Two-dimensioned bi-directional object layout

Abstract

C++ object layout schemes rely on (sometimes numerous) compiler generated fields. We describe a new language-independent object layout scheme, which is space optimal, i.e., objects are contiguous, and contain no compiler generated fields other than a single type identifier. As in C++ and other multiple inheritance languages such as Cecil and Dylan, the new scheme sometimes requires extra levels of indirection to access some of the fields. Using a data set of 28 hierarchies, totaling almost 50,000 types, we show that the new scheme improves field access efficiency over standard implementations, and competes favorably with (the non-space optimal) highly optimized C++ specific implementations. The benchmark includes a new analytical model for computing the frequency of indirections in a sequence of field access operations. Our layout scheme relies on whole-program analysis, which requires about 10 microseconds per type on a contemporary architecture (Pentium III, 900Mhz, 256MB machine), even in very large hierarchies. © Springer-Verlag Berlin Heidelberg 2003.