Bug 815161: Use caching getproperty to fetch prototype to create this, r=jandem

js/src/ion/IonBuilder.cpp

@@ -3517,15 +3517,29 @@ MDefinition *
 IonBuilder::createThisScripted(MDefinition *callee)
 {
     // Get callee.prototype.
+    //
     // This instruction MUST be idempotent: since it does not correspond to an
-    // explicit operation in the bytecode, we cannot use resumeAfter(). But
-    // calling GetProperty can trigger a GC, and thus invalidation.
-    MCallGetProperty *getProto = MCallGetProperty::New(callee, cx->names().classPrototype);
-
-    // Getters may not override |prototype| fetching, so this is repeatable.
-    getProto->markUneffectful();
+    // explicit operation in the bytecode, we cannot use resumeAfter().
+    // Getters may not override |prototype| fetching, so this operation is indeed idempotent.
+    // - First try an idempotent property cache.
+    // - Upon failing idempotent property cache, we can't use a non-idempotent cache,
+    //   therefore we fallback to CallGetProperty
+    //
+    // Note: both CallGetProperty and GetPropertyCache can trigger a GC,
+    //       and thus invalidation.
+    MInstruction *getProto;
+    if (!invalidatedIdempotentCache()) {
+        MGetPropertyCache *getPropCache = MGetPropertyCache::New(callee, cx->names().classPrototype);
+        getPropCache->setIdempotent();
+        getProto = getPropCache;
+    } else {
+        MCallGetProperty *callGetProp = MCallGetProperty::New(callee, cx->names().classPrototype);
+        callGetProp->setIdempotent();
+        getProto = callGetProp;
+    }
     current->add(getProto);
 
+    // Create this from prototype
     MCreateThis *createThis = MCreateThis::New(callee, getProto, NULL);
     current->add(createThis);
 

js/src/ion/MIR.h

@@ -4823,11 +4823,11 @@ class MCallGetProperty
     public BoxInputsPolicy
 {
     CompilerRootPropertyName name_;
-    bool markEffectful_;
+    bool idempotent_;
 
     MCallGetProperty(MDefinition *value, HandlePropertyName name)
       : MUnaryInstruction(value), name_(name),
-        markEffectful_(true)
+        idempotent_(false)
     {
         setResultType(MIRType_Value);
     }
@@ -4851,11 +4851,11 @@ class MCallGetProperty
     // Constructors need to perform a GetProp on the function prototype.
     // Since getters cannot be set on the prototype, fetching is non-effectful.
     // The operation may be safely repeated in case of bailout.
-    void markUneffectful() {
-        markEffectful_ = false;
+    void setIdempotent() {
+        idempotent_ = true;
     }
     AliasSet getAliasSet() const {
-        if (markEffectful_)
+        if (!idempotent_)
             return AliasSet::Store(AliasSet::Any);
         return AliasSet::None();
     }