Studying optimal spilling in the light of SSA

Recent developments in register allocation, mostly linked to static single assignment (SSA) form, have shown that it is possible to decouple the problem in two successive phases: a first spilling phase places load and store instructions so that the register pressure at all program points is small enough, a second assignment and coalescing phase maps the remaining variables to physical registers and reduces the number of move instructions among registers. This paper focuses on the first phase, for which many open questions remain: in particular, we study the notion of optimal spilling (what can be expressed?) and the impact of SSA form (does it help?).