[Assignment Tracking] Elide a map copy in some cases

Restructure AssignmentTrackingLowering::join to avoid a map copy in the case
where BB has more than one pred.

We only need to perform a copy of a pred LiveOut if there's exactly one
already-visited pred (Result = PredLiveOut). With more than one pred the result
is built by calling Result = join(std::move(Result), PredLiveOut) for each
subsequent pred, where join parameters are const &. i.e. with more than 1 pred
we can avoid copying by referencing the first two pred LiveOuts in the first
join and then using a move + reference for the rest.

This reduces compile time for CTMark LTO-O3-g builds.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D144732

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

@@ -1759,48 +1759,72 @@ AssignmentTrackingLowering::joinBlockInfo(const BlockInfo &A,
 
 bool AssignmentTrackingLowering::join(
     const BasicBlock &BB, const SmallPtrSet<BasicBlock *, 16> &Visited) {
-  BlockInfo BBLiveIn;
-  bool FirstJoin = true;
-  // LiveIn locs for BB is the join of the already-processed preds' LiveOut
-  // locs.
+
+  SmallVector<const BasicBlock *> VisitedPreds;
+  // Ignore backedges if we have not visited the predecessor yet. As the
+  // predecessor hasn't yet had locations propagated into it, most locations
+  // will not yet be valid, so treat them as all being uninitialized and
+  // potentially valid. If a location guessed to be correct here is
+  // invalidated later, we will remove it when we revisit this block. This
+  // is essentially the same as initialising all LocKinds and Assignments to
+  // an implicit ⊥ value which is the identity value for the join operation.
   for (auto I = pred_begin(&BB), E = pred_end(&BB); I != E; I++) {
-    // Ignore backedges if we have not visited the predecessor yet. As the
-    // predecessor hasn't yet had locations propagated into it, most locations
-    // will not yet be valid, so treat them as all being uninitialized and
-    // potentially valid. If a location guessed to be correct here is
-    // invalidated later, we will remove it when we revisit this block. This
-    // is essentially the same as initialising all LocKinds and Assignments to
-    // an implicit ⊥ value which is the identity value for the join operation.
     const BasicBlock *Pred = *I;
-    if (!Visited.count(Pred))
-      continue;
+    if (Visited.count(Pred))
+      VisitedPreds.push_back(Pred);
+  }
 
-    auto PredLiveOut = LiveOut.find(Pred);
-    // Pred must have been processed already. See comment at start of this loop.
-    assert(PredLiveOut != LiveOut.end());
+  // No preds visited yet.
+  if (VisitedPreds.empty()) {
+    auto It = LiveIn.try_emplace(&BB, BlockInfo());
+    bool DidInsert = It.second;
+    if (DidInsert)
+      It.first->second.init(TrackedVariablesVectorSize);
+    return /*Changed*/ DidInsert;
+  }
+
+  // Exactly one visited pred. Copy the LiveOut from that pred into BB LiveIn.
+  if (VisitedPreds.size() == 1) {
+    const BlockInfo &PredLiveOut = LiveOut.find(VisitedPreds[0])->second;
+    auto CurrentLiveInEntry = LiveIn.find(&BB);
 
-    // Perform the join of BBLiveIn (current live-in info) and PrevLiveOut.
-    if (FirstJoin)
-      BBLiveIn = PredLiveOut->second;
+    // Check if there isn't an entry, or there is but the LiveIn set has
+    // changed (expensive check).
+    if (CurrentLiveInEntry == LiveIn.end())
+      LiveIn.insert(std::make_pair(&BB, PredLiveOut));
+    else if (PredLiveOut != CurrentLiveInEntry->second)
+      CurrentLiveInEntry->second = PredLiveOut;
     else
-      BBLiveIn = joinBlockInfo(std::move(BBLiveIn), PredLiveOut->second);
-    FirstJoin = false;
+      return /*Changed*/ false;
+    return /*Changed*/ true;
   }
 
-  if (FirstJoin)
-    BBLiveIn.init(TrackedVariablesVectorSize);
+  // More than one pred. Join LiveOuts of blocks 1 and 2.
+  assert(VisitedPreds.size() > 1);
+  const BlockInfo &PredLiveOut0 = LiveOut.find(VisitedPreds[0])->second;
+  const BlockInfo &PredLiveOut1 = LiveOut.find(VisitedPreds[1])->second;
+  BlockInfo BBLiveIn = joinBlockInfo(PredLiveOut0, PredLiveOut1);
+
+  // Join the LiveOuts of subsequent blocks.
+  ArrayRef Tail = ArrayRef(VisitedPreds).drop_front(2);
+  for (const BasicBlock *Pred : Tail) {
+    const auto &PredLiveOut = LiveOut.find(Pred);
+    assert(PredLiveOut != LiveOut.end() &&
+           "block should have been processed already");
+    BBLiveIn = joinBlockInfo(std::move(BBLiveIn), PredLiveOut->second);
+  }
 
+  // Save the joined result for BB.
   auto CurrentLiveInEntry = LiveIn.find(&BB);
   // Check if there isn't an entry, or there is but the LiveIn set has changed
   // (expensive check).
-  if (CurrentLiveInEntry == LiveIn.end() ||
-      BBLiveIn != CurrentLiveInEntry->second) {
-    LiveIn[&BB] = std::move(BBLiveIn);
-    // A change has occured.
-    return true;
-  }
-  // No change.
-  return false;
+  if (CurrentLiveInEntry == LiveIn.end())
+    LiveIn.try_emplace(&BB, std::move(BBLiveIn));
+  else if (BBLiveIn != CurrentLiveInEntry->second)
+    CurrentLiveInEntry->second = std::move(BBLiveIn);
+  else
+    return /*Changed*/ false;
+  return /*Changed*/ true;
 }
 
 /// Return true if A fully contains B.