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

[Jonas Maebe]

On 14 Feb 2011, at 14:36, Jeppe Johansen wrote:

> Den 14-02-2011 14:00, Jonas Maebe skrev:
>>
>> It's not the same.
>>
>> "weak" (without external) would be a definition, which means that  
>> it would be used in a situation like this:
>>
>>  procedure Test; weak;
>>    begin
>>      writeln('test');
>>    end;
>>
>> And the compiler would then generate something like this:
>>
>>  .weak Test
>> Test:
>> <code for test>
>>
>>
>> This would however be invalid code:
>>
>>  procedure Test; weak; external; // or "weakexternal" instead of  
>> "weak; external;"
>>    begin
>>    end;
>>
>> And this would be an incomplete declaration (missing function body):
>>
>>  procedure Test; weak;
>>
>> That's why it was not a good idea (from me) to introduce  
>> "weakexternal", since "weak" combined with "external" would be the  
>> same (even if "weak" by itself weren't supported immediately).  
>> Furthermore, the whole "default 'initialvalue'" stuff is actually  
>> superfluous, since it basically is already part of the  
>> weak(external) declaration:
>>
>>  procedure Test; weakexternal name 'aliasedTest';
>>
>> could be defined to generate this:
>>
>>  .weak Test
>>  .set Test,aliasedTest
>>
>> After all, the above states that references to "Test" should be  
>> treated as weak references to a symbol called "aliasedTest". It  
>> would also be equivalent to what C does. I also don't think it  
>> would break much, if any, existing code that uses "weakexternal" if  
>> the semantics were to be redefined in this way.
>>
>
> Sure, the weak declaration as a procedure directive could be added,  
> and it would indeed be something different from a weakexternal  
> procedure. Weakexternal should stay as it is, a procedure  
> declaration directive. I can see how a "weak" directive could be  
> useful, but I don't need that. I need a linker directive that points  
> to one of two external functions
>
> The reason weakexternal makes sense is because it's a weak undefined  
> symbol reference to some external symbol,

A "weakexternal" declaration by itself is not a reference (weak/ 
undefined or not), neither at the Pascal, C nor at the assembler/ 
object file level. It only says that *if* there is a reference to that  
symbol, then this reference should be treated as a weak reference.

> which - if it doesn't exist - points to some other external  
> reference. A weak procedure would be almost the same, you just add  
> the default reference as code explicitly

It's the same as the difference between these two declarations in C:

int SayHello(void) __attribute__((weak))
{
   ...
}

and

extern int SayHello() __attribute__((weak));
or
extern int SayHello() __attribute__((weak,alias 'OtherSayHello'));

(this second "alias" form is not supported by all object formats  
though -- e.g. gcc on Mac OS X warns that it's ignored --, but at the  
Pascal level it can be emulated by the compiler just like for non-weak  
alias definitions)

> The problem with weak is that it's tagged onto a piece of code. So  
> in a case like the one that originally provoked the proposal
>
>  procedure EmptyFunc;
>    begin // Empty function
>    end;
>
> procedure Func1; weakexternal name 'Func1' default 'EmptyFunc';
> procedure Func2; weakexternal name 'Func2' default 'EmptyFunc';
>
> using weak you would need two procedures.

No, because you would be able to combine "weak" and "external" to get  
the same result as with "weakexternal". Furthermore, there's also the  
"alias" modifier to add symbol aliases to a definition.

I'm also not sure why you made this remark, because at no point I  
suggested removing the functionality currently offered by  
"weakexternal". I only proposed to make the syntax more orthogonal,  
and to extend the semantics of "weakexternal;"/"weak; external;" so  
that the "default 'xxx'" specifier is not required while still getting  
the same effect.


Jonas