diff -r 9d17f29fe16a -r 23a804fa1d3e src/BytecodeCompiler.cpp
--- a/src/BytecodeCompiler.cpp	Sun Sep 16 22:49:54 2012 -0700
+++ b/src/BytecodeCompiler.cpp	Sun Sep 16 23:15:13 2012 -0700
@@ -987,10 +987,17 @@
             }
         }
     } else if (type->isStruct()) {
-        Local<Map> map(zone_, type->newMap());
+        Local<StructMap> map(zone_, StructMap::cast(type->newMap()));
+
+        // Because structs can contain arrays, any sort of composition
+        // triggers non-LIFO allocation. This is just for correctness.
+        // We could do better by:
+        //  (1) Limiting to structs that just (recursively) contain arrays,
+        //  (2) Swap Stack for Default tenure in Struct::New
+        //  (3) JIT analysis
         if (var->scope()->kind() == Scope::GLOBAL)
             __ newstruct(map, Heap::Tenure_Old);
-        else if (var->needsHeapAllocation())
+        else if (var->needsHeapAllocation() || map->composed())
             __ newstruct(map, Heap::Tenure_Default);
         else
             __ newstruct(map, Heap::Tenure_Stack);