[Dominators] Teach LoopDeletion to use the new incremental API

Summary:
This patch makes LoopDeletion use the incremental DominatorTree API.

We modify LoopDeletion to perform the deletion in 5 steps:
1. Create a new dummy edge from the preheader to the exit, by adding a conditional branch.
2. Inform the DomTree about the new edge.
3. Remove the conditional branch and replace it with an unconditional edge to the exit. This removes the edge to the loop header, making it unreachable.
4. Inform the DomTree about the deleted edge.
5. Remove the unreachable block from the function.

Creating the dummy conditional branch is necessary to perform incremental DomTree update.
We should consider using the batch updater when it's ready.

Reviewers: dberlin, davide, grosser, sanjoy

Reviewed By: dberlin, grosser

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D35391

llvm-svn: 309850

llvm/include/llvm/Support/GenericDomTree.h

@@ -481,11 +481,15 @@ class DominatorTreeBase {
 
   /// Inform the dominator tree about a CFG edge deletion and update the tree.
   ///
-  /// This function has to be called just after making the update
-  /// on the actual CFG. There cannot be any other updates that the dominator
-  /// tree doesn't know about. The only exception is when the deletion that the
-  /// tree is informed about makes some (dominator) subtree unreachable -- in
-  /// this case, it is fine to perform deletions within this subtree.
+  /// This function has to be called just after making the update on the actual
+  /// CFG. An internal functions checks if the edge doesn't exist in the CFG in
+  /// DEBUG mode. There cannot be any other updates that the
+  /// dominator tree doesn't know about.
+  ///
+  /// However, it is fine to perform multiple CFG deletions that make different
+  /// subtrees forward-unreachable and to inform the DomTree about them all at
+  /// the same time, as the incremental algorithm doesn't walk the tree above
+  /// the NearestCommonDominator of a deleted edge
   ///
   /// Note that for postdominators it automatically takes care of deleting
   /// a reverse edge internally (so there's no need to swap the parameters).

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

@@ -239,17 +239,44 @@ static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
 
   auto *ExitBlock = L->getUniqueExitBlock();
   assert(ExitBlock && "Should have a unique exit block!");
-
   assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
 
-  // Connect the preheader directly to the exit block.
+  auto *OldBr = dyn_cast<BranchInst>(Preheader->getTerminator());
+  assert(OldBr && "Preheader must end with a branch");
+  assert(OldBr->isUnconditional() && "Preheader must have a single successor");
+  // Connect the preheader to the exit block. Keep the old edge to the header
+  // around to perform the dominator tree update in two separate steps
+  // -- #1 insertion of the edge preheader -> exit and #2 deletion of the edge
+  // preheader -> header.
+  //
+  //
+  // 0.  Preheader          1.  Preheader           2.  Preheader
+  //        |                    |   |                   |
+  //        V                    |   V                   |
+  //      Header <--\            | Header <--\           | Header <--\
+  //       |  |     |            |  |  |     |           |  |  |     |
+  //       |  V     |            |  |  V     |           |  |  V     |
+  //       | Body --/            |  | Body --/           |  | Body --/
+  //       V                     V  V                    V  V
+  //      Exit                   Exit                    Exit
+  //
+  // By doing this is two separate steps we can perform the dominator tree
+  // update without using the batch update API.
+  //
   // Even when the loop is never executed, we cannot remove the edge from the
   // source block to the exit block. Consider the case where the unexecuted loop
   // branches back to an outer loop. If we deleted the loop and removed the edge
   // coming to this inner loop, this will break the outer loop structure (by
   // deleting the backedge of the outer loop). If the outer loop is indeed a
   // non-loop, it will be deleted in a future iteration of loop deletion pass.
-  Preheader->getTerminator()->replaceUsesOfWith(L->getHeader(), ExitBlock);
+  IRBuilder<> Builder(OldBr);
+  Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
+  // Remove the old branch. The conditional branch becomes a new terminator.
+  OldBr->eraseFromParent();
+
+  // Update the dominator tree by informing it about the new edge from the
+  // preheader to the exit.
+  DT.insertEdge(Preheader, ExitBlock);
 
   // Rewrite phis in the exit block to get their inputs from the Preheader
   // instead of the exiting block.
@@ -276,25 +303,19 @@ static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
     ++BI;
   }
 
-  // Update the dominator tree and remove the instructions and blocks that will
-  // be deleted from the reference counting scheme.
-  SmallVector<DomTreeNode*, 8> ChildNodes;
-  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
-       LI != LE; ++LI) {
-    // Move all of the block's children to be children of the Preheader, which
-    // allows us to remove the domtree entry for the block.
-    ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
-    for (DomTreeNode *ChildNode : ChildNodes) {
-      DT.changeImmediateDominator(ChildNode, DT[Preheader]);
-    }
+  // Disconnect the loop body by branching directly to its exit.
+  Builder.SetInsertPoint(Preheader->getTerminator());
+  Builder.CreateBr(ExitBlock);
+  // Remove the old branch.
+  Preheader->getTerminator()->eraseFromParent();
 
