From: Peter Zijlstra <peterz@infradead.org>
To: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
x86@kernel.org, linux-kernel@vger.kernel.org,
linux@rasmusvillemoes.dk, andriy.shevchenko@intel.com,
dan.j.williams@intel.com
Subject: Re: [PATCH v2 1/2] x86: fix bitops.h warning with a moved cast
Date: Thu, 20 Feb 2020 19:12:36 +0100 [thread overview]
Message-ID: <20200220181236.GC18400@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200220173722.2034546-1-jesse.brandeburg@intel.com>
On Thu, Feb 20, 2020 at 09:37:21AM -0800, Jesse Brandeburg wrote:
> Fix many sparse warnings when building with C=1.
>
> 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)
>
> @@ -72,9 +74,11 @@ static __always_inline void
> arch_clear_bit(long nr, volatile unsigned long *addr)
> {
> if (__builtin_constant_p(nr)) {
> + u8 cmaski = ~CONST_MASK(nr);
> +
> asm volatile(LOCK_PREFIX "andb %1,%0"
> : CONST_MASK_ADDR(nr, addr)
> - : "iq" ((u8)~CONST_MASK(nr)));
> + : "iq" (cmaski));
> } else {
> asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
> : : RLONG_ADDR(addr), "Ir" (nr) : "memory");
Urgh, that's sad. So why doesn't this still generate a warning, ~ should
promote your u8 to int, and then you down-cast to u8 on assignment
again.
So now you have more lines, more ugly casts and exactly the same
generated code; where the win?
Perhaps you should write it like:
: "iq" (0xFF ^ CONST_MASK(nr))
hmm?
next prev parent reply other threads:[~2020-02-20 18:12 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-20 17:37 [PATCH v2 1/2] x86: fix bitops.h warning with a moved cast Jesse Brandeburg
2020-02-20 17:37 ` [PATCH v2 2/2] lib: make a test module with set/clear bit Jesse Brandeburg
2020-02-20 18:10 ` Peter Zijlstra
2020-02-20 19:03 ` Jesse Brandeburg
2020-02-20 18:12 ` Peter Zijlstra [this message]
2020-02-20 22:32 ` [PATCH v2 1/2] x86: fix bitops.h warning with a moved cast Jesse Brandeburg
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