Re: Crosspost: Help wanted from comp.compilers

["Robert I. Eachus" <rieachus@attbi.com>]

Preben Randhol wrote:

> Perhaps I misunderstand you, but if you make the .ali files read-only
> the library won't be recompiled when you with a unit in your program. If
> it needs to be due to change of compiler or some other reason the
> compiler will stop compiling and tell you it is needed. But I guess you
> were refering to something else?

You misunderstood.  The specifications of all units, usually packages, 
that you with are recompiled everytime.  This does not affect the status 
of that unit.  It is just that it is faster and easier to do those 
recompiles than to store an intermediate representation of the spec.

To compare the two approaches:

Old way:

When you compile a package specification, a library entry is created 
containing a processed version of the package spec.

When you compile a unit that withs that package, the processed version 
is found and loaded into the compiler.

GNAT way:

IF you compile a package specification, the compiler checks it for 
errors and does nothing else.  (Well it gives you a message saying no 
code generated.  Go do gnatmake foo.ads for some package specification 
you have lying around to try it.)

When you compile a body (foo.adb) the complier creates a transitive 
closure of the with clauses, compiling each specification as found.  The 
compiler then compiles the body, including any subunits, creates an 
object file (and other files) and puts them in the directory (program 
library).

Are there cases where doing things the old way would be faster?  Sure. 
But in the majority of cases the new GNAT approach, shared by some other 
Ada 95 compilers is faster.  And as far as I am concerned, it is better 
for actual Ada programmers.  This goes back to a very old paper, 
"Nesting in Ada is for the Birds,"  by  L. Clarke, J. Wileden & A. Wolf, 
  ACM SIGPLAN Notices 15, 11 (1980), 139-145.  (How's that for Googling 
an old reference out of nowhere. ;-)  In Ada trees tend to become 
shorter and bushier as programmers get more familiar with Ada style.  So 
for all I know, compiling specs into the library was the right model for 
Ada 83, but the new model is better now.

-- 

                                                        Robert I. Eachus

�In an ally, considerations of house, clan, planet, race are 
insignificant beside two prime questions, which are: 1. Can he shoot? 2. 
Will he aim at your enemy?� -- from the Laiden novels by Sharon Lee and 
Steve Miller.