chore: Separate unconstrained functions during monomorphization (#6894)

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

compiler/noirc_driver/src/lib.rs

@@ -590,10 +590,17 @@ pub fn compile_no_check(
     cached_program: Option<CompiledProgram>,
     force_compile: bool,
 ) -> Result<CompiledProgram, CompileError> {
+    let force_unconstrained = options.force_brillig;
+
     let program = if options.instrument_debug {
-        monomorphize_debug(main_function, &mut context.def_interner, &context.debug_instrumenter)?
+        monomorphize_debug(
+            main_function,
+            &mut context.def_interner,
+            &context.debug_instrumenter,
+            force_unconstrained,
+        )?
     } else {
-        monomorphize(main_function, &mut context.def_interner)?
+        monomorphize(main_function, &mut context.def_interner, force_unconstrained)?
     };
 
     if options.show_monomorphized {
@@ -632,7 +639,6 @@ pub fn compile_no_check(
             }
         },
         enable_brillig_logging: options.show_brillig,
-        force_brillig_output: options.force_brillig,
         print_codegen_timings: options.benchmark_codegen,
         expression_width: if options.bounded_codegen {
             options.expression_width.unwrap_or(DEFAULT_EXPRESSION_WIDTH)

compiler/noirc_evaluator/src/ssa.rs

@@ -56,9 +56,6 @@ pub struct SsaEvaluatorOptions {
 
     pub enable_brillig_logging: bool,
 
-    /// Force Brillig output (for step debugging)
-    pub force_brillig_output: bool,
-
     /// Pretty print benchmark times of each code generation pass
     pub print_codegen_timings: bool,
 
@@ -99,7 +96,6 @@ pub(crate) fn optimize_into_acir(
     let builder = SsaBuilder::new(
         program,
         options.ssa_logging.clone(),
-        options.force_brillig_output,
         options.print_codegen_timings,
         &options.emit_ssa,
     )?;
@@ -154,15 +150,16 @@ pub(crate) fn optimize_into_acir(
 /// Run all SSA passes.
 fn optimize_all(builder: SsaBuilder, options: &SsaEvaluatorOptions) -> Result<Ssa, RuntimeError> {
     Ok(builder
+        .run_pass(Ssa::remove_unreachable_functions, "Removing Unreachable Functions")
         .run_pass(Ssa::defunctionalize, "Defunctionalization")
         .run_pass(Ssa::remove_paired_rc, "Removing Paired rc_inc & rc_decs")
-        .run_pass(Ssa::separate_runtime, "Runtime Separation")
         .run_pass(Ssa::resolve_is_unconstrained, "Resolving IsUnconstrained")
         .run_pass(|ssa| ssa.inline_functions(options.inliner_aggressiveness), "Inlining (1st)")
         // Run mem2reg with the CFG separated into blocks
         .run_pass(Ssa::mem2reg, "Mem2Reg (1st)")
         .run_pass(Ssa::simplify_cfg, "Simplifying (1st)")
         .run_pass(Ssa::as_slice_optimization, "`as_slice` optimization")
+        .run_pass(Ssa::remove_unreachable_functions, "Removing Unreachable Functions")
         .try_run_pass(
             Ssa::evaluate_static_assert_and_assert_constant,
             "`static_assert` and `assert_constant`",
@@ -275,19 +272,12 @@ pub fn create_program(
         (generated_acirs, generated_brillig, brillig_function_names, error_types),
         ssa_level_warnings,
     ) = optimize_into_acir(program, options)?;
-    if options.force_brillig_output {
-        assert_eq!(
-            generated_acirs.len(),
-            1,
-            "Only the main ACIR is expected when forcing Brillig output"
-        );
-    } else {
-        assert_eq!(
-            generated_acirs.len(),
-            func_sigs.len(),
-            "The generated ACIRs should match the supplied function signatures"
-        );
-    }
+
+    assert_eq!(
+        generated_acirs.len(),
+        func_sigs.len(),
+        "The generated ACIRs should match the supplied function signatures"
+    );
 
     let error_types = error_types
         .into_iter()
@@ -450,11 +440,10 @@ impl SsaBuilder {
     fn new(
         program: Program,
         ssa_logging: SsaLogging,
-        force_brillig_runtime: bool,
         print_codegen_timings: bool,
         emit_ssa: &Option<PathBuf>,
     ) -> Result<SsaBuilder, RuntimeError> {
-        let ssa = ssa_gen::generate_ssa(program, force_brillig_runtime)?;
+        let ssa = ssa_gen::generate_ssa(program)?;
         if let Some(emit_ssa) = emit_ssa {
             let mut emit_ssa_dir = emit_ssa.clone();
             // We expect the full package artifact path to be passed in here,

compiler/noirc_evaluator/src/ssa/function_builder/mod.rs

@@ -483,29 +483,33 @@ impl FunctionBuilder {
     ///
     /// Returns whether a reference count instruction was issued.
     fn update_array_reference_count(&mut self, value: ValueId, increment: bool) -> bool {
-        match self.type_of_value(value) {
-            Type::Numeric(_) => false,
-            Type::Function => false,
-            Type::Reference(element) => {
-                if element.contains_an_array() {
-                    let reference = value;
-                    let value = self.insert_load(reference, element.as_ref().clone());
-                    self.update_array_reference_count(value, increment);
-                    true
-                } else {
-                    false
+        if self.current_function.runtime().is_brillig() {
+            match self.type_of_value(value) {
+                Type::Numeric(_) => false,
+                Type::Function => false,
+                Type::Reference(element) => {
+                    if element.contains_an_array() {
+                        let reference = value;
+                        let value = self.insert_load(reference, element.as_ref().clone());
+                        self.update_array_reference_count(value, increment);
+                        true
+                    } else {
+                        false
+                    }
                 }
-            }
-            Type::Array(..) | Type::Slice(..) => {
-                // If there are nested arrays or slices, we wait until ArrayGet
-                // is issued to increment the count of that array.
-                if increment {
-                    self.insert_inc_rc(value);
-                } else {
-                    self.insert_dec_rc(value);
+                Type::Array(..) | Type::Slice(..) => {
+                    // If there are nested arrays or slices, we wait until ArrayGet
+                    // is issued to increment the count of that array.
+                    if increment {
+                        self.insert_inc_rc(value);
+                    } else {
+                        self.insert_dec_rc(value);
+                    }
+                    true
                 }
-                true
             }
+        } else {
+            false
         }
     }
 

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

@@ -51,9 +51,8 @@ fn value(function: &Function, id: ValueId) -> String {
         }
         Value::Function(id) => id.to_string(),
         Value::Intrinsic(intrinsic) => intrinsic.to_string(),
-        Value::Param { .. } | Value::Instruction { .. } | Value::ForeignFunction(_) => {
-            id.to_string()
-        }
+        Value::ForeignFunction(function) => function.clone(),
+        Value::Param { .. } | Value::Instruction { .. } => id.to_string(),
     }
 }
 

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

@@ -1550,7 +1550,7 @@ mod test {
     fn deduplicates_side_effecting_intrinsics() {
         let src = "
         // After EnableSideEffectsIf removal:
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: Field, v1: Field, v2: u1):
             v4 = call is_unconstrained() -> u1
             v7 = call to_be_radix(v0, u32 256) -> [u8; 1]    // `a.to_be_radix(256)`;
@@ -1567,7 +1567,7 @@ mod test {
         ";
         let ssa = Ssa::from_str(src).unwrap();
         let expected = "
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: Field, v1: Field, v2: u1):
             v4 = call is_unconstrained() -> u1
             v7 = call to_be_radix(v0, u32 256) -> [u8; 1]

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

@@ -637,10 +637,12 @@ mod test {
     use std::sync::Arc;
 
     use im::vector;
+    use noirc_frontend::monomorphization::ast::InlineType;
 
     use crate::ssa::{
         function_builder::FunctionBuilder,
         ir::{
+            function::RuntimeType,
             map::Id,
             types::{NumericType, Type},
         },
@@ -742,7 +744,7 @@ mod test {
     #[test]
     fn keep_paired_rcs_with_array_set() {
         let src = "
-            acir(inline) fn main f0 {
+            brillig(inline) fn main f0 {
               b0(v0: [Field; 2]):
                 inc_rc v0
                 v2 = array_set v0, index u32 0, value u32 0
@@ -759,7 +761,7 @@ mod test {
 
     #[test]
     fn keep_inc_rc_on_borrowed_array_store() {
-        // acir(inline) fn main f0 {
+        // brillig(inline) fn main f0 {
         //     b0():
         //       v1 = make_array [u32 0, u32 0]
         //       v2 = allocate
@@ -776,6 +778,7 @@ mod test {
 
         // Compiling main
         let mut builder = FunctionBuilder::new("main".into(), main_id);
+        builder.set_runtime(RuntimeType::Brillig(InlineType::Inline));
         let zero = builder.numeric_constant(0u128, NumericType::unsigned(32));
         let array_type = Type::Array(Arc::new(vec![Type::unsigned(32)]), 2);
         let v1 = builder.insert_make_array(vector![zero, zero], array_type.clone());
@@ -810,7 +813,7 @@ mod test {
 
