[cfe-dev] Implementing __builtin_LINE(x) using a clang plugin

[John McCall via cfe-dev]

On 22 Dec 2019, at 3:17, Blaise Tine via cfe-dev wrote:
> Hi all,
> I have  C++ DSL that defines its own primitive types and operators 
> and I want to provide some debugging facilities during compilation to 
> display the source location (filename, line number) for uninitialized 
> DSL objects.To capture the source line information for a given DSL 
> object, I'm using __builtin_LINE(x) inside the class constructor, 
> where 'x' is the function nesting depth (see below).

`__builtin_LINE()` does not take an argument in either Clang or GCC, as 
far as I can tell, so I’ll assume you’re asking whether that would 
be a reasonable extension.  And I don’t know why a *nesting* depth 
(i.e. the number of functions/structs/whatever  that the current source 
location is nested within) would affect the result of 
`__builtin_LINE()`, so I’ll also assume you actually mean a *stack* 
depth (i.e. you would like to return the line number of the caller).  If 
that’s correct, then the answer is simple: it is not reasonable to 
implement such a feature in either a plugin or in the normal compiler.  
I can see two ways to implement it: (1) emit a specialized version of 
the function for every caller or (2) dynamically look up line-table 
information associated with the return address.  Both approaches would 
be highly complex and have serious drawbacks.

Fortunately, you don’t really need any of that, because the whole 
point of `__builtin_LINE()` is to be used in a default argument 
expression, where it will be evaluated in the context of the caller:

```
class DSLObject {
public:
   DSLObject(unsigned lineno = __builtin_LINE()) : lineno_(lineno) {}
private:
   int lineno_;
};
```

John.
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