[fpc-devel] Proposal: Supporting initial references for weakexternal

[Jeppe Johansen]

While trying to find a nice solution for generating interrupt vector 
tables on ARM, I found that the usually employed solution is to generate 
weak references to symbols, that are initialized to a single handler. 
This way an application can override the weak reference, by declaring a 
new symbol with the same name. While it's possible to do that currently 
by using some external assembler code, I think a simpler solution could 
be made by using the weakexternal directive. The infrastructure is 
already there, but it's used in a slightly different way.

An added bonus of using this new way is that IRQs will branch directly 
to the handler, instead of first branching to some code that loads the 
handler address, and then branching to the handler

I've modified the syntax for this directive slightly, so it can take the 
form "weakexternal ['library'] [name 'name'] [set 'initialvalue'];" 
instead of "weakexternal ['library'] [name 'name'];"

A short example:
     procedure NoHandler;
     begin
     end;

     procedure TestProc; weakexternal name 'TestProc' set 'NoHandler';
     ...
     begin
        TestProc; // This will call NoHandler
     end.

If we modify the example:
     procedure NoHandler;
     begin
     end;

     procedure TestProcHandler; [public, alias: 'TestProc'];
     begin
     end;

     procedure TestProc; weakexternal name 'TestProc' set 'NoHandler';
     ...
     begin
        TestProc; // This will call TestProcHandler
     end.

All this is done either at compile time, or link time

What do you think about this proposal?

I've put a patch up here: http://j-software.dk/new/weakexternal.txt
And a small example of how lpc21x4.pp, for example, could be modified to 
use the new functionality: http://j-software.dk/new/lpc21x4.txt