feat(ssa): Immediately simplify away RefCount instructions in ACIR functions

compiler/noirc_evaluator/src/acir/mod.rs

@@ -769,7 +769,8 @@ impl<'a> Context<'a> {
                 unreachable!("Expected all load instructions to be removed before acir_gen")
             }
             Instruction::IncrementRc { .. } | Instruction::DecrementRc { .. } => {
-                // Do nothing. Only Brillig needs to worry about reference counted arrays
+                // Only Brillig needs to worry about reference counted arrays
+                unreachable!("Expected all Rc instructions to be removed before acir_gen")
             }
             Instruction::RangeCheck { value, max_bit_size, assert_message } => {
                 let acir_var = self.convert_numeric_value(*value, dfg)?;

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

@@ -5,7 +5,7 @@ use crate::ssa::{function_builder::data_bus::DataBus, ir::instruction::SimplifyR
 use super::{
     basic_block::{BasicBlock, BasicBlockId},
     call_stack::