[clang] [llvm] [DebugInfo] Place local ODR-uniqued types in decl DISubprograms (PR #142166)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> > How does this work for LTO? (we'd want all inlined instances to refer to a singular abstract definition) Yeah, looks like that's broken (the abstract origins are not shared - they're duplicated in every translation unit).
> 
> But the notion of "abstract" subprogram pops up only during DWARF generation, doesn’t it? On the level of LLVM IR, we don’t have a way to distinguish between abstract and concrete DISubprogram, if I get it right.

I think it might be useful to think of it that we only have abstract subprograms - concrete and inlined subprograms exist as a consequence of the instructions/locations in the llvm functions that define them.

(long time ago we had subprograms that referred to functions (which became problematic in LTO - with multiple subprograms referring to the same deduplicated function, or optimized out functions still having lingering subprograms), so we inverted it - functions refer to their subprogram - at which point I think we sort of created the lazily-referenced "abstract" subprogram by construction, it can be referred to by both/either the concrete function, or the inlined instructions - it seems to me it is an abstract/shared description across both of those cases)

> > We should fix that. We have mangled names on functions like we do for types, so we could use the same approach to deduplicate them via mangled name as a key? (for types this mangled name field was introduced specifically as a deduplication key (I think initially for type units, then for LTO merging, but maybe it was the other way around) - currently the linkage name for functions isn't a load bearing key, but it could become one, or perhaps needs to be a distinct key compared to the mangled name - in the case of internal functions/types they might have a mangled name, but not allow deduplication (for internal linkage types, we omit the mangled name/deduplication key))
> 
> It seems something similar is done for declarations of composite type member subprograms, according to https://llvm.org/docs/LangRef.html#disubprogramdeclaration :
> 
> > When spFlags: DISPFlagDefinition is not present, subprograms describe a declaration in the type tree as opposed to a definition of a function. In this case, the declaration field must be empty. If the scope is a composite type with an ODR identifier: and that does not set flags: DIFwdDecl, then the subprogram declaration is uniqued based only on its linkageName: and scope:.

Oh, I think that was added for call-site declarations which are deduplicated correctly/not emitted as unique, etc.... so they're probably uniqued with each other, but not with the unique definitions from other translation units.

So maybe the solution is as simple as removing the "unique" from DISubprogram definitions, and the existing infrastructure will handle it? (maybe have to expand it to allow it to work for things with DISPFlagDefinition... )

(I'm a bit confused about some of that spec wording though - DIFwdDecl is /not/ set but also DISPFlagDefinition is not present... (a: why do we have both of those, they seem redundant with each other, b: both being not-present seems contradictory))

https://github.com/llvm/llvm-project/pull/142166