Constant propagator should revisit phis when it visits predecessor block.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index 358c2122de2b8148f94bfed9f1053675b45390f1..39a1d22f8e0e81629632a34473b84c84a94025dd 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -3663,18 +3663,12 @@ void ConstantPropagator::VisitGraphEntry(GraphEntryInstr* block) {
 
 
 void ConstantPropagator::VisitJoinEntry(JoinEntryInstr* block) {
-  ZoneGrowableArray<PhiInstr*>* phis = block->phis();
-  if (phis != NULL) {
-    for (intptr_t phi_idx = 0; phi_idx < phis->length(); ++phi_idx) {
-      PhiInstr* phi = (*phis)[phi_idx];
-      if (phi == NULL) continue;
-      phi->Accept(this);
-    }
-  }
-
+  // Phis are visited when visiting predecessor. See VisitSuccessorPhis.
   for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
     it.Current()->Accept(this);
   }
+
+  VisitSuccessorPhis(block);
 }
 
 
@@ -3682,6 +3676,8 @@ void ConstantPropagator::VisitTargetEntry(TargetEntryInstr* block) {
   for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
     it.Current()->Accept(this);
   }
+
+  VisitSuccessorPhis(block);
 }
 
 
@@ -3691,6 +3687,18 @@ void ConstantPropagator::VisitParallelMove(ParallelMoveInstr* instr) {
 }
 
 
+void ConstantPropagator::VisitSuccessorPhis(BlockEntryInstr* block) {
+  // Phi value depends on the reachability of a predecessor. We have
+  // to revisit phis every time a predecessor becomes reachable.
+  GotoInstr* goto_instr = block->last_instruction()->AsGoto();
+  if (goto_instr != NULL) {
+    for (PhiIterator it(goto_instr->successor()); !it.Done(); it.Advance()) {
+      it.Current()->Accept(this);
+    }
+  }
+}
+
+
 // --------------------------------------------------------------------------
 // Analysis of control instructions.  Unconditional successors are
 // reachable.  Conditional successors are reachable depending on the