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
next prev 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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