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xen-devel
Re: [Xen-devel] code question?
Keir Fraser wrote:
On 11 Aug 2005, at 18:03, M.A. Williamson wrote:
I think that:
!!a & !b === (a != 0 ) && ( b == 0 )
It's just more efficient to do it directly using bitwise ops than to
use the full logical operators (can gcc really not optimise this sort
of thing???).
It can't optimise out short-circuit semantics.
Yes it can, if the second branch of the short-circuit is guaranteed to
terminate with no side effects when it has defined behaviour.
The function thread_jump in
<http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cfgcleanup.c?rev=1.140.2.7>
attempts to prove that a basic block satisfies this property, although I'm
not sure whether gcc is capable of doing the specific optimization we're
talking about here. I know that some other C compilers are.
--
David Hopwood <david.nospam.hopwood@xxxxxxxxxxxxxxxx>
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