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From: Ingo Molnar <mingo@kernel.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Rusty Russell <rusty@rustcorp.com.au>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/2] x86/bitops: implement __test_bit
Date: Mon, 31 Aug 2015 08:05:49 +0200	[thread overview]
Message-ID: <20150831060549.GB7093@gmail.com> (raw)
In-Reply-To: <1440776707-22016-1-git-send-email-mst@redhat.com>


* Michael S. Tsirkin <mst@redhat.com> wrote:

> +static __always_inline int __constant_test_bit(long nr, const unsigned long *addr)
> +{
> +	return ((1UL << (nr & (BITS_PER_LONG-1))) &
> +		(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
> +}
> +
> +static inline int __variable_test_bit(long nr, const unsigned long *addr)
> +{
> +	int oldbit;
> +
> +	asm volatile("bt %2,%1\n\t"
> +		     "sbb %0,%0"
> +		     : "=r" (oldbit)
> +		     : "m" (*addr), "Ir" (nr));
> +
> +	return oldbit;
> +}

Color me confused, why use assembly for this at all?

Why not just use C for testing the bit (i.e. turn __constant_test_bit() into 
__test_bit()) - that would also allow the compiler to propagate the result, 
potentially more optimally than we can do it via SBB...

Thanks,

	Ingo

  parent reply	other threads:[~2015-08-31  6:05 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-08-30  8:38 [PATCH 1/2] x86/bitops: implement __test_bit Michael S. Tsirkin
2015-08-30  8:38 ` [PATCH 2/2] kvm/x86: use __test_bit Michael S. Tsirkin
2015-08-31  6:05 ` Ingo Molnar [this message]
2015-08-31  6:13   ` [PATCH 1/2] x86/bitops: implement __test_bit H. Peter Anvin
2015-08-31  7:56     ` Michael S. Tsirkin
2015-08-31  7:59       ` Ingo Molnar
2015-08-31  8:15         ` yalin wang
2015-08-31  8:19           ` Ingo Molnar
2015-08-31  8:15         ` Ingo Molnar
2015-08-31 11:19         ` Michael S. Tsirkin
2015-09-01  9:24           ` Ingo Molnar
2015-09-01  9:40             ` Michael S. Tsirkin
2015-09-01 11:39               ` Ingo Molnar
2015-09-01 15:03                 ` Michael S. Tsirkin
2015-09-01 23:48                   ` H. Peter Anvin

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