Reducing the Impact of Spill Code
Harvey, Timothy J.
This work was also published as a Rice University thesis/dissertation: http://hdl.handle.net/1911/17173
All graph-coloring register allocators rely on heuristics to arrive at a "good" answer to the NP-complete problem of register allocation, resulting in suboptimal code due to spill code. We look at a post-pass to the allocator that 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' spill-code insertion algorithm that sometimes inserts an unnecessary number of spill instructions. This fix 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 with 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.