[clang][ASTContext] Try to exit early before loading serialize comments from AST files

[jkorous]

Summary

Loading external comments is expensive. This change probably doesn't apply to common cases but is for free and would save some work in case none of the declaration needs external comments to be loaded.

[jkorous]

Thanks @gribozavr!

clang/lib/AST/ASTContext.cpp

 
 using namespace clang;
 
 enum FloatingRank {
   Float16Rank, HalfRank, FloatRank, DoubleRank, LongDoubleRank, Float128Rank
 };
 
 RawComment *ASTContext::getRawCommentForDeclNoCache(const Decl *D) const {
-  if (!CommentsLoaded && ExternalSource) {
-    ExternalSource->ReadComments();
-
-#ifndef NDEBUG
-    ArrayRef<RawComment *> RawComments = Comments.getComments();
-    assert(std::is_sorted(RawComments.begin(), RawComments.end(),
-                          BeforeThanCompare<RawComment>(SourceMgr)));
-#endif
-
-    CommentsLoaded = true;
-  }
-
   assert(D);
 
+  // If we already tried to load comments but there are none,
+  // we won't find anything.
+  if (CommentsLoaded && Comments.getComments().empty())
+    return nullptr;
+
   // User can not attach documentation to implicit declarations.
   if (D->isImplicit())
     return nullptr;
 
   // User can not attach documentation to implicit instantiations.
   if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
     if (FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return nullptr;
RawComment *ASTContext::getRawCommentForDeclNoCache(const Decl *D) const {
 
   // TODO: we could look up template parameter documentation in the template
   // documentation.
   if (isa<TemplateTypeParmDecl>(D) ||
       isa<NonTypeTemplateParmDecl>(D) ||
       isa<TemplateTemplateParmDecl>(D))
     return nullptr;
 
-  ArrayRef<RawComment *> RawComments = Comments.getComments();
-
-  // If there are no comments anywhere, we won't find anything.
-  if (RawComments.empty())
-    return nullptr;
-
   // Find declaration location.
   // For Objective-C declarations we generally don't expect to have multiple
   // declarators, thus use declaration starting location as the "declaration
   // location".
   // For all other declarations multiple declarators are used quite frequently,
   // so we use the location of the identifier as the "declaration location".
   SourceLocation DeclLoc;
   if (isa<ObjCMethodDecl>(D) || isa<ObjCContainerDecl>(D) ||
else {
     }
   }
 
   // If the declaration doesn't map directly to a location in a file, we
   // can't find the comment.
   if (DeclLoc.isInvalid() || !DeclLoc.isFileID())
     return nullptr;
 
+  if (!CommentsLoaded && ExternalSource) {
+    ExternalSource->ReadComments();
+
+#ifndef NDEBUG
+    ArrayRef<RawComment *> RawComments = Comments.getComments();
+    assert(std::is_sorted(RawComments.begin(), RawComments.end(),
+                          BeforeThanCompare<RawComment>(SourceMgr)));
+#endif
+
+    CommentsLoaded = true;
+  }
+
+  ArrayRef<RawComment *> RawComments = Comments.getComments();
+  // If there are no comments anywhere, we won't find anything.
+  if (RawComments.empty())
+    return nullptr;
+
   // Find the comment that occurs just after this declaration.
   ArrayRef<RawComment *>::iterator Comment;
   {
     // When searching for comments during parsing, the comment we are looking
     // for is usually among the last two comments we parsed -- check them
     // first.
     RawComment CommentAtDeclLoc(
         SourceMgr, SourceRange(DeclLoc), LangOpts.CommentOpts, false);