[ASTImporter] Properly delete decls from SavedImportPaths

Summary:
We see a significant regression (~40% slower on large codebases) in expression evaluation after https://reviews.llvm.org/rL364771. A sampling profile shows the extra time is spent in SavedImportPathsTy::operator[] when called from ASTImporter::Import. I believe this is because ASTImporter::Import adds an element to the SavedImportPaths map for each decl unconditionally (see https://github.com/llvm/llvm-project/blob/7b81c3f8793d30a4285095a9b67dcfca2117916c/clang/lib/AST/ASTImporter.cpp#L8256).

To fix this, we call SavedImportPathsTy::erase on the declaration rather than clearing its value vector. That way we do not accidentally introduce new empty elements.  (With this patch the performance is restored, and we do not see SavedImportPathsTy::operator[] in the profile anymore.)

Reviewers: martong, teemperor, a.sidorin, shafik

Reviewed By: martong

Subscribers: rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73166

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 1f2ce30..9dd20e2 100644 (file)
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -8253,7 +8253,7 @@ Expected<Decl *> ASTImporter::Import(Decl *FromD) {
           // FIXME Should we remove these Decls from the LookupTable,
           // and from ImportedFromDecls?
       }
-    SavedImportPaths[FromD].clear();
+    SavedImportPaths.erase(FromD);
 
     // Do not return ToDOrErr, error was taken out of it.
     return make_error<ImportError>(ErrOut);
@@ -8286,7 +8286,7 @@ Expected<Decl *> ASTImporter::Import(Decl *FromD) {
   Imported(FromD, ToD);
 
   updateFlags(FromD, ToD);
-  SavedImportPaths[FromD].clear();
+  SavedImportPaths.erase(FromD);
   return ToDOrErr;
 }