Messages in this thread | | | Date | Sun, 4 Jun 2006 23:07:32 -0400 | From | Chuck Ebbert <> | Subject | Re: [minor fix] radixtree: regulate tag get return value |
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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
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