Lambda the Ultimate

"Guaranteed Optimization", in Active Libraries and Universal Languages, by Todd L. Veldhuizen. PhD thesis, 2004.

This chapter describes a new technique for designing optimizers that provide proven guarantees of what optimizations they perform. We use rewrite rules to capture common patterns of `adding abstraction' to programs, and develop a proof technique for compilers based on superanalysis (Chapter 2) to prove that any sequence of rule applications is undone in a single step. Such compilers address the problem of `abstraction penalty' --- see Section~1.4.1 --- and offers programmers an intuitive guarantee of what improvements the compiler will perform. We also show that compilers based on this approach find `optimal programs' in a carefully defined sense.

I know it's a little unusual to link to a chapter of a thesis instead of a paper, but on his web page the author said the paper I was planning on linking to ("Guaranteed Optimization: Proving Nullspace Properties of Compilers") had been superceded.

Anyway, the basic idea well illustrates the adage that a small change of POV can yield a lot of insight. Here, Veldhuizen changes the POV of a compiler writer from "optimization" to "de-pessimization", and gets a really nice result: an exact characterization of the specific kinds of inefficient patterns the compiler will eliminate. The value of this kind of predictability can be very large; functional programmers can rely on tail call optimization because we know exactly when the compiler will do the transformation.

Charles Nutter:

If you are interested in the future of non-Java languages on the JVM, you should be on this list. Yes, we talk about a lot of JVM lanuage implementation challenges, we discuss compilers and stack frames and call-site optimizations, but we also talk about features peripheral to language implementation like package indexing and retrofitting Java 5+ code. We need your help.