[llvm] [SampleFDO] Improve stale profile matching by diff algorithm (PR #87375)

[via llvm-commits]

================
@@ -171,43 +215,37 @@ void SampleProfileMatcher::runStaleProfileMatching(
   // Use function's beginning location as the initial anchor.
   int32_t LocationDelta = 0;
   SmallVector<LineLocation> LastMatchedNonAnchors;
-
   for (const auto &IR : IRAnchors) {
     const auto &Loc = IR.first;
-    auto CalleeName = IR.second;
+    [[maybe_unused]] StringRef CalleeName = IR.second.stringRef();
     bool IsMatchedAnchor = false;
-    // Match the anchor location in lexical order.
-    if (!CalleeName.empty()) {
-      auto CandidateAnchors =
-          CalleeToCallsitesMap.find(getRepInFormat(CalleeName));
-      if (CandidateAnchors != CalleeToCallsitesMap.end() &&
-          !CandidateAnchors->second.empty()) {
-        auto CI = CandidateAnchors->second.begin();
-        const auto Candidate = *CI;
-        CandidateAnchors->second.erase(CI);
-        InsertMatching(Loc, Candidate);
-        LLVM_DEBUG(dbgs() << "Callsite with callee:" << CalleeName
-                          << " is matched from " << Loc << " to " << Candidate
-                          << "\n");
-        LocationDelta = Candidate.LineOffset - Loc.LineOffset;
-
-        // Match backwards for non-anchor locations.
-        // The locations in LastMatchedNonAnchors have been matched forwards
-        // based on the previous anchor, spilt it evenly and overwrite the
-        // second half based on the current anchor.
-        for (size_t I = (LastMatchedNonAnchors.size() + 1) / 2;
-             I < LastMatchedNonAnchors.size(); I++) {
-          const auto &L = LastMatchedNonAnchors[I];
-          uint32_t CandidateLineOffset = L.LineOffset + LocationDelta;
-          LineLocation Candidate(CandidateLineOffset, L.Discriminator);
-          InsertMatching(L, Candidate);
-          LLVM_DEBUG(dbgs() << "Location is rematched backwards from " << L
-                            << " to " << Candidate << "\n");
-        }
 
-        IsMatchedAnchor = true;
-        LastMatchedNonAnchors.clear();
+    // Match the anchor location in lexical order.
+    auto R = MatchedAnchors.find(Loc);
+    if (R != MatchedAnchors.end()) {
+      const auto &Candidate = R->second;
+      InsertMatching(Loc, Candidate);
+      LLVM_DEBUG(dbgs() << "Callsite with callee:" << CalleeName
----------------
WenleiHe wrote:

nit: fold `IR.second.stringRef()` to use here so you don't have to use `[[maybe_unused]]` to suppress warning for non-debug builds. 

https://github.com/llvm/llvm-project/pull/87375