fix(ssa): Use number of SSA instructions for the Brillig unrolling by…

…tecode size limit (noir-lang#7242)

Co-authored-by: Tom French <[email protected]>

compiler/noirc_evaluator/src/ssa/ir/function.rs

@@ -211,6 +211,16 @@ impl Function {
 
         unreachable!("SSA Function {} has no reachable return instruction!", self.id())
     }
+
+    pub(crate) fn num_instructions(&self) -> usize {
+        self.reachable_blocks()
+            .iter()
+            .map(|block| {
+                let block = &self.dfg[*block];
+                block.instructions().len() + block.terminator().is_some() as usize
+            })
+            .sum()
+    }
 }
 
 impl Clone for Function {

compiler/noirc_evaluator/src/ssa/opt/unrolling.rs

@@ -24,10 +24,6 @@ use acvm::{acir::AcirField, FieldElement};
 use im::HashSet;
 
 use crate::{
-    brillig::{
-        brillig_gen::{brillig_globals::convert_ssa_globals, convert_ssa_function},
-        brillig_ir::brillig_variable::BrilligVariable,
-    },
     errors::RuntimeError,
     ssa::{
         ir::{
@@ -60,8 +56,6 @@ impl Ssa {
         mut self,
         max_bytecode_increase_percent: Option<i32>,
     ) -> Result<Ssa, RuntimeError> {
-        let mut global_cache = None;
-
         for function in self.functions.values_mut() {
             let is_brillig = function.runtime().is_brillig();
 
@@ -78,20 +72,9 @@ impl Ssa {
             // to the globals and a mutable reference to the function at the same time, both part of the `Ssa`.
             if has_unrolled && is_brillig {
                 if let Some(max_incr_pct) = max_bytecode_increase_percent {
-                    if global_cache.is_none() {
-                        let globals = (*function.dfg.globals).clone();
-                        let used_globals = &globals.values_iter().map(|(id, _)| id).collect();
-                        let globals_dfg = DataFlowGraph::from(globals);
-                        // DIE is run at the end of our SSA optimizations, so we mark all globals as in use here.
-                        let (_, brillig_globals, _) =
-                            convert_ssa_globals(false, &globals_dfg, used_globals, function.id());
-                        global_cache = Some(brillig_globals);
-                    }
-                    let brillig_globals = global_cache.as_ref().unwrap();
-
                     let orig_function = orig_function.expect("took snapshot to compare");
-                    let new_size = brillig_bytecode_size(function, brillig_globals);
-                    let orig_size = brillig_bytecode_size(&orig_function, brillig_globals);
+                    let new_size = function.num_instructions();
+                    let orig_size = function.num_instructions();
                     if !is_new_size_ok(orig_size, new_size, max_incr_pct) {
                         *function = orig_function;
                     }
@@ -1022,25 +1005,6 @@ fn simplify_between_unrolls(function: &mut Function) {
     function.mem2reg();
 }
 
-/// Convert the function to Brillig bytecode and return the resulting size.
-fn brillig_bytecode_size(
-    function: &Function,
-    globals: &HashMap<ValueId, BrilligVariable>,
-) -> usize {
-    // We need to do some SSA passes in order for the conversion to be able to go ahead,
-    // otherwise we can hit `unreachable!()` instructions in `convert_ssa_instruction`.
-    // Creating a clone so as not to modify the originals.
-    let mut temp = function.clone();
-
-    // Might as well give it the best chance.
-    simplify_between_unrolls(&mut temp);
-
-    // This is to try to prevent hitting ICE.
-    temp.dead_instruction_elimination(false, true);
-
-    convert_ssa_function(&temp, false, globals).byte_code.len()
-}
-
 /// Decide if the new bytecode size is acceptable, compared to the original.
 ///
 /// The maximum increase can be expressed as a negative value if we demand a decrease.