[TailCallElim] Preserve DT and PDT

Summary:
Previously, in the NewPM pipeline, TailCallElim recalculates the DomTree when it modifies any instruction in the Function.
For example,
```
CallInst *CI = dyn_cast<CallInst>(&I);
...
CI->setTailCall();
Modified = true;
...
if (!Modified || ...)
  return PreservedAnalyses::all();
```
After applying this patch, the DomTree only recalculates if needed (plus an extra insertEdge() + an extra deleteEdge() call).

When optimizing SQLite with `-passes="default<O3>"` pipeline of the newPM, the number of DomTree recalculation decreases by 6.2%, the number of nodes visited by DFS decreases by 2.9%. The time used by DomTree will decrease approximately 1%~2.5% after applying the patch.
 
Statistics:
```
Before the patch:
 23010 dom-tree-stats               - Number of DomTree recalculations
489264 dom-tree-stats               - Number of nodes visited by DFS -- DomTree
After the patch:
 21581 dom-tree-stats               - Number of DomTree recalculations
475088 dom-tree-stats               - Number of nodes visited by DFS -- DomTree
```

Reviewers: kuhar, dmgreen, brzycki, grosser, davide

Reviewed By: kuhar, brzycki

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49982

llvm-svn: 338954

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 7f61477d961..94f37daa7e6 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -646,7 +646,8 @@ void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
 }
 
 ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
-                                             BasicBlock *Pred) {
+                                             BasicBlock *Pred,
+                                             DomTreeUpdater *DTU) {
   Instruction *UncondBranch = Pred->getTerminator();
   // Clone the return and add it to the end of the predecessor.
   Instruction *NewRet = RI->clone();
@@ -680,6 +681,10 @@ ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
   // longer branch to them.
   BB->removePredecessor(Pred);
   UncondBranch->eraseFromParent();
+
+  if (DTU)
+    DTU->deleteEdge(Pred, BB);
+
   return cast<ReturnInst>(NewRet);
 }