[DFAJumpThreading] Update domtree lazily (#162240)

Author: XChy

llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp

@@ -158,7 +158,8 @@ class DFAJumpThreading {
   void
   unfoldSelectInstrs(DominatorTree *DT,
                      const SmallVector<SelectInstToUnfold, 4> &SelectInsts) {
-    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+    // TODO: Have everything use a single lazy DTU
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
     SmallVector<SelectInstToUnfold, 4> Stack(SelectInsts);
 
     while (!Stack.empty()) {
@@ -977,8 +978,6 @@ struct TransformDFA {
 
   /// Transform each threading path to effectively jump thread the DFA.
   void createAllExitPaths() {
-    DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Eager);
-
     // Move the switch block to the end of the path, since it will be duplicated
     BasicBlock *SwitchBlock = SwitchPaths->getSwitchBlock();
     for (ThreadingPath &TPath : SwitchPaths->getThreadingPaths()) {
@@ -995,15 +994,18 @@ struct TransformDFA {
     SmallPtrSet<BasicBlock *, 16> BlocksToClean;
     BlocksToClean.insert_range(successors(SwitchBlock));
 
-    for (const ThreadingPath &TPath : SwitchPaths->getThreadingPaths()) {
-      createExitPath(NewDefs, TPath, DuplicateMap, BlocksToClean, &DTU);
-      NumPaths++;
-    }
+    {
+      DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Lazy);
+      for (const ThreadingPath &TPath : SwitchPaths->getThreadingPaths()) {
+        createExitPath(NewDefs, TPath, DuplicateMap, BlocksToClean, &DTU);
+        NumPaths++;
+      }
 
-    // After all paths are cloned, now update the last successor of the cloned
-    // path so it skips over the switch statement
-    for (const ThreadingPath &TPath : SwitchPaths->getThreadingPaths())
-      updateLastSuccessor(TPath, DuplicateMap, &DTU);
+      // After all paths are cloned, now update the last successor of the cloned
+      // path so it skips over the switch statement
+      for (const ThreadingPath &TPath : SwitchPaths->getThreadingPaths())
+        updateLastSuccessor(TPath, DuplicateMap, &DTU);
+    }
 
     // For each instruction that was cloned and used outside, update its uses
     updateSSA(NewDefs);