Draft Comparing Two Garbage Collectors for C++

Our research is concerned with compiler-independent, tag-free garbage collection for the C ++ programming language. This paper presents a mark-and-sweep collector , and explains how it ameliorates shortcomings of a previous copy collector. The new collector, like the old, uses C ++ 's facilities for creating abstract data types to deene a tracked reference type, called roots, at the level of the application program. A programmer wishing to utilize the garbage collection service uses these roots in place of normal, raw pointers. We present a detailed study of the cost of using roots, as compared to both normal pointers and reference counted pointers, in terms of instruction counts. We examine the eeciency of a small C++ application using roots, reference counting, manual reclamation, and conservative collection. Coding the application to use garbage collection, and analyzing the resulting ee-ciency, helped us identify a number of memory leaks and ineeciencies in the original, manually reclaimed version. We nd that for this program, garbage collection using roots is much more eecient than reference counting, though less eecient than manual reclamation. It is hard to directly compare our collector to the conservative collector because of the diiering ee-ciencies of their respective memory allocators. This paper is a longer, more thorough version of a paper that appears in POPL '92 under the title, A mark-and-sweep collector for C++.