-    ChildNodes.clear();
-    DT.eraseNode(*LI);
+  // Inform the dominator tree about the removed edge.
+  DT.deleteEdge(Preheader, L->getHeader());
 
-    // Remove the block from the reference counting scheme, so that we can
-    // delete it freely later.
-    (*LI)->dropAllReferences();
-  }
+  // Remove the block from the reference counting scheme, so that we can
+  // delete it freely later.
+  for (auto *Block : L->blocks())
+    Block->dropAllReferences();
 
   // Erase the instructions and the blocks without having to worry
   // about ordering because we already dropped the references.

llvm/test/Transforms/LoopDeletion/2017-07-11-incremental-dt.ll

@@ -0,0 +1,56 @@
+; RUN: opt < %s -loop-deletion -S
+; RUN: opt < %s -loop-deletion -analyze -domtree 2>&1 | FileCheck -check-prefix=DT %s
+; RUN: opt < %s -loop-deletion -analyze -verify-dom-info
+
+; CHECK: for.body
+; CHECK-NOT: for.cond1
+
+; Verify only the important parts of the DomTree.
+; DT: [1] %entry
+; DT:   [2] %for.cond
+; DT:     [3] %lbl63A679E5
+; DT:     [3] %for.cond9
+; DT:     [3] %lbl64774A9B
+; DT:     [3] %for.body
+; DT:       [4] %for.cond3.loopexit
+
+define i32 @fn1() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %entry
+  br i1 undef, label %lbl63A679E5, label %for.body
+
+for.body:                                         ; preds = %for.cond
+  br label %for.cond1
+
+for.cond1:                                        ; preds = %for.cond1, %for.body
+  br i1 undef, label %for.cond1, label %for.cond3.loopexit
+
+for.cond3.loopexit:                               ; preds = %for.cond1
+  br label %for.cond3
+
+for.cond3:                                        ; preds = %for.cond9, %for.cond3.loopexit
+  br i1 undef, label %for.body4, label %for.cond17
+
+for.body4:                                        ; preds = %for.cond3
+  br label %for.cond5
+
+for.cond5:                                        ; preds = %lbl63A679E5, %for.body4
+  br label %for.cond9
+
+lbl63A679E5:                                      ; preds = %for.cond
+  br label %for.cond5
+
+for.cond9:                                        ; preds = %for.end14.split, %for.cond5
+  br i1 undef, label %for.cond3, label %lbl64774A9B
+
+lbl64774A9B:                                      ; preds = %for.cond17, %for.cond9
+  br label %for.end14.split
+
+for.end14.split:                                  ; preds = %lbl64774A9B
+  br label %for.cond9
+
+for.cond17:                                       ; preds = %for.cond3
+  br label %lbl64774A9B
+}

llvm/unittests/IR/DominatorTreeTest.cpp

@@ -797,6 +797,36 @@ TEST(DominatorTree, DeleteUnreachable) {
   }
 }
 
+TEST(DominatorTree, DeletionsInSubtrees) {
+  CFGHolder Holder;
+  std::vector<CFGBuilder::Arc> Arcs = {{"0", "1"}, {"1", "2"}, {"1", "3"},
+                                       {"1", "6"}, {"3", "4"}, {"2", "5"},
+                                       {"5", "2"}};
+
+  // It is possible to perform multiple deletions and inform the
+  // DominatorTree about them at the same time, if the all of the
+  // deletions happen in different subtrees.
+  std::vector<CFGBuilder::Update> Updates = {{Delete, {"1", "2"}},
+                                             {Delete, {"1", "3"}}};
+  CFGBuilder B(Holder.F, Arcs, Updates);
+  DominatorTree DT(*Holder.F);
+  EXPECT_TRUE(DT.verify());
+
+  Optional<CFGBuilder::Update> LastUpdate;
+  while ((LastUpdate = B.applyUpdate()))
+    ;
+
+  DT.deleteEdge(B.getOrAddBlock("1"), B.getOrAddBlock("2"));
+  DT.deleteEdge(B.getOrAddBlock("1"), B.getOrAddBlock("3"));
+
+  EXPECT_TRUE(DT.verify());
+  EXPECT_EQ(DT.getNode(B.getOrAddBlock("2")), nullptr);
+  EXPECT_EQ(DT.getNode(B.getOrAddBlock("3")), nullptr);
+  EXPECT_EQ(DT.getNode(B.getOrAddBlock("4")), nullptr);
+  EXPECT_EQ(DT.getNode(B.getOrAddBlock("5")), nullptr);
+  EXPECT_NE(DT.getNode(B.getOrAddBlock("6")), nullptr);
+}
+
 TEST(DominatorTree, InsertDelete) {
   std::vector<CFGBuilder::Arc> Arcs = {
       {"1", "2"}, {"2", "3"}, {"3", "4"},  {"4", "5"},  {"5", "6"},  {"5", "7"},