> Is it an NP complete problem our own heuristics are currently better at estimating?

Some of the important problems are NP-complete (like register allocation). Another problem is that compilers aren't that good at telling fast-paths from slow-paths (and keeping everything in registers for the fast paths). For more info see this message from the author of LuaJIT: http://article.gmane.org/gmane.comp.lang.lua.general/75426