Fix bug in finding arity for verify attributes

[bakaq]

This fixes a bug in the current attributed variables implementation that creates a lot of strange bugs that seem like miscompilations.

Closes #2706, closes #2809, closes #2632.

[adri326]

I don't have much experience with the logic behind verify_attributes, but I couldn't find any issue with this fix.

I wonder how hard it would be to store somewhere else the number of temporary variables, so that the WAM instructions don't need to be scanned for every attributed variable unification.

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I do believe that the current method of modifying self.code to eventually call verify_attr_interrupt is iffy. It feels flimsy and it will make JIT impossible in its current form.

[adri326]

You forgot these in :)

[adri326]

This can also work:

Suggested change

	while p < self.code.len()
	&& current_pred_limit.map(|x| p < x).unwrap_or(true)
	let current_pred_limit = current_pred_limit.unwrap_or(self.code.len());
	while p < self.code.len() && p < current_pred_limit {

I also think that both loops could be joined together, simplifying the logic a bit (or at least making it less imperative in nature):

• grab a slice of self.code: let predicate_instrs = &self.code[p..current_pred_limit.min(self.code.len())];
• find p_interrupt: let p_interrupt = p + predicate_instrs.partition_point(Instruction::is_head_instr);
• find the arity:

let arity = predicate_instrs
    .flat_map(Instruction::registers)
    .flat_map(|r| {
        match r {
            RegType::Temp(t) => Some(t),
            _ => None
        }
    })
    .max()
    .unwrap_or(0);

[adri326]

Ah I'm not sure if partition_point can be used, since it does a binary search.

[bakaq]

I wonder how hard it would be to store somewhere else the number of temporary variables, so that the WAM instructions don't need to be scanned for every attributed variable unification.

I do believe that the current method of modifying self.code to eventually call verify_attr_interrupt is iffy. It feels flimsy and it will make JIT impossible in its current form.

I think ideally this should all be done in the compilation step, although maybe the verify attributes interrupt really needs to be this dynamic.