Method optimize_pure_enode

cranelift/codegen/src/egraph/mod.rs:347–454

Optimizes an enode by applying any matching mid-end rewrite rules (or store-to-load forwarding, which is a special case), unioning together all possible optimized (or rewritten) forms of this expression into an eclass and returning the `Value` that represents that eclass.

(&mut self, inst: Inst)

    /// of this expression into an eclass and returning the `Value`
    /// that represents that eclass.
    fn optimize_pure_enode(&mut self, inst: Inst) -> Value {
        // A pure node always has exactly one result.
        let orig_value = self.func.dfg.first_result(inst);

        let mut guard = TakeAndReplace::new(self, |x| &mut x.optimized_values);
        let (ctx, optimized_values) = guard.get();

        // Limit rewrite depth. When we apply optimization rules, they
        // may create new nodes (values) and those are, recursively,
        // optimized eagerly as soon as they are created. So we may
        // have more than one ISLE invocation on the stack. (This is
        // necessary so that as the toplevel builds the
        // right-hand-side expression bottom-up, it uses the "latest"
        // optimized values for all the constituent parts.) To avoid
        // infinite or problematic recursion, we bound the rewrite
        // depth to a small constant here.
        const REWRITE_LIMIT: usize = 5;
        if ctx.rewrite_depth >= REWRITE_LIMIT {
            ctx.stats.rewrite_depth_limit += 1;
            return orig_value;
        }
        ctx.rewrite_depth += 1;
        trace!("Incrementing rewrite depth; now {}", ctx.rewrite_depth);

        // Invoke the ISLE toplevel constructor, getting all new
        // values produced as equivalents to this value.
        trace!("Calling into ISLE with original value {}", orig_value);
        ctx.stats.rewrite_rule_invoked += 1;
        debug_assert!(optimized_values.is_empty());
        crate::opts::generated_code::constructor_simplify(
            &mut IsleContext { ctx },
            orig_value,
            optimized_values,
        );

        ctx.stats.rewrite_rule_results += optimized_values.len() as u64;

        // It's not supposed to matter what order `simplify` returns values in.
        ctx.ctrl_plane.shuffle(optimized_values);

        let num_matches = optimized_values.len();
        if num_matches > MATCHES_LIMIT {
            trace!(
                "Reached maximum matches limit; too many optimized values \
                 ({num_matches} > {MATCHES_LIMIT}); ignoring rest.",
            );
            optimized_values.truncate(MATCHES_LIMIT);
        }

        // Sort and deduplicate optimized values, in case multiple
        // rules produced the same simplification.
        optimized_values.sort_unstable();
        optimized_values.dedup();

        trace!("  -> returned from ISLE: {orig_value} -> {optimized_values:?}");

        // Construct a union-node tree representing the new eclass
        // that results from rewriting. If any returned value was