[Dominators] Refine the logic of recalculate() in the DomTreeUpdater

Summary:
This patch refines the logic of `recalculate()` in the `DomTreeUpdater` in the following two aspects:
1. Previously, `recalculate()` tests whether there are pending updates/BBs awaiting deletion and then do recalculation under Lazy UpdateStrategy; and do recalculation immediately under Eager UpdateStrategy. (The former behavior is inherited from the `DeferredDominance` class). This is an inconsistency between two strategies and there is no obvious reason to do this. So the behavior is changed to always recalculate available trees when calling `recalculate()`.
2. Fix the issue of when DTU under Lazy UpdateStrategy holds nothing but with BBs awaiting deletion, after calling `recalculate()`, BBs awaiting deletion aren't flushed. An additional unittest is added to cover this case.

Reviewers: kuhar, dmgreen, brzycki, grosser, davide

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50173

llvm-svn: 338822

1 files changed, 12 insertions, 17 deletions

diff --git a/llvm/lib/IR/DomTreeUpdater.cpp b/llvm/lib/IR/DomTreeUpdater.cpp
index f035a86edda..b72c1b77c2c 100644
--- a/llvm/lib/IR/DomTreeUpdater.cpp
+++ b/llvm/lib/IR/DomTreeUpdater.cpp
@@ -152,39 +152,34 @@ bool DomTreeUpdater::forceFlushDeletedBB() {
   return true;
 }
 
-bool DomTreeUpdater::recalculate(Function &F) {
-  if (!DT && !PDT)
-    return false;
+void DomTreeUpdater::recalculate(Function &F) {
 
   if (Strategy == UpdateStrategy::Eager) {
     if (DT)
       DT->recalculate(F);
     if (PDT)
       PDT->recalculate(F);
-    return true;
+    return;
   }
 
+  // There is little performance gain if we pend the recalculation under
+  // Lazy UpdateStrategy so we recalculate available trees immediately.
+
   // Prevent forceFlushDeletedBB() from erasing DomTree or PostDomTree nodes.
   IsRecalculatingDomTree = IsRecalculatingPostDomTree = true;
 
   // Because all trees are going to be up-to-date after recalculation,
   // flush awaiting deleted BasicBlocks.
-  if (forceFlushDeletedBB() || hasPendingUpdates()) {
-    if (DT)
-      DT->recalculate(F);
-    if (PDT)
-      PDT->recalculate(F);
-
-    // Resume forceFlushDeletedBB() to erase DomTree or PostDomTree nodes.
-    IsRecalculatingDomTree = IsRecalculatingPostDomTree = false;
-    PendDTUpdateIndex = PendPDTUpdateIndex = PendUpdates.size();
-    dropOutOfDateUpdates();
-    return true;
-  }
+  forceFlushDeletedBB();
+  if (DT)
+    DT->recalculate(F);
+  if (PDT)
+    PDT->recalculate(F);
 
   // Resume forceFlushDeletedBB() to erase DomTree or PostDomTree nodes.
   IsRecalculatingDomTree = IsRecalculatingPostDomTree = false;
-  return false;
+  PendDTUpdateIndex = PendPDTUpdateIndex = PendUpdates.size();
+  dropOutOfDateUpdates();
 }
 
 bool DomTreeUpdater::hasPendingUpdates() const {