diff --git a/js/src/jit-test/tests/asm.js/testGlobals.js b/js/src/jit-test/tests/asm.js/testGlobals.js
index 2a75d938b27..e596d582368 100644
--- a/js/src/jit-test/tests/asm.js/testGlobals.js
+++ b/js/src/jit-test/tests/asm.js/testGlobals.js
@@ -104,3 +104,8 @@ var p = new Proxy(global,
                    getOwnPropertyDescriptor:function(name) { return {value:Int32Array}}});
 new Int32Array(ab)[4] = 42;
 assertEq(f1(p, null, ab)(), 42);
+
+// GVN checks
+assertEq(asmLink(asmCompile(USE_ASM + "var g=0; function f() { var x = 0; g = 1; x = g; return (x+g)|0 } return f"))(), 2);
+assertEq(asmLink(asmCompile(USE_ASM + "var g=0; function f() { var x = 0; g = 1; x = g; g = 2; return (x+g)|0 } return f"))(), 3);
+assertEq(asmLink(asmCompile(USE_ASM + "var g=0; var h=0; function f() { var x = 0; g = 1; x = g; h = 3; return (x+g)|0 } return f"))(), 2);
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 28e5c4d0d15..9aa8d5dbda4 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -2432,6 +2432,26 @@ MLoadFixedSlot::mightAlias(MDefinition *store)
     return true;
 }
 
+bool
+MAsmJSLoadGlobalVar::mightAlias(MDefinition *def)
+{
+    if (def->isAsmJSStoreGlobalVar()) {
+        MAsmJSStoreGlobalVar *store = def->toAsmJSStoreGlobalVar();
+        return store->globalDataOffset() == globalDataOffset_;
+    }
+    return true;
+}
+
+bool
+MAsmJSLoadGlobalVar::congruentTo(MDefinition *ins) const
+{
+    if (ins->isAsmJSLoadGlobalVar()) {
+        MAsmJSLoadGlobalVar *load = ins->toAsmJSLoadGlobalVar();
+        return globalDataOffset_ == load->globalDataOffset_;
+    }
+    return false;
+}
+
 bool
 MLoadSlot::mightAlias(MDefinition *store)
 {
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index ac41db67fb4..e1e60fe74bc 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -217,10 +217,11 @@ class AliasSet {
         FixedSlot         = 1 << 3, // A member of obj->fixedSlots().
         TypedArrayElement = 1 << 4, // A typed array element.
         DOMProperty       = 1 << 5, // A DOM property
-        Last              = DOMProperty,
+        AsmJSGlobalVar    = 1 << 6, // An asm.js global var
+        Last              = AsmJSGlobalVar,
         Any               = Last | (Last - 1),
 
-        NumCategories     = 6,
+        NumCategories     = 7,
 
         // Indicates load or store.
         Store_            = 1 << 31
@@ -8446,8 +8447,7 @@ class MAsmJSLoadGlobalVar : public MNullaryInstruction
     {
         JS_ASSERT(type == MIRType_Int32 || type == MIRType_Double);
         setResultType(type);
-        if (isConstant)
-            setMovable();
+        setMovable();
     }
 
     unsigned globalDataOffset_;
@@ -8462,12 +8462,13 @@ class MAsmJSLoadGlobalVar : public MNullaryInstruction
 
     unsigned globalDataOffset() const { return globalDataOffset_; }
 
+    bool congruentTo(MDefinition *ins) const;
+
     AliasSet getAliasSet() const {
-        if (isConstant_)
-            return AliasSet::None();
-        else
-            return AliasSet::Store(AliasSet::Any);
+        return AliasSet::Load(AliasSet::AsmJSGlobalVar);
     }
+
+    bool mightAlias(MDefinition *def);
 };
 
 class MAsmJSStoreGlobalVar : public MUnaryInstruction
@@ -8487,6 +8488,10 @@ class MAsmJSStoreGlobalVar : public MUnaryInstruction
 
     unsigned globalDataOffset() const { return globalDataOffset_; }
     MDefinition *value() const { return getOperand(0); }
+
+    AliasSet getAliasSet() const {
+        return AliasSet::Store(AliasSet::AsmJSGlobalVar);
+    }
 };
 
 class MAsmJSLoadFuncPtr : public MUnaryInstruction