--- a/js/src/ion/MIR.h
+++ b/js/src/ion/MIR.h
@@ -6184,31 +6184,37 @@ class MFunctionEnvironment
   : public MUnaryInstruction,
     public SingleObjectPolicy
 {
   public:
     MFunctionEnvironment(MDefinition *function)
         : MUnaryInstruction(function)
     {
         setResultType(MIRType_Object);
+        setMovable();
     }
 
     INSTRUCTION_HEADER(FunctionEnvironment)
 
     static MFunctionEnvironment *New(MDefinition *function) {
         return new MFunctionEnvironment(function);
     }
 
     MDefinition *function() const {
         return getOperand(0);
     }
 
     TypePolicy *typePolicy() {
         return this;
     }
+
+    // A function's environment is fixed.
+    AliasSet getAliasSet() const {
+        return AliasSet::None();
+    }
 };
 
 // Loads the current js::ForkJoinSlice*.
 // Only applicable in ParallelExecution.
 class MParSlice
   : public MNullaryInstruction
 {
   public: