Optimization pass epic

[jeaye]

General

• Constant propagation
• Constant folding (compile-time eval of pure expressions with literals)
  • Requires tracking pure fns

Vars

• If a var is already dereferenced once in a fn, use that for every other call in that fn

Dead code elimination

• Dead branches
• Pure statements

Boxing

• Arithmetic unboxing for numeric locals
• Unboxed numeric function parameters (requires either hinting or monomorphization)
• Unboxing for non-numeric locals (strings, maps, etc -- a la Project Valhalla)
  • If the local is never returned or closed over, it can be on the stack
  • Requires tracking capturing fns
    • https://llvm.org/docs/LangRef.html#pointercapture
  • C# does some of this: https://em-tg.github.io/csborrow/

Startup

• Group all fns into a single module to load
• Box globals in static memory, not with the GC

Form rewriting

• Auto-transient
• Auto-transducer
• Zero-cost map destructuring (see here)
• General loop unrolling
• Specific loop unrolling (i.e. get-in)
• Faster str usage (a la stringer)

AOT-only

• Direct linking (dynamic call)
• Direct linking (direct call)
• Unused var removal (includes defns)
• Flag to elide var meta
• Remove unused args from fns

References

• LLVM front-end tips: https://llvm.org/docs/Frontend/PerformanceTips.html
• Optimizing compiler: https://en.wikipedia.org/wiki/Optimizing_compiler
• Clojure optimization discussion: https://clojureverse.org/t/do-clojure-still-have-rooms-to-improve-at-compiler-level/7802
• Orbit, optimizing Scheme compiler: https://www.cs.purdue.edu/homes/suresh/502-Fall2008/papers/orbit.pdf
• Lisp compiler optimizations: https://healeycodes.com/lisp-compiler-optimizations