[NFC] Use find_last_of when seraching for code in getRawCommentForDeclNoCacheImpl

[kuganv]

Summary

ASTContext::getRawCommentForDeclNoCacheImpl in this case, extracts the text between
the comment and declaration to make sure if there are no other declarations or preprocessor
directives between comment or declarations.

While using Text.find_first_of or Text.find_last_of is functionally the same,
using Text.find_last_of terminates fast in the presence of such.

Especially in generated code with sparse comments, it takes longer
to bailout when there is code in-between. Searching from last
(with find_last_of) bails out faster.

This shows up in perf profiles with clangd in some auto-generated code.
ASTContext::getRawCommentForDeclNoCacheImpl showing as much as 18.2% in this
case. With find_last_of, this drops to 2.8%.

[kuganv]

Updating the summary.

[kuganv]

Rebasing the diff to latest main

[kuganv]

Rebasing branch.

[kuganv]

Rebase

[aaron.ballman]

LGTM! You should probably put "NFC" into the patch title when landing the changes though, so it's more clear as to why there's no test coverage.

Do you think this warrants adding a release note to let users know about the performance improvement?

[kuganv]

In D141950#4141455, @aaron.ballman wrote:

LGTM! You should probably put "NFC" into the patch title when landing the changes though, so it's more clear as to why there's no test coverage.

Do you think this warrants adding a release note to let users know about the performance improvement?

Thanks for the review. I have updated the title with NFC. This may not be significant enough to require update on release notes.

clang/lib/AST/ASTContext.cpp

if (Invalid)
     return nullptr;
 
   // Extract text between the comment and declaration.
   StringRef Text(Buffer + CommentEndOffset,
                  DeclLocDecomp.second - CommentEndOffset);
 
   // There should be no other declarations or preprocessor directives between
   // comment and declaration.
-  if (Text.find_first_of(";{}#@") != StringRef::npos)
+  if (Text.find_last_of(";{}#@") != StringRef::npos)
     return nullptr;
 
   return CommentBeforeDecl;
 }
 
 RawComment *ASTContext::getRawCommentForDeclNoCache(const Decl *D) const {
   const SourceLocation DeclLoc = getDeclLocForCommentSearch(D, SourceMgr);