[llvm] [DFAJumpThreading] Update domtree lazily (PR #162240)

[Hongyu Chen via llvm-commits]

https://github.com/XChy updated https://github.com/llvm/llvm-project/pull/162240

>From abbaaac1fa7c0a25f48971404f412cc9469b189f Mon Sep 17 00:00:00 2001
From: XChy <xxs_chy at outlook.com>
Date: Tue, 7 Oct 2025 16:08:31 +0800
Subject: [PATCH 1/2] [DFAJumpThreading] Update domtree lazily

---
 .../Transforms/Scalar/DFAJumpThreading.cpp    | 23 ++++++++++---------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
index 7968a5d71fdff..2bfe693b852d6 100644
--- a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
@@ -158,7 +158,7 @@ class DFAJumpThreading {
   void
   unfoldSelectInstrs(DominatorTree *DT,
                      const SmallVector<SelectInstToUnfold, 4> &SelectInsts) {
-    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
     SmallVector<SelectInstToUnfold, 4> Stack(SelectInsts);
 
     while (!Stack.empty()) {
@@ -975,8 +975,6 @@ 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()) {
@@ -993,15 +991,18 @@ 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);

>From b21f1e38e3d1761ae2fb3bf00e873390b0d2cb16 Mon Sep 17 00:00:00 2001
From: XChy <xxs_chy at outlook.com>
Date: Tue, 7 Oct 2025 17:58:40 +0800
Subject: [PATCH 2/2] add todo

---
 llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
index 2bfe693b852d6..b4fb591a85432 100644
--- a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
@@ -158,6 +158,7 @@ class DFAJumpThreading {
   void
   unfoldSelectInstrs(DominatorTree *DT,
                      const SmallVector<SelectInstToUnfold, 4> &SelectInsts) {
+    // TODO: Have everything use a single lazy DTU
     DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
     SmallVector<SelectInstToUnfold, 4> Stack(SelectInsts);