[lldb-dev] A bit of extra-polish for my lldb plugin

[Jim Ingham via lldb-dev]

Because we try as much as possible to let the compiler figure this sort of thing out for us, we implement the transparent lookups for this & self by compiling our expression in a context that poses as a method of the class whose method we are stopped in.  For instance, for ObjC, we construct a category on the class, and put the expression text in a method in that category, and then compile all that and call the method.

That's done in ExpressionSourceCode::GetText.

The one hitch to this is that this works because the compiler that is linked into lldb knows about the ObjC model we are emulating.  An unmodified clang won't do transparent lookup into some random argument that happens to be called _param.  So you would have to build lldb with a version of clang that understands your lookup rules for this to work.

If that's not possible then you can try to do this by monkeying with the lookup rules implemented by lldb's ClangASTSource to return "_param.a" when it is just looking up "a".  That's how we inject variables and types that are present in the local context into the expression as it is getting parsed.  But at that point you are getting your hands into some fairly deep magic, and clang's Aslan is not as forgiving as Narnia's...

Jim


> On May 26, 2017, at 2:27 PM, Nat! via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Let me show you a snippet of a lldb debug session in progress in my ObjC
> variant:
> 
> ```
> -100000,100000,v,18.48
> Process 45774 stopped
> * thread #1, queue = 'com.apple.main-thread', stop reason = step in
>    frame #0: 0x0000000100000e2a multiple.debug`+[Foo
> long:int:char:float:](self=Foo, _cmd=<no value available>,
> _param=0x00007fff5fbff948) at multiple.m:15
>   12  	         char:(char) c
>   13  	        float:(float) d
>   14  	{
>   15  	   printf( "%ld,%d,%c,%.2f\n", a, b, c, d);
> -> 16  	}
>   17  	
>   18  	@end
> (lldb) p *_param
> (p.long:int:char:float:) $2 = (a = -100000, b = 100000, c = 'v', d =
> 18.4799995)
> ```
> 
> You can see that the parameter values `a,b,c,d` are actually fields of
> a struct parameter `_param`. `_param` uniformly appears as the third
> parameter after `self` and `_cmd`. `p _param->a` works of course, but it
> would be nice to be able to say 'p a', since in the source code one sees
> only `a`. `_param` is more or less an implementation detail.
> 
> A clue how to achieve this, would be very much appreciated.
> 
> Ciao
>   Nat!
> 
> 
> [*] except, if it's a picture of thousand words :)
> 
> https://www.mulle-kybernetik.com/weblog/2015/mulle_objc_meta_call_convention.html
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