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Date: Sun, 4 Jun 2006 23:07:32 -0400
From: Chuck Ebbert <76306.1226@compuserve.com>
Subject: Re: [minor fix] radixtree: regulate tag get return value
To: Andrew Morton <akpm@osdl.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
        Wu Fengguang <wfg@mail.ustc.edu.cn>
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In-Reply-To: <20060604131824.e2d1c934.akpm@osdl.org>

On Sun, 4 Jun 2006 13:18:24 -0700, Andrew Morton wrote:

> test_bit() returns (1 & (expr)) - it _has_ to return 0 or 1.

On i386, test_bit(nr, addr) will return 0 or 1 only when nr is
a constant -- it uses (expr != 0).

If nr is a variable it will return either 0 or -1 because it uses:

        __asm__ __volatile__(
                "btl %2,%1\n\tsbbl %0,%0"
                :"=r" (oldbit)
                :"m" (*addr),"Ir" (nr));
        return oldbit;

btl %2,%1 sets the carry flag if the selected bit is set.

sbbl %0,%0 subtracts a register from itself and then subtracts one
from the result if the carry flag is set.

-- 
Chuck