[Dominators] Take fast path when applying <=1 updates

Summary:
This patch teaches `DT.applyUpdates` to take the fast when applying zero or just one update and makes it not run the internal batch updater machinery.

With this patch, it should no longer make sense to have a special check in user's code that checks the update sequence size before applying them, e.g.
```
if (!MyUpdates.empty())
  DT.applyUpdates(MyUpdates);
```
or
```
if (MyUpdates.size() == 1)
  if (...)
    DT.insertEdge(...)
  else
    DT.deleteEdge(...)
```

Reviewers: dberlin, brzycki, davide, grosser, sanjoy

Reviewed By: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38541

llvm-svn: 314917

llvm/include/llvm/Support/GenericDomTree.h

@@ -522,7 +522,9 @@ class DominatorTreeBase {
   ///
   /// Batch updates should be generally faster when performing longer sequences
   /// of updates than calling insertEdge/deleteEdge manually multiple times, as
-  /// they can reorder the updates and remove redundant ones internally.
+  /// it can reorder the updates and remove redundant ones internally.
+  /// The batch updater is also able to detect sequences of zero and exactly one
+  /// update -- it's optimized to do less work in these cases.
   ///
   /// Note that for postdominators it automatically takes care of applying
   /// updates on reverse edges internally (so there's no need to swap the

llvm/include/llvm/Support/GenericDomTreeConstruction.h

@@ -1122,6 +1122,22 @@ struct SemiNCAInfo {
   //~~
 
   static void ApplyUpdates(DomTreeT &DT, ArrayRef<UpdateT> Updates) {
+    const size_t NumUpdates = Updates.size();
+    if (NumUpdates == 0)
+      return;
+
+    // Take the fast path for a single update and avoid running the batch update
+    // machinery.
+    if (NumUpdates == 1) {
+      const auto &Update = Updates.front();
+      if (Update.getKind() == UpdateKind::Insert)
+        DT.insertEdge(Update.getFrom(), Update.getTo());
+      else
+        DT.deleteEdge(Update.getFrom(), Update.getTo());
+
+      return;
+    }
+
     BatchUpdateInfo BUI;
     LegalizeUpdates(Updates, BUI.Updates);