Revert "NewGVN: Make unreachable blocks be marked with unreachable"

This reverts commit r293196

Besides making things look nicer, ATM, we'd like to preserve analysis
more than we'd like to destroy the CFG.  We'll probably revisit in the future

llvm-svn: 293501

1 files changed, 18 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index c17b9f7fcf2..259a4eade0d 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -2106,21 +2106,26 @@ void NewGVN::deleteInstructionsInBlock(BasicBlock *BB) {
   DEBUG(dbgs() << "  BasicBlock Dead:" << *BB);
   ++NumGVNBlocksDeleted;
 
-  // Change to unreachable does not handle destroying phi nodes. We just replace
-  // the users with undef.
-  if (BB->empty())
+  // Check to see if there are non-terminating instructions to delete.
+  if (isa<TerminatorInst>(BB->begin()))
     return;
-  auto BBI = BB->begin();
-  while (auto *Phi = dyn_cast<PHINode>(BBI)) {
-    Phi->replaceAllUsesWith(UndefValue::get(Phi->getType()));
-    ++BBI;
-  }
 
-  Instruction *ToKill = &*BBI;
-  // Nothing but phi nodes, so nothing left to remove.
-  if (!ToKill)
-    return;
-  NumGVNInstrDeleted += changeToUnreachable(ToKill, false);
+  // Delete the instructions backwards, as it has a reduced likelihood of having
+  // to update as many def-use and use-def chains. Start after the terminator.
+  auto StartPoint = BB->rbegin();
+  ++StartPoint;
+  // Note that we explicitly recalculate BB->rend() on each iteration,
+  // as it may change when we remove the first instruction.
+  for (BasicBlock::reverse_iterator I(StartPoint); I != BB->rend();) {
+    Instruction &Inst = *I++;
+    if (!Inst.use_empty())
+      Inst.replaceAllUsesWith(UndefValue::get(Inst.getType()));
+    if (isa<LandingPadInst>(Inst))
+      continue;
+
+    Inst.eraseFromParent();
+    ++NumGVNInstrDeleted;
+  }
 }
 
 void NewGVN::markInstructionForDeletion(Instruction *I) {