+/* Byte code objects. These are fixed size objects with pointers to
+ * four arrays, designed so that a BCO can be easily "re-linked" to
+ * other BCOs, to facilitate GHC's intelligent recompilation. The
+ * array of instructions is static and not re-generated when the BCO
+ * is re-linked, but the other 3 arrays will be regenerated.
+ *
+ * A BCO represents either a function or a stack frame. In each case,
+ * it needs a bitmap to describe to the garbage collector the
+ * pointerhood of its arguments/free variables respectively, and in
+ * the case of a function it also needs an arity. These are stored
+ * directly in the BCO, rather than in the instrs array, for two
+ * reasons:
+ * (a) speed: we need to get at the bitmap info quickly when
+ * the GC is examining APs and PAPs that point to this BCO
+ * (b) a subtle interaction with the compacting GC. In compacting
+ * GC, the info that describes the size/layout of a closure
+ * cannot be in an object more than one level of indirection
+ * away from the current object, because of the order in
+ * which pointers are updated to point to their new locations.
+ */
+
+typedef struct {
+ StgHeader header;
+ StgArrWords *instrs; // a pointer to an ArrWords
+ StgArrWords *literals; // a pointer to an ArrWords
+ StgMutArrPtrs *ptrs; // a pointer to a MutArrPtrs
+ StgArrWords *itbls; // a pointer to an ArrWords
+ StgHalfWord arity; // arity of this BCO
+ StgHalfWord size; // size of this BCO (in words)
+ StgWord bitmap[FLEXIBLE_ARRAY]; // an StgLargeBitmap
+} StgBCO;
+
+#define BCO_BITMAP(bco) ((StgLargeBitmap *)((StgBCO *)(bco))->bitmap)
+#define BCO_BITMAP_SIZE(bco) (BCO_BITMAP(bco)->size)
+#define BCO_BITMAP_BITS(bco) (BCO_BITMAP(bco)->bitmap)
+#define BCO_BITMAP_SIZEW(bco) ((BCO_BITMAP_SIZE(bco) + BITS_IN(StgWord) - 1) \
+ / BITS_IN(StgWord))
+