[JumpThreading] Preservation of DT and LVI across the pass

Summary:
See D37528 for a previous (non-deferred) version of this
patch and its description.

Preserves dominance in a deferred manner using a new class
DeferredDominance. This reduces the performance impact of
updating the DominatorTree at every edge insertion and
deletion. A user may call DDT->flush() within JumpThreading
for an up-to-date DT. This patch currently has one flush()
at the end of runImpl() to ensure DT is preserved across
the pass.

LVI is also preserved to help subsequent passes such as
CorrelatedValuePropagation. LVI is simpler to maintain and
is done immediately (not deferred). The code to perfom the
preversation was minimally altered and was simply marked
as preserved for the PassManager to be informed.

This extends the analysis available to JumpThreading for
future enhancements. One example is loop boundary threading.

Reviewers: dberlin, kuhar, sebpop

Reviewed By: kuhar, sebpop

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40146

llvm-svn: 320612

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 606bd8b..a44746d 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -45,16 +45,22 @@
 
 using namespace llvm;
 
-void llvm::DeleteDeadBlock(BasicBlock *BB) {
+void llvm::DeleteDeadBlock(BasicBlock *BB, DeferredDominance *DDT) {
   assert((pred_begin(BB) == pred_end(BB) ||
          // Can delete self loop.
          BB->getSinglePredecessor() == BB) && "Block is not dead!");
   TerminatorInst *BBTerm = BB->getTerminator();
+  std::vector<DominatorTree::UpdateType> Updates;
 
   // Loop through all of our successors and make sure they know that one
   // of their predecessors is going away.
-  for (BasicBlock *Succ : BBTerm->successors())
+  if (DDT)
+    Updates.reserve(BBTerm->getNumSuccessors());
+  for (BasicBlock *Succ : BBTerm->successors()) {
     Succ->removePredecessor(BB);
+    if (DDT)
+      Updates.push_back({DominatorTree::Delete, BB, Succ});
+  }
 
   // Zap all the instructions in the block.
   while (!BB->empty()) {
@@ -69,8 +75,12 @@ void llvm::DeleteDeadBlock(BasicBlock *BB) {
     BB->getInstList().pop_back();
   }
 
-  // Zap the block!
-  BB->eraseFromParent();
+  if (DDT) {
+    DDT->applyUpdates(Updates);
+    DDT->deleteBB(BB); // Deferred deletion of BB.
+  } else {
+    BB->eraseFromParent(); // Zap the block!
+  }
 }
 
 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,