Forth language features that support or inhibit optimization

General comments

• What's important depends on the architecture (e.g., for Intel it's keeping the stuff in registers).
• Exploiting instruction-level parallelism (on VLIW and superscalar processors) is an increasingly important challenge.
• Using Forth source code is quite different for the workshop topic from using a stack-based intermediate language.

Existing features

• + stack items, locals can go into registers
  • - stack effects not fixed at compile time require representing the stack in memory
  • + definition before use helps determining the stack effect
  • - EXECUTE, deferred words have no fixed stack effect (proposed remedy)
• + the addr-len string representation allows more parallelism and optimizations than zero-terminated strings.
• - compilation speed requirements prohibit many optimizations.
• - >body ! inhibites optimization of does>-defined words (proposed remedy).

New features for supporting optimization

• @+ c@+ for exploiting autoincrement addressing modes.
• INVARIANT allows optimizing @ from unchanging ranges of memory.
• Frequency annotations (produced by the programmer and/or a profiler) allow the compiler to optimize for the frequently occuring case.