[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

1 files changed, 41 insertions, 20 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index ac4dd44a0e9..cf452aa6396 100644
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/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.