In a computer system that uses a generational garbage collector in which objects are promoted from a "young" generation to an "old" generation, a compiler output designates certain dynamic-allocation instructions as being ones whose resultant allocated objects will be considered "pinned." The compiler associates with such allocation instructions respective segments of the code following the instructions and objects allocated within one of those segments are considered to remain pinned until program execution passes beyond that segment. The garbage collector refrains from promoting any pinned object, and as a consequence, an instruction that writes a reference into an object field while that object is pinned does not need to be accompanied by a write barrier.