Practical and effective higher-order optimizations

Abstract

Inlining is an optimization that replaces a call to a function with that function's body. This optimization not only reduces the overhead of a function call, but can expose additional optimization opportunities to the compiler, such as removing redundant operations or unused conditional branches. Another optimization, copy propagation, replaces a redundant copy of a still-live variable with the original. Copy propagation can reduce the total number of live variables, reducing register pressure and memory usage, and possibly eliminating redundant memory-to-memory copies. In practice, both of these optimizations are implemented in nearly every modern compiler.These two optimizations are practical to implement and effective in first-order languages, but in languages with lexically-scoped first-class functions (aka, closures), these optimizations are not available to code programmed in a higher-order style. With higherorder functions, the analysis challenge has been that the environment at the call site must be the same as at the closure capture location, up to the free variables, or the meaning of the program may change. Olin Shivers' 1991 dissertation called this family of optimizations Super-β and he proposed one analysis technique, called reflow, to support these optimizations. Unfortunately, reflow has proven too expensive to implement in practice. Because these higher-order optimizations are not available in functional-language compilers, programmers studiously avoid uses of higher-order values that cannot be optimized (particularly in compiler benchmarks).This paper provides the first practical and effective technique for Super-β (higher-order) inlining and copy propagation, which we call unchanged variable analysis. We show that this technique is practical by implementing it in the context of a real compiler for an ML-family language and showing that the required analyses have costs below 3% of the total compilation time. This technique's effectiveness is shown through a set of benchmarks and example programs, where this analysis exposes additional potential optimization sites. * Portions of this work were performed while the author was at the University of Chicago.[Copyright notice will appear here once 'preprint' option is removed.]

Staged abstract interpreters: fast and modular whole-program analysis via meta-programming

“…In general, k-CFA (where k > 0) is considered impractical to be used as a compiler pass, due to its EXPTIME lower bound [Van Horn and Mairson 2008]. However, several variants of 0-CFA were developed and shown to be useful in practice [Adams et al 2011;Ashley and Dybvig 1998;Bergstrom et al 2014;Reppy 2006].…”

Partial aborts for transactions via first-class continuations

“…The translation from direct style to continuation-passing style eliminates the special handling of continuations, so that capturing a continuation is effectively a variable-variable copy and subject to copy propagation, and makes control flow explicit. Using higher-order control-flow analysis, we perform a number of further optimizations on the CPS IR program, such as arity-raising [6] and aggressive inlining [5].…”