From: Jakub Kuderski <kubakuderski@gmail.com>
Date: Wed, 4 Oct 2017 17:32:55 +0000 (+0000)
Subject: [Dominators] Take fast path when applying <=1 updates
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=90edfc7134ca94cd3aaf4fd79bcc37f2f8e0a04f;p=llvm

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314917 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/include/llvm/Support/GenericDomTree.h b/include/llvm/Support/GenericDomTree.h
index 8d9640f76fd..635c87a106f 100644
--- a/include/llvm/Support/GenericDomTree.h
+++ b/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
diff --git a/include/llvm/Support/GenericDomTreeConstruction.h b/include/llvm/Support/GenericDomTreeConstruction.h
index 34c1fe6800a..460110eb70b 100644
--- a/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/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);