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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: In-Out Parameters for functions
Date: Wed, 25 Feb 2004 14:41:40 -0600
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References: 
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Date: 2004-02-25T14:41:40-06:00
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"Stephen Leake" <stephen_leake@acm.org> wrote in message
news:mailman.20.1077669870.327.comp.lang.ada@ada-france.org...
> 2) Does anyone have a real example of a compiler taking advantage of
>    the evaluation order freedom to speed up a program?

Janus/Ada will take advantage of this to reduce pending floating point
results on the Intel CPUs. These are quite expensive because the floating
point stack is very short, so it has to be clear before any subprogram call.
Thus the pending results have to be spilled to memory (and later reloaded),
rather than just using registers.

So, if you have an expression like:

    B := V + F(...);

Janus/Ada may make the call first, in order to avoid the pending floating
point operation.

Of course, if F changes the value of V, then you'll get a different answer.
Of course, the compiler does not know what's in the body of F. Indeed, even
if it could see the body of F, it couldn't assume that it doesn't modify V,
as it could be changed and recompiled without the call code being changed.

Thus, requiring a particular order of evaluation would make this
optimization impossible. (Even if V is a local variable, it could be changed
by overlays or other nasty code.)

                   Randy.