Simplify reachable brillig function analysis

Author: jfecher

crates/noirc_evaluator/src/brillig/mod.rs

@@ -6,13 +6,10 @@ use self::{
     brillig_ir::artifact::{BrilligArtifact, Label},
 };
 use crate::ssa::{
-    ir::{
-        function::{Function, FunctionId, RuntimeType},
-        value::Value,
-    },
+    ir::function::{Function, FunctionId, RuntimeType},
     ssa_gen::Ssa,
 };
-use std::collections::{HashMap, HashSet};
+use std::collections::{BTreeSet, HashMap};
 
 /// Context structure for the brillig pass.
 /// It stores brillig-related data required for brillig generation.
@@ -53,45 +50,11 @@ impl Ssa {
     pub(crate) fn to_brillig(&self, enable_debug_trace: bool) -> Brillig {
         // Collect all the function ids that are reachable from brillig
         // That means all the functions marked as brillig and ACIR functions called by them
-        let mut brillig_reachable_function_ids: HashSet<FunctionId> = HashSet::new();
-
-        // Initialize the queue with all the functions marked as brillig
-        let mut reachability_queue: Vec<FunctionId> = self
+        let brillig_reachable_function_ids = self
             .functions
             .iter()
-            .filter_map(
-                |(id, func)| {
-                    if func.runtime() == RuntimeType::Brillig {
-                        Some(*id)
-                    } else {
-                        None
-                    }
-                },
-            )
-            .collect();
-
-        while let Some(func_id) = reachability_queue.pop() {
-            let func = &self.functions[&func_id];
-            brillig_reachable_function_ids.insert(func.id());
-
-            // Explore all functions that are reachable from this function
-            for (_, value) in func.dfg.values_iter() {
-                // All reachable functions appear as literals after defunctionalization of the SSA
-                let reachable_function = match value {
-                    Value::Function(function_id) => function_id,
-                    _ => continue,
-                };
-
-                // If the function is already reachable by brillig or enqueued, skip it.
-                if brillig_reachable_function_ids.contains(reachable_function)
-                    || reachability_queue.contains(reachable_function)
-                {
-                    continue;
-                }
-
-                reachability_queue.push(*reachable_function);
-            }
-        }
+            .filter_map(|(id, func)| (func.runtime() == RuntimeType::Brillig).then_some(*id))
+            .collect::<BTreeSet<_>>();
 
         let mut brillig = Brillig::default();
         for brillig_function_id in brillig_reachable_function_ids {