[NFC] Unblock Lazy Member Loading For Import-As-Member

Lazy member loading had an antagonistic relationship with the import-as-member facilities. The member tables were stored in a hash map that is keyed by serialized declaration context.  While this was good for importing the entire member set of a given extension, it's in the complete wrong order for lazy member loading, which wants the same data keyed by base name.

Given that it is annoying to redo the globals-as-member tables to support one use case or the other, coupled with the fact that optimizing for one use-case automatically pessimizes the other, just take a page from rdar://18696086 and store the same information twice in two separate formats each optimized for the task at hand.

Preliminary benchmarks indicate that this leads to a 5% reduction in Clang-Imported entities which will drastically speed up most apps that use Dispatch and CoreGraphics.

Author: CodaFi

lib/ClangImporter/ClangImporter.cpp

@@ -3010,7 +3010,7 @@ void ClangImporter::loadExtensions(NominalTypeDecl *nominal,
       // FIXME: If we already looked at this for this generation,
       // skip.
 
-      for (auto entry : table.lookupGlobalsAsMembers(effectiveClangContext)) {
+      for (auto entry : table.allGlobalsAsMembersInContext(effectiveClangContext)) {
         // If the entry is not visible, skip it.
         if (!isVisibleClangEntry(clangCtx, entry)) continue;
 
@@ -3763,17 +3763,6 @@ ClangImporter::Implementation::loadNamedMembers(
       return None;
   }
 
-  // Also bail out if there are any global-as-member mappings for this context;
-  // we can support some of them lazily but the full set of idioms seems
-  // prohibitively complex (also they're not stored in by-name lookup, for
-  // reasons unclear).
-  if (isa<ExtensionDecl>(D) && !checkedGlobalsAsMembers.insert(IDC).second) {
-    if (forEachLookupTable([&](SwiftLookupTable &table) -> bool {
-        return (!table.lookupGlobalsAsMembers(effectiveClangContext).empty());
-      }))
-      return None;
-  }
-
   // There are 3 cases:
   //
   //  - The decl is from a bridging header, CMO is Some(nullptr)
@@ -3823,6 +3812,27 @@ ClangImporter::Implementation::loadNamedMembers(
     }
   }
 
+  for (auto entry : table->lookupGlobalsAsMembers(SerializedSwiftName(N),
+                                                  effectiveClangContext)) {
+    if (!entry.is<clang::NamedDecl *>()) continue;
+    auto member = entry.get<clang::NamedDecl *>();
+    if (!isVisibleClangEntry(clangCtx, member)) continue;
+
+     // Skip Decls from different clang::DeclContexts
+    if (member->getDeclContext() != CDC) continue;
+
+    SmallVector<Decl*, 4> tmp;
+    insertMembersAndAlternates(member, tmp);
+    for (auto *TD : tmp) {
+      if (auto *V = dyn_cast<ValueDecl>(TD)) {
+        // Skip ValueDecls if they import under different names.
+        if (V->getBaseName() == N) {
+          Members.push_back(V);
+        }
+      }
+    }
+  }
+
   if (N == DeclBaseName::createConstructor()) {
     if (auto *classDecl = dyn_cast<ClassDecl>(D)) {
       SmallVector<Decl *, 4> ctors;

lib/ClangImporter/ImportDecl.cpp

@@ -8544,7 +8544,7 @@ void ClangImporter::Implementation::loadAllMembersIntoExtension(
   startedImportingEntity();
 
   // Load the members.
-  for (auto entry : table->lookupGlobalsAsMembers(effectiveClangContext)) {
+  for (auto entry : table->allGlobalsAsMembersInContext(effectiveClangContext)) {
     auto decl = entry.get<clang::NamedDecl *>();
 
     // Only include members in the same submodule as this extension.