Two-dimensional bidirectional object layout

  • Authors:

  • Joseph (Yossi) Gil;William Pugh;Grant E. Weddell;Yoav Zibin

  • Affiliations:

  • Technion—Israel Institute of Technology;Dept. of Computer Science, University of Maryland, College Park;University of Waterloo;Technion—Israel Institute of Technology

  • Venue:
  • ACM Transactions on Programming Languages and Systems (TOPLAS)
  • Year:
  • 2008

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Abstract

Object layout schemes used in C++ and other languages rely on (sometimes numerous) compiler generated fields. We describe a language-independent object layout scheme, which is space optimal, that is, 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 this scheme improves field access efficiency over standard implementations, and competes favorably with (the non-space-optimal) highly optimized C++ specific implementations. The benchmark includes an 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. We also present a layout scheme for separate compilation using the user-annotation of virtual inheritance edge that is used in C++.