ART: Update DCE to work with try/catch

Dead block elimination was previously disabled because it needed
to be updated. With this patch, try/catch blocks can be removed
as a result of a dead if/switch branch.

Change-Id: I3261060bf24fd5fe7bb0f989247f0ef62ec5fd7b

1 files changed, 31 insertions, 8 deletions

diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index 68fb0acf7f..4d79b55771 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -366,7 +366,11 @@ void HGraph::ComputeTryBlockInformation() {
     HBasicBlock* first_predecessor = block->GetPredecessors()[0];
     DCHECK(!block->IsLoopHeader() || !block->GetLoopInformation()->IsBackEdge(*first_predecessor));
     const HTryBoundary* try_entry = first_predecessor->ComputeTryEntryOfSuccessors();
-    if (try_entry != nullptr) {
+    if (try_entry != nullptr &&
+        (block->GetTryCatchInformation() == nullptr ||
+         try_entry != &block->GetTryCatchInformation()->GetTryEntry())) {
+      // We are either setting try block membership for the first time or it
+      // has changed.
       block->SetTryCatchInformation(new (arena_) TryCatchInformation(*try_entry));
     }
   }
@@ -1372,13 +1376,30 @@ void HBasicBlock::DisconnectAndDelete() {
   // instructions.
   for (HBasicBlock* predecessor : predecessors_) {
     HInstruction* last_instruction = predecessor->GetLastInstruction();
+    if (last_instruction->IsTryBoundary() && !IsCatchBlock()) {
+      // This block is the only normal-flow successor of the TryBoundary which
+      // makes `predecessor` dead. Since DCE removes blocks in post order,
+      // exception handlers of this TryBoundary were already visited and any
+      // remaining handlers therefore must be live. We remove `predecessor` from
+      // their list of predecessors.
+      DCHECK_EQ(last_instruction->AsTryBoundary()->GetNormalFlowSuccessor(), this);
+      while (predecessor->GetSuccessors().size() > 1) {
+        HBasicBlock* handler = predecessor->GetSuccessors()[1];
+        DCHECK(handler->IsCatchBlock());
+        predecessor->RemoveSuccessor(handler);
+        handler->RemovePredecessor(predecessor);
+      }
+    }
+
     predecessor->RemoveSuccessor(this);
     uint32_t num_pred_successors = predecessor->GetSuccessors().size();
     if (num_pred_successors == 1u) {
       // If we have one successor after removing one, then we must have
-      // had an HIf or HPackedSwitch, as they have more than one successor.
-      // Replace those with a HGoto.
-      DCHECK(last_instruction->IsIf() || last_instruction->IsPackedSwitch());
+      // had an HIf, HPackedSwitch or HTryBoundary, as they have more than one
+      // successor. Replace those with a HGoto.
+      DCHECK(last_instruction->IsIf() ||
+             last_instruction->IsPackedSwitch() ||
+             (last_instruction->IsTryBoundary() && IsCatchBlock()));
       predecessor->RemoveInstruction(last_instruction);
       predecessor->AddInstruction(new (graph_->GetArena()) HGoto(last_instruction->GetDexPc()));
     } else if (num_pred_successors == 0u) {
@@ -1387,10 +1408,12 @@ void HBasicBlock::DisconnectAndDelete() {
       // SSAChecker fails unless it is not removed during the pass too.
       predecessor->RemoveInstruction(last_instruction);
     } else {
-      // There are multiple successors left.  This must come from a HPackedSwitch
-      // and we are in the middle of removing the HPackedSwitch. Like above, leave
-      // this alone, and the SSAChecker will fail if it is not removed as well.
-      DCHECK(last_instruction->IsPackedSwitch());
+      // There are multiple successors left. The removed block might be a successor
+      // of a PackedSwitch which will be completely removed (perhaps replaced with
+      // a Goto), or we are deleting a catch block from a TryBoundary. In either
+      // case, leave `last_instruction` as is for now.
+      DCHECK(last_instruction->IsPackedSwitch() ||
+             (last_instruction->IsTryBoundary() && IsCatchBlock()));
     }
   }
   predecessors_.clear();