[LLVMdev] [Debug Info + LTO] Type Uniquing for C types?

[Eric Christopher]

> With C++'s ODR, we are able to unique C++ types by using type identifiers to
> refer to types.
> Type identifiers are generated by C++ mangler. What about languages without
> ODR? Should we unique C types as well?
>

We can, but the identifier will need to be constructed on, likely, a
language dependent basis to ensure uniqueness.

> One solution for C types is to generate a cross-CU unique identifier for C
> types. And before linking, we update all type identifiers in a source module
> with the corresponding hash of the C types, then linking can continue as
> usual.
>

Yes.

> This requires clang to generate a cross-CU unique identifier for C types
> (one simple scheme is using a identifier that is unique within the CU and
> concatenating the CU's file name). And it also requires hashing of C types
> at DebugInfo IR level. We can add an API such as
> updateTypeIdentifiers(Module *), linker can call it right before linking in
> a source module.
>

I think the easiest design you'll get for uniquing C types that are
named the same thing (i.e. type defined in a .h file) is to use the
name of the struct combined with the file (and possibly line/column)
as an identifier. If you want to unify by structure then you'll need
to do something the equivalent to the type hashing that we're
implementing in the back end, but that'll be more difficult to
construct via the front end - it may be possible though.

-eric