notes

Lowering Whitespace with strictness

Lowering instructions to statements:

• push n -> abstracted
• dup -> abstracted
• copy n -> abstracted or stack_ref_lazy
• swap -> abstracted
• drop -> abstracted
• slide n -> abstracted
• add -> add_lazy
• sub -> sub_lazy
• mul -> mul_lazy
• div -> div_lazy
• mod -> mod_lazy
• store -> eval and store
• retrieve -> eval_lazy and retrieve
• label -> nop
• call -> call
• jmp -> jmp
• jz -> eval and br zero
• jn -> eval and br neg
• ret -> guard calls and ret
• end -> exit
• printc -> eval and print char
• printi -> eval and print int
• readc -> eval and read char
• readi -> eval and read int

Strictness analysis

For each basic block, iterate the instructions from the start and rewrite each eval and eval_lazy. For evaluation of a lazy arithmetic operation (add_lazy, sub_lazy, mul_lazy, div_lazy), move the operation and its dependents to to the place of eval. If moving it would cause ordering issues (e.g., when used as the value of an ordered store), then instead, place an eval_lazy after the op. For evaluation of stack_ref_lazy, replace it with a guard underflow and stack_ref.

This needs further thoughts. In particular, the mechanism for eager expression evaluation with lazy exception handling needs refinement. It should probably be a sea-of-nodes IR to more easily express this partial order.

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