[Frontend] Destroy compiling compiler instance before read

[bnbarham]

llvm#134887 added a clone for the compiler instance in compileModuleAndReadASTImpl, which would then be destroyed after the corresponding read in the importing instance.

Swift has a SwiftNameLookupExtension module extension which updates (effectively) global state - populating the lookup table for a module on read and removing it when the module is destroyed.

With newly cloned instance, we would then see:

• Module compiled with cloned instance
• Module read with importing instance
• Lookup table for that module added
• Cloned instance destroyed
• Module from that cloned instance destroyed
• Lookup table for that module name removed

Depending on the original semantics is incredibly fragile, but for now it's good enough to ensure that the read in the importing instance is after the cloned instanced is destroyed. Ideally we'd only ever add to the lookup tables in the original importing instance, never its clones.

[bnbarham]

Just on stable/20250601 for now as I'm unsure this is actually the change we want upstream

[bnbarham]

@swift-ci please test llvm

[bnbarham]

@swift-ci please test

[jansvoboda11]

This makes sense to me. Can you clarify what acts as the effectively global state in SwiftNameLookupExtension ?

[bnbarham]

Can you clarify what acts as the effectively global state in SwiftNameLookupExtension ?

SwiftNameLookupExtension::createExtensionReader adds to lookupTables (whose lifetime is that of the ClangImporter) after reading the module. It also passes a OnRemove (to remove the module from the lookup table) down to SwiftLookupTableReader::create, which is called on ~SwiftLookupTableReader.

And readers have the lifetime of the module they were created for:

      if (auto Reader = Known->second->createExtensionReader(Metadata, *this,
                                                             F, Stream)) {
        F.ExtensionReaders.push_back(std::move(Reader));
      }

[bnbarham]

swiftlang/swift#82708

@swift-ci please test

[bnbarham]

Ah, that branch was main :( I'm just going to merge, tested it locally.