Bug 1122723 - Fold MConcat when used with the empty string. r=h4writer

js/src/jit/MIR.cpp

@@ -1142,6 +1142,18 @@ MStringLength::foldsTo(TempAllocator &alloc)
     return this;
 }
 
+MDefinition *
+MConcat::foldsTo(TempAllocator &alloc)
+{
+    if (lhs()->isConstantValue() && lhs()->constantValue().toString()->empty())
+        return rhs();
+
+    if (rhs()->isConstantValue() && rhs()->constantValue().toString()->empty())
+        return lhs();
+
+    return this;
+}
+
 static bool
 EnsureFloatInputOrConvert(MUnaryInstruction *owner, TempAllocator &alloc)
 {

js/src/jit/MIR.h

@@ -6123,6 +6123,7 @@ class MConcat
         return new(alloc) MConcat(left, right);
     }
 
+    MDefinition *foldsTo(TempAllocator &alloc) MOZ_OVERRIDE;
     bool congruentTo(const MDefinition *ins) const MOZ_OVERRIDE {
         return congruentIfOperandsEqual(ins);
     }