1. For employing a computer to compile source code that specifies operation of a mutator, which includes a loop including at least one reference-modifying instruction that modifies a reference stored in an array of references, into object code for execution by a computer system, which includes a memory of which at least a portion is logically partitioned into cards, together with a garbage collector that relies on the mutator's execution of write-barrier code to keep track of at least some reference modifications, a method comprising: (A) determining whether execution of the loop included in the mutator would result in modifications, by the least one reference-modifying instruction included in the loop, of at least one reference stored in the array of references within each card spanned by the array of references; (B) deferring, in response to determining that execution of the loop would result in at least one reference stored in the array of references being modified within each card spanned by the array of references, emission of write-barrier code corresponding to the reference modifications made to the references contained in the array of references; (C) emitting write-barrier code that executes subsequent to the execution of the object code implementing the loop, the write-barrier code executing a write barrier corresponding to each card spanned by the array of references; and (D) providing, in response to determining that execution of the loop would result in at least one reference stored in the array of references being modified within each card spanned by the array of references, a data structure containing an indication of a location that stores the memory address of the array of references, the data structure being accessible to the collector.