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

[Jeppe Johansen]

After thinking about this proposal for some time, I've decided that I 
like the original proposal better than the one that involved typechecked 
qualified procedure identifiers

I think it might be a bad idea to apply high-level semantics to it, 
because it's not crossplatform(COFF doesn't seem to support it), and 
it's already something that only makes sense to use in a limit amount of 
cases.

I did a little change to the patch though. I changed the syntax to be 
"weakexternal ['library'] [name 'name'] [default 'initialvalue'];" since 
I figured default made more sense than set, in the given context. I 
still think using weakexternal makes sense(over adding a new "weak" 
keyword), since the meaning and functionality still is the same

You can find the slightly modified patch here: 
http://j-software.dk/weakexternal3.patch.txt

Den 12-01-2011 06:29, Jeppe Johansen skrev:
> 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
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