. Many high-level language compilers generate C code and then invoke a C compiler to do code generation, register allocation, stack management, and low-level optimization. To date, most of these compilers link the resulting code against a conservative mark-sweep garbage collector in order to reclaim unused memory. We introduce a new collector, MCC, based on mostly-copying collection, and characterize the conditions that favor such a collector over a mark-sweep collector. In particular we demonstrate that mostly-copying collection outperforms conservative mark-sweep under the same conditions that accurate copying collection outperforms accurate mark-sweep: Specifically, MCC meets or exceeds the performance of a mature mark-sweep collector when allocation rates are high, and physical memory is large relative to the live data. 1 High Level Overview Languages such as C [1] and C++ [27] are GC-unfriendly because they allow programs to violate the key premise of tracing garbage collection--...