rjmccall added a comment. Sorry for the delay. That seems like a reasonable summary of our discussion. Let me try to lay out the right architecture for this as I see it.
Conceptually, we can split the translation unit into a sequence of partial translation units (PTUs). Every declaration will be associated with a unique PTU that owns it. The first key insight here is that the owning PTU isn't always the "active" (most recent) PTU, and it isn't always the PTU that the declaration "comes from". A new declaration (that isn't a redeclaration or specialization of anything) does belong to the active PTU. A template specialization, however, belongs to the most recent PTU of all the declarations in its signature — mostly that means that it can be pulled into a more recent PTU by its template arguments. The second key insight is that processing a PTU might extend an earlier PTU. Rolling back the later PTU shouldn't throw that extension away. For example, if the second PTU defines a template, and the third PTU requires that template to be instantiated at `float`, that template specialization is still part of the second PTU. Similarly, if the fifth PTU uses an inline function belonging to the fourth, that definition still belongs to the fourth. When we go to emit code in a new PTU, we map each declaration we have to emit back to its owning PTU and emit it in a new module for just the extensions to that PTU. We keep track of all the modules we've emitted for a PTU so that we can unload them all if we decide to roll it back. Most declarations/definitions will only refer to entities from the same or earlier PTUs. However, it is possible (primarily by defining a previously-declared entity, but also through templates or ADL) for an entity that belongs to one PTU to refer to something from a later PTU. We will have to keep track of this and prevent unwinding to later PTU when we recognize it. Fortunately, this should be very rare; and crucially, we don't have to do the bookkeeping for this if we've only got one PTU, e.g. in normal compilation. Otherwise, PTUs after the first just need to record enough metadata to be able to revert any changes they've made to declarations belonging to earlier PTUs, e.g. to redeclaration chains or template specialization lists. It should even eventually be possible for PTUs to provide their own slab allocators which can be thrown away as part of rolling back the PTU. We can maintain a notion of the active allocator and allocate things like Stmt/Expr nodes in it, temporarily changing it to the appropriate PTU whenever we go to do something like instantiate a function template. More care will be required when allocating declarations and types, though. We would want the PTU to be efficiently recoverable from a `Decl`; I'm not sure how best to do that. An easy option that would cover most declarations would be to make multiple `TranslationUnitDecl`s and parent the declarations appropriately, but I don't think that's good enough for things like member function templates, since an instantiation of that would still be parented by its original class. Maybe we can work this into the DC chain somehow, like how lexical DCs are. CHANGES SINCE LAST ACTION https://reviews.llvm.org/D96033/new/ https://reviews.llvm.org/D96033 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
