A practical system fljr intermodule code optimization at link-time

We have developed a system called OM to explore the problem of code optimization at link-time. OM takes a collection of object modules constituting the entire program, and converts the object code into a symbolic Register Transfer Language (RTL) form that can be easily manipulated. This RTL is then transformed by intermodule optimization and finally converted back into object form. Although much high-level information about the program is gone at link-time, this approach enables us to perform optimizations that a compiler looking at a single module cannot see. Since object modules are more or less independent of the particular source language or compiler, this also gives us the chance to improve the code in ways that some compilers might simply have missed. To test the concept, we have used OM to build an optimizer that does interprocedural code motion. It moves simple loop-invariant code out of loops, even when the loop body extends across many procedures and the loop control is in a different procedure from the invariant code. Our technique also easily handles ‘‘loops’’ induced by recursion rather than iteration. Our code motion technique makes use of an interprocedural liveness analysis to discover dead registers that it can use to hold loop-invariant results. This liveness analysis also lets us perform interprocedural dead code elimination. We applied our code motion and dead code removal to SPEC benchmarks compiled with optimization using the standard compilers for the DECstation 5000. Our system improved the performance by 5% on average and by more than 14% in one case. More improvement should be possible soon; at present we move only simple load and load-address operations out of loops, and we scavenge registers to hold these values, rather than completely reallocating them. This paper will appear in the March issue of Journal of Programming Languages. It replaces Technical Note TN-31, an earlier version of the same material.