[LLVMdev] Parameter names in IR and debug info

[David Blaikie]

On Thu, Feb 19, 2015 at 8:18 AM, Rodney M. Bates <rodney_bates at lcwb.coop>
wrote:

> Have I correctly inferred below, how I build IR and debug info for
> a function type and a function (value), in particular, how to supply
> the names of the formal parameters?
>

Generally good advice: http://llvm.org/docs/tutorial/LangImpl8.html


> To create a function in llvm IR and give names to its formal parameters,
> I must:
>
> 1. Build a LLVMTypeRef for the type of each formal and the function result.
>

Sounds like you're using the C API. I'm not especially familiar with that,
so answers may be vague.


> 2. Build a function type using LLVMFunctionType, from the results of 1.
> 3. Build a function (an LLVMValueRef), using LLVMAddFunction, from the
> result of 2.
> 4. Get the LLVMValueRef for each formal (apparently, these are constructed
> inside
>    LLVMAddFunction), using LLVMGetParam, from the result of 3.
> 5. Set the formal name using LLVMSetValueName, from each result of 5.
>

The names of LLVM IR values are purely aids for LLVM developers (such as
yourself), they should never have any impact on the result of LLVM (in
terms of machine asm/code - the textual LLVM IR will include the names, but
again, this is just a debugging aid for you, the LLVM developer (it has no
impact on the DWARF debug info LLVM emits))


> Which appears to imply that the formal names are part of the function,
> not the function type, and thus the function type could be reused for
> another
> function whose signature differs only in the names of the formals.  Also
> the
> function type could be used as the referent of a pointer type, which could
> then be used as the type of a variable, without any actual function at all.
>

Sure.


>
> To build corresponding debug info, I must:
>
> 6. Build a llvm::DIArray, using llvm::getOrCreateArray, from the results
> of 4.
> 7. Build a llvm::DIComposite type for the function, using
>    llvm::createSubroutineType, from the result of 6.
> 8. Build a llvm::DIFunction using llvm::createFunction, from the result of
> 7.
>
> Here, I need the formal values, with names, first, before building the
> function
> type.


I don't think you should need parameter names for createSubroutineType -
it's just a type (composed of other types, no variable names, just type
names).


> This appears to imply that, in debug info, the formal names are also part
> of the function type,


Shouldn't be. But the actual DWARF output doesn't necessarily have explicit
function types - it just has a function with some formal parameters, each
with a type and in a specified order.


> which thus cannot be reused for a different function with
> different formal names.
>
> Can I build a DI function type without having an actual function of that
> type?
> This happens in my language.


Not sure I understand. You mean your language has, say, a function pointer
even though you have no function of that type. Certainly clang does this
(try compiling something simple like "void (*x)();" in clang and look at
the LLVM IR it produces - you'd want to produce something similar).

- David


>
>
> --
> Rodney Bates
> rodney.m.bates at acm.org
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