Re: Compile time executed functions

[Mats Karlssohn <mats@mida.se>]

Robert A Duff wrote:
> 
> Mats Karlssohn <mats@mida.se> writes:
> 
> > I agree on the location issue (RAM vs. ROM), but the control over how
> > and when the code is executed is already a language issue I think.
> > We already have generics in the language wich controls how code should
> > be generated.
> 
> I don't think so.  Some compilers generate a separate piece of code for
%<

OK, understood, let's not waste time on that issue.

> >...We also have pragma Inline and representation pragmas
> > that controls the layout of code and data.
> 
> Pragma Inline doesn't *require* anything (it's just a hint).
> Representation pragmas and clauses specify data layout, but not code
> layout.

Maybe you could see my proposed pragma as a representation pragma
for code then ?
 
> >... So why couldn't we also
> > have pragma(s) to _hint_ to the compiler that this piece of code is
> > only used for initializing constants and is therefore NOT needed in
> > the runtime image if the compiler executes it.
> 
> Oh, hint?  Like pragma Inline?  Well, maybe.  But why shouldn't the
> compiler just do the optimization?  I mean, if I write "2 + 2", I want
> the compiler to generate "4" in the machine code.  Likewise, if I have a
> function call, and the function returns "2 + 2", and the function is
> inlined, I want the compiler to do the same.  I don't think any hint is
> needed here.  And anyway, the compiler can always ignore hints.

This leads me to another thing that I've sometimes have found annoy�ng:
Why isn't it required by the language that there should be a n option
to force the compiler to tell you wich pragma;s it cant obey ?
I just hate auditing machine code just to check that the compiler
hasn't generated silly code. Although I must admit that this stuff has
only occured a few times during the past 7 years.
 
%<
> Some compilers are capable of guessing when inlining is a good idea,
> even *without* pragma Inline.  If the guesses are good, then that's
> better than using the pragma.

Hmmm, actually, no. I want to be able to control what goes where
and how, in wich form it goes there. But I want to know the
compilers opinion on my chioses to. Perhaps I should state it like:
When I have said someting to the compiler, like a representation
pragma or pragma Inline, the compiler shall damn well do as I tell
it, it may Warn or cause an error, but when it produces output it
shall be of the form I've asked. As for stuff I havn't said enything
about it should just do what it can.

%<
> Compilers already do some of what you want (if you inline).  And adding
> hint pragmas won't make them do better.  An open checkbook, on the other
> hand, might cause a compiler writer to do certain table lookups (or
> whatever it is you wanted) at compile time.  ;-)

Obviously my experience with different compilers is not great
enough, I havn't come across one. Would you care to name one ?


-- 
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