Re: Compile time executed functions

["Ken Garlington" <Ken.Garlington@computer.org>]

I think the key statement is "The constant is really a constant and should
therefore be placed in ROM."

AFAIK, you can create an _Ada_ constant from a function. Whether it's placed
in ROM is a concept outside the scope of the language, and is a function of
compiler optimizations. For example, my company spent a lot of time and
money in order to get the result of an Unchecked_Conversion between a
System.Address and a compatible access type placed into ROM for a particular
Ada 83 compiler/linker. Before we paid for this, the result would be placed
in RAM. Both implementations were legal with respect to the language
definition.

A more general solution, as you noted, is to write an Ada program that
writes an easier-to-optimize Ada program (or, in some cases, assembly
instructions). We have several of those for commonly-used capabilities in
our domain.

"Mats Karlssohn" <mats@mida.se> wrote in message
news:3AC03CCE.70E3C2D5@mida.se...
: After following the thread on C style preprocessing, an old wish of
: mine popped up in my brain (or whatever is inside the scull :).
:
: Sometimes, especially when doing embedded programming, I feel a great
: need to execute a function to get literal initialization a constant.
: That is I'd like to have the compiler to execute a function for me (at
: compiletime!) and use the return of that function to initialize the
: constant.
:
: Example (pleas, this is just an example, it hasn't been compiled):
:
: type Byte is mod 2**8;
: type Word is mod 2**16;
: type CRC_Lookup_Table is array(0..255) of Byte;
:
: constant CRC_Polynom := 4711; -- replace with a 'correct' polunom
:
: function Generate_CRC_Table(Polynom : in Word) return CRC_Lookup_Table
: is
: begin
:    -- Implementation
: end Generate_CRC_Table;
:
: package Package_Using_CRC is
:    constant CRC_Lookup_Table := Generate_CRC_Table(CRC16_Polynom);
:
:    --
: end Package_Using_CRC;
:
:
: - The constant is really a constant an should therefore be placed in
: ROM.
: - Therefore the function Generate_CRC_Table isn't needed in the output
:   at all and since it may be quite large and/or time consuming it would
:   be nice to do the job once, at compile time.
:
:
: The way I'm currently doing this is by building a small external program
: that generates an appropriate code fragment, that gets pasted into the
: right place. This gives both maintenance and configuration management
: headaches.
:
: I must admit that I havn't researched enough to say that I can't get the
: same effect using generice and/or other techniques in Ada.
:
: Now, first I wonder have somebody out there solved this issue in a way
: that complies with the first to points above ?
:
: I realize that a lot of limitations must be put on the
: Generate_CRC_Table function, but, have something along these lines been
: considered by the language designers.
:
: I have hoped to get the time to either build a small preprocessor to do
: this kind of execution/substitution trick, or to add the same trick into
: GNAT using some pragma(Compiler_Executed, func). Sorry to say, I don't
: see that ammount of time in the near future.
:
:
: Finally, as usual, I beg you to bear with my english.
:
: --
: 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