Section: New Results

“Optimal” formulation of register spilling

Participants : Alain Darte, Quentin Colombet, Fabrice Rastello.

This work is part of the contract (see Section  7.2 ) with the cec team at stm icroelectronics. The motivation of this work was to develop an optimal spilling algorithm, based on integer linear programming (ILP), to be used to evaluate heuristics and to better understand the traps in which they can fall.

Optimizing the placement of load and store instructions (spill) is the key to get good performances in compute-intensive codes and to avoid memory transfers, which impact time and power consumption. Performing register allocation in two decoupled phases enables the design of faster and better spilling heuristics. We developed an ILP formulation for “optimal” spilling optimization under SSA, which models the problem in a finer way than previous approaches. In particular, we can model the fact that a given variable can reside in more than one storage location (different registers and in memory). This formulation has been fully implemented in the LAO back-end compiler. Several difficulties have been revealed, which were expected but never precisely identified: they are due to the move instructions that propagate values between registers and to interactions with post optimization phases, such as some peephole optimizations and post-pass scheduling. More work has to be done to improve our formulation even further and to derive, from a study of benchmarks, some good criteria to drive spilling heuristics.