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Date: Mon, 31 Aug 2015 08:05:49 +0200
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
Message-ID: <20150831060549.GB7093@gmail.com>
References: <1440776707-22016-1-git-send-email-mst@redhat.com>
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* 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