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Date: Mon, 07 Jan 2002 13:21:26 -0500
From: Hyman Rosen <hyrosen@mail.com>
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Newsgroups: comp.lang.ada
Subject: Re: A case where Ada defaults to unsafe?
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Date: 2002-01-07T13:21:26-05:00
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FGD wrote:

> Any way you look at it, better performance is obtained when 
> short-circuit and is used only when absolutely necessary. It's only 
> natural for Ada to "default" to the commutative and.


What many of you seem to forget is that the commutative form requires
that both operands are evaluated. Of course the compiler can elide by
the as-if rule, but not if there is a chance of overflow:

	if a + b > 3 and	x + y < 4	-- case 1
	if a + b > 3 and then	x + y < 4	-- case 2

In the first case, if there is a chance that evaluating x + y could
lead to overflow, the compiler cannot forgo the evaluation even in
the case of a + b <= 3, becuase the exception must be generated. So
we are not making a simple trade-off between letting the programmer
or the compiler choose the order.