Reland r45243.

runtime/vm/constant_propagator.cc

Index: runtime/vm/constant_propagator.cc
diff --git a/runtime/vm/constant_propagator.cc b/runtime/vm/constant_propagator.cc
index 67dc17b350b507004c16acfaad8e8b6a759c3446..35ee3ccd7f9d6537a569dfd3bf94af24a738e673 100644
--- a/runtime/vm/constant_propagator.cc
+++ b/runtime/vm/constant_propagator.cc
@@ -318,7 +318,16 @@ void ConstantPropagator::VisitPhi(PhiInstr* instr) {
 
 
 void ConstantPropagator::VisitRedefinition(RedefinitionInstr* instr) {
-  SetValue(instr, instr->value()->definition()->constant_value());
+  // Ensure that we never remove redefinition of a constant unless we are also
+  // are guaranteed to fold away code paths that correspond to non-matching
+  // class ids. Otherwise LICM might potentially hoist incorrect code.
+  const Object& value = instr->value()->definition()->constant_value();
+  if (IsConstant(value) &&
+      CheckClassInstr::IsImmutableClassId(value.GetClassId())) {

[Florian Schneider, 2015/04/20 09:33:32]

Works for now. Could we use the knowledge that the cid of a string can only
change from StringCid -> ExternalStringCid? i.e. even though the cid is not
considered constant, it can only be one of two values.

+    SetValue(instr, value);
+  } else {
+    SetValue(instr, non_constant_);
+  }
 }
 
 
@@ -788,10 +797,14 @@ void ConstantPropagator::VisitLoadClassId(LoadClassIdInstr* instr) {
     SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
     return;
   }
+
   const Object& object = instr->object()->definition()->constant_value();
   if (IsConstant(object)) {
-    SetValue(instr, Smi::ZoneHandle(Z, Smi::New(object.GetClassId())));
-    return;
+    cid = object.GetClassId();
+    if (CheckClassInstr::IsImmutableClassId(cid)) {
+      SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
+      return;
+    }
   }
   SetValue(instr, non_constant_);
 }