Revert "A simpler scheme for garbage collection of ureachable phi inputs."

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index eea3bd6a60ed9e1f5462f5a63f0f3042e58f398c..a76b8dd953705261a9af13c323f6ea5c1d36a5e9 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -148,7 +148,7 @@ const Object& Value::BoundConstant() const {
 
 
 GraphEntryInstr::GraphEntryInstr(TargetEntryInstr* normal_entry)
-    : BlockEntryInstr(0, CatchClauseNode::kInvalidTryIndex),
+    : BlockEntryInstr(CatchClauseNode::kInvalidTryIndex),
       normal_entry_(normal_entry),
       catch_entries_(),
       initial_definitions_(),
@@ -481,19 +481,6 @@ RawAbstractType* PushArgumentInstr::CompileType() const {
 }
 
 
-void JoinEntryInstr::AddPredecessor(BlockEntryInstr* predecessor) {
-  // Require the predecessors to be sorted by block_id to make managing
-  // their corresponding phi inputs simpler.
-  intptr_t pred_id = predecessor->block_id();
-  intptr_t index = 0;
-  while ((index < predecessors_.length()) &&
-         (predecessors_[index]->block_id() < pred_id)) {
-    ++index;
-  }
-  predecessors_.InsertAt(index, predecessor);
-}
-
-
 intptr_t JoinEntryInstr::IndexOfPredecessor(BlockEntryInstr* pred) const {
   for (intptr_t i = 0; i < predecessors_.length(); ++i) {
     if (predecessors_[i] == pred) return i;
@@ -502,6 +489,54 @@ intptr_t JoinEntryInstr::IndexOfPredecessor(BlockEntryInstr* pred) const {
 }
 
 
+void JoinEntryInstr::EliminateUnreachablePhiInputs() {
+  if (phis_ == NULL || phis_->is_empty()) return;
+
+  // Loop over the predecessors, reorganize phi inputs.
+  // TODO(kmillikin): Replace phis that have a single remaining input with
+  // the input.  This requires being a bit careful about use lists.
+  intptr_t input_count = predecessors_.length();
+  for (intptr_t new_idx = 0; new_idx < input_count; ++new_idx) {
+    BlockEntryInstr* pred = predecessors_[new_idx];
+    // Linear search for the old predecessor index.  We can't directly
+    // compare block entries, because unreachable code elimination has
+    // replaced some targets with joins.
+    intptr_t old_idx = 0;
+    for (; old_idx < stale_predecessors_.length(); ++old_idx) {
+      if (stale_predecessors_[old_idx]->next() == pred->next()) break;
+    }
+    ASSERT(old_idx < stale_predecessors_.length());
+    // If the index has changed, adjust all phi inputs.
+    if (old_idx != new_idx) {
+      ASSERT(new_idx < old_idx);
+      // Swap each phi's inputs so the input at new_idx is correct.
+      // Preserve the previous value in case it is from a reachable
+      // predecessor.
+      for (intptr_t phi_idx = 0; phi_idx < phis_->length(); ++phi_idx) {
+        PhiInstr* phi = (*phis_)[phi_idx];
+        if (phi == NULL) continue;
+        Value* temp = phi->InputAt(new_idx);
+        phi->SetInputAt(new_idx, phi->InputAt(old_idx));
+        phi->SetInputAt(old_idx, temp);
+      }
+      // The old input at new_idx is now found at old_idx.  It may be a
+      // reachable predecessor so swap the old predecessors too.
+      BlockEntryInstr* temp = stale_predecessors_[new_idx];
+      stale_predecessors_[new_idx] = stale_predecessors_[old_idx];
+      stale_predecessors_[old_idx] = temp;
+    }
+  }
+  // Now truncate each phi if necessary.
+  if (input_count < stale_predecessors_.length()) {
+    for (intptr_t phi_idx = 0; phi_idx < phis_->length(); ++phi_idx) {
+      PhiInstr* phi = (*phis_)[phi_idx];
+      if (phi == NULL) continue;
+      phi->inputs_.TruncateTo(input_count);
+    }
+  }
+}
+
+
 // ==== Recording assigned variables.
 void Definition::RecordAssignedVars(BitVector* assigned_vars,
                                     intptr_t fixed_parameter_count) {