[clangd] Make advanceTo() faster on Posting Lists

[kbobyrev]

Summary

If the current element is already beyond advanceTo()'s DocID, just return instead of doing binary search. This simple optimization saves up to 6-7% performance,

[ilya-biryukov]

LGTM

clang-tools-extra/clangd/index/dex/Iterator.cpp

41

Maybe add a short comment that this gives performance boost?
Otherwise this looks redundant when reading the code

[kbobyrev]

Add a short comment on why calling lower_bound is redundant. Also, don't spend extra line on return by inverting the condition.

clang-tools-extra/clangd/index/dex/Iterator.cpp

void advance() override {
     assert(!reachedEnd() && "DOCUMENT iterator can't advance() at the end.");
     ++Index;
   }
 
   /// Applies binary search to advance cursor to the next item with DocID equal
   /// or higher than the given one.
   void advanceTo(DocID ID) override {
     assert(!reachedEnd() && "DOCUMENT iterator can't advance() at the end.");
+    if (peek() >= ID)
+      return;
     Index = std::lower_bound(Index, std::end(Documents), ID);
   }
 
   DocID peek() const override {
     assert(!reachedEnd() && "DOCUMENT iterator can't peek() at the end.");
     return *Index;
   }