Re: Compile time executed functions

[Mats Karlssohn <mats@mida.se>]

Ken Garlington wrote:
%<
> : The other issue is that I feel that within Ada's expressive powers
> : it would be useful to have _some_ of the possibilities that is
> : availible when using C++ templates and/or C preprocessor macros.
> : (Please, no flames on the two 'bad words' above!)
> 
> Is this a different subject than resolving complex functions as simple ROM
> constants? I'm having trouble seeing how a complex function written as a
> macro consistently ends up as a constant...

I'll try to explain a little more. But first, forget about the RAM/ROM
issue, it was a mistake to include that in the original post.

Say I want to create a table of data. The data is constant and must not
change during the programs lifetime. Further more, the data is quite
time consuming to calculate.

In C++ (bad word again :) I can (with some effort) write a template that
does all the calculations in the compiler at compile time.

In Ada, my experience is that I end up with something like:

package BlaBlah is

   -- declarations needed

   MyLookupData : constant LookupTable := GenerateTable(parameter);

end BlahBlah;

GenerateTable will be called at package elaboration time, and therefore
consume time on the target. Moreover, the GenerateTable code is not used
by anything else but is included into the loadmodule anyway.

Now IF I haf a pragma that tagged GenerateTable as a 'compiler
executed function' it would allow the compiler to compile a native
version of Generatetable, execute it and use the result as a literal
when compiling package BlahBlah. Furthermore the compiler would know
that GenerateTable isn't needed on the target and thus eliminate it's
code.

As I noted in the original post, this can be (and is today) solved by
manually putting GenerateTable into its own program and the pasting the
output into the source of package BlahBlah. This has caused management
problems and I expect it to continue to do so.


Am I plain stupid since I worry about when what's included in my
output and when time is consumed ? 

-- 
Mats Karlssohn, developer                         mailto:mats@mida.se  
Mida Systemutveckling AB                          http://www.mida.se
Box 64, S-732 22 ARBOGA, SWEDEN
Phone: +46-(0)589-89808   Fax: +46-(0)589-89809