[clang] [clang][Lex] Optimize the FileCheckPoints search in Preprocessor (PR #206356)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Hardik Kumar (hardikxk)

<details>
<summary>Changes</summary>

Resolves a FIXME and improved linear search by replacing with a binary search on the FileCheckPoints SmallVector in Lex/Preprocessor.

- used std::upper_bound to find the upper bound of the Start position.
- handle special case if the Start is less than all elements in FileCheckPoints so returns a nullptr;
- else return the dereferenced value of the CheckPoint just before Start point.

---
Full diff: https://github.com/llvm/llvm-project/pull/206356.diff


1 Files Affected:

- (modified) clang/lib/Lex/Preprocessor.cpp (+5-8) 


``````````diff
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index c69d084d6514f..3f83546ea7956 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -1746,16 +1746,13 @@ void Preprocessor::removePPCallbacks() {
 const char *Preprocessor::getCheckPoint(FileID FID, const char *Start) const {
   if (auto It = CheckPoints.find(FID); It != CheckPoints.end()) {
     const SmallVector<const char *> &FileCheckPoints = It->second;
-    const char *Last = nullptr;
-    // FIXME: Do better than a linear search.
-    for (const char *P : FileCheckPoints) {
-      if (P > Start)
-        break;
-      Last = P;
+    auto P =
+        std::upper_bound(FileCheckPoints.begin(), FileCheckPoints.end(), Start);
+    if (P == FileCheckPoints.begin()) {
+      return nullptr;
     }
-    return Last;
+    return *std::prev(P);
   }
-
   return nullptr;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/206356