST_MASK(nr)));
> +			: "iq" (CONST_MASK(nr) ^ 0xff));
>  	} else {
>  		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
>  			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");
> 
> base-commit: 8b614cb8f1dcac8ca77cf4dd85f46ef3055f8238
> -- 
> 2.24.1
> 

  parent reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10 22:17 Jesse Brandeburg
2020-03-10 22:17 ` [PATCH v6 2/2] lib: make a test module with set/clear bit Jesse Brandeburg
2020-04-15 14:46   ` Andy Shevchenko
2020-03-11  4:37 ` [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast Luc Van Oostenryck
2020-03-17 19:25 ` Peter Zijlstra
2020-03-18 21:59 ` [tip: x86/asm] x86: Fix " tip-bot2 for Jesse Brandeburg
2020-05-04 19:37 ` Nick Desaulniers [this message]
2020-05-05  1:14   ` [PATCH v6 1/2] x86: fix " Jesse Brandeburg
2020-05-05 15:14     ` Andy Shevchenko
2020-05-05 17:29       ` Nick Desaulniers
2020-05-05 17:47         ` Nick Desaulniers

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