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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: This can't be done in Ada...or?
Date: 11 Feb 2005 13:27:11 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2005-02-11T13:27:11-05:00
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Jeff C <jcreem@yahoo.com> writes:

> Per Lindquist wrote:
> > 3. does *not* cause any execution overhead if disabled.
...
> > I say it can't be done in Ada. Please prove me wrong!


> 3) Pragma Inline the procedure calls.  Put the "if Logging_Enabled"
> inside the logging procedures. Make the Logging_Enabled a static
> constant. Compiler should optimize away the entire procedure call if
> done properly (it does on GNAT and VADS for the cases I have tried)

The compiler can optimize away everything inside the "if
Logging_Enabled".  But it can't optimize away the evaluation of the
parameters unless it can prove the absence of side effects.
For example:

    Trace.Error(..., "Bad value of X " & Debug_Info(X));

where Debug_Info is some user-defined function that produces useful
debug info about some complicated data structure called X.
It has no side effects, but the compiler doesn't know that, usually.

I suppose you could always write Debug_Info like this:

    function Debug_Info(...) return String;
    pragma Inline(Debug_Info);
    -- Don't call this unless Logging_Enabled is True!

    function Debug_Info(...) return String is
    begin
        if Logging_Enabled then
            return The_Real_Debug_Info(...);
        else
            raise Program_Error;
        end if;
    end Debug_Info;

But then you have to write two versions of Debug_Info
for every type.  So I'd say the original poster's statement,
"it can't be done in Ada" is pretty true, if zero run-time overhead
is required.

In my code, I usually do as you suggested, but *also* add "if
Logging_Enabled" or whatever around calls if the parameter evaluation
could be expensive.  This provides *almost* what the original poster
asked for -- you need the annoying "if Logging_Enabled" only sometimes.

- Bob