     #[test]
     fn keep_inc_rc_on_borrowed_array_set() {
-        // acir(inline) fn main f0 {
+        // brillig(inline) fn main f0 {
         //     b0(v0: [u32; 2]):
         //       inc_rc v0
         //       v3 = array_set v0, index u32 0, value u32 1
@@ -821,7 +824,7 @@ mod test {
         //       return v4
         //   }
         let src = "
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: [u32; 2]):
             inc_rc v0
             v3 = array_set v0, index u32 0, value u32 1
@@ -837,7 +840,7 @@ mod test {
         // We expect the output to be unchanged
         // Except for the repeated inc_rc instructions
         let expected = "
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: [u32; 2]):
             inc_rc v0
             v3 = array_set v0, index u32 0, value u32 1
@@ -875,7 +878,7 @@ mod test {
     #[test]
     fn remove_inc_rcs_that_are_never_mutably_borrowed() {
         let src = "
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: [Field; 2]):
             inc_rc v0
             inc_rc v0
@@ -893,7 +896,7 @@ mod test {
         assert_eq!(main.dfg[main.entry_block()].instructions().len(), 5);
 
         let expected = "
-        acir(inline) fn main f0 {
+        brillig(inline) fn main f0 {
           b0(v0: [Field; 2]):
             v2 = array_get v0, index u32 0 -> Field
             return v2

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

@@ -14,7 +14,6 @@ mod tests {
         let options = &SsaEvaluatorOptions {
             ssa_logging: SsaLogging::None,
             enable_brillig_logging: false,
-            force_brillig_output: false,
             print_codegen_timings: false,
             expression_width: ExpressionWidth::default(),
             emit_ssa: None,

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

@@ -20,8 +20,8 @@ mod rc;
 mod remove_bit_shifts;
 mod remove_enable_side_effects;
 mod remove_if_else;
+mod remove_unreachable;
 mod resolve_is_unconstrained;
-mod runtime_separation;
 mod simplify_cfg;
 mod unrolling;
 

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

@@ -181,7 +181,9 @@ impl IdMaps {
             }
 
             Value::Function(id) => {
-                let new_id = self.function_ids[id];
+                let new_id = *self.function_ids.get(id).unwrap_or_else(|| {
+                    unreachable!("Unmapped function with id {id}")
+                });
                 new_function.dfg.import_function(new_id)
             }
 

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

@@ -0,0 +1,80 @@
+use std::collections::BTreeSet;
+
+use fxhash::FxHashSet as HashSet;
+
+use crate::ssa::{
+    ir::{
+        function::{Function, FunctionId},
+        instruction::Instruction,
+        value::Value,
+    },
+    ssa_gen::Ssa,
+};
+
+impl Ssa {
+    /// Removes any unreachable functions from the code. These can result from
+    /// optimizations making existing functions unreachable, e.g. `if false { foo() }`,
+    /// or even from monomorphizing an unconstrained version of a constrained function
+    /// where the original constrained version ends up never being used.
+    pub(crate) fn remove_unreachable_functions(mut self) -> Self {
+        let mut used_functions = HashSet::default();
+
+        for function_id in self.functions.keys() {
+            if self.is_entry_point(*function_id) {
+                collect_reachable_functions(&self, *function_id, &mut used_functions);
+            }
+        }
+
+        self.functions.retain(|id, _| used_functions.contains(id));
+        self
+    }
+}
+
+fn collect_reachable_functions(
+    ssa: &Ssa,
+    current_func_id: FunctionId,
+    reachable_functions: &mut HashSet<FunctionId>,
+) {
+    if reachable_functions.contains(&current_func_id) {
+        return;
+    }
+    reachable_functions.insert(current_func_id);
+
+    // If the debugger is used, its possible for function inlining
+    // to remove functions that the debugger still references
+    let Some(func) = ssa.functions.get(&current_func_id) else {
+        return;
+    };
+
+    let used_functions = used_functions(func);
+
+    for called_func_id in used_functions.iter() {
+        collect_reachable_functions(ssa, *called_func_id, reachable_functions);
+    }
+}
+
+fn used_functions(func: &Function) -> BTreeSet<FunctionId> {
+    let mut used_function_ids = BTreeSet::default();
+
+    let mut find_functions = |value| {
+        if let Value::Function(function) = func.dfg[func.dfg.resolve(value)] {
+            used_function_ids.insert(function);
+        }
+    };
+
+    for block_id in func.reachable_blocks() {
+        let block = &func.dfg[block_id];
+
+        for instruction_id in block.instructions() {
+            let instruction = &func.dfg[*instruction_id];
+
+            if matches!(instruction, Instruction::Store { .. } | Instruction::Call { .. }) {
+                instruction.for_each_value(&mut find_functions);
+            }
+        }
+
+        block.unwrap_terminator().for_each_value(&mut find_functions);
+    }
+
+    used_function_ids
+}