Reducing the impact of spill code

Abstract

All graph-coloring register allocators rely on heuristics to arrive at a "good" answer to the NP-complete problem of allocation, resulting in suboptimal code. We look at a post-pass to the allocator which removes unnecessary spill code by finding places where the availability of an unused register allows us to "promote" a spill to a register. We explain and correct an error in Briggs' code that sometimes inserts an excessive and unnecessary number of spill instructions. The fix to this bug has an insignificant impact on the runtime of the compiler and never causes a degradation in runtime of the code produced. We suggest minimizing the impact of the spill code through the use of a small separate memory dedicated to spills and under the exclusive control of the compiler. We show an algorithm and experimental results which suggest that this hardware construct would significantly decrease the runtime of the code