[modules] Cache 'acceptable decl' lookups for namespaces. In projects with

thousands of modules, each of which declares the same namespace, linearly
scanning the redecl chain looking for a visible declaration (once for each leaf
module, for each use) performs very poorly. Namespace visibility can only
decrease when we leave a module during a module build step, and we never care
*which* visible declaration of a namespace we find, so we can cache this very
effectively.

This results in a 35x speedup on one of our internal build steps (2m -> 3.5s),
but is hard to unit test because it requires a very large number of modules.
Ideas for a test appreciated! No functionality change intended other than the
speedup.

llvm-svn: 261161

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 1a415b01dac..3345a1bbecb 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -14826,6 +14826,9 @@ void Sema::ActOnModuleEnd(SourceLocation DirectiveLoc, Module *Mod) {
     VisibleModules = std::move(VisibleModulesStack.back());
     VisibleModulesStack.pop_back();
     VisibleModules.setVisible(Mod, DirectiveLoc);
+    // Leaving a module hides namespace names, so our visible namespace cache
+    // is now out of date.
+    VisibleNamespaceCache.clear();
   }
 }