+
+ /*
+ * Carry always fits on 31 bits; we want to keep it in a
+ * 32-bit register on 32-bit architectures (on a 64-bit
+ * architecture, cast down from 64 to 32 bits means
+ * clearing the high bits, which is not free; on a 32-bit
+ * architecture, the same operation really means ignoring
+ * the top register, which has negative or zero cost).
+ */
+#if BR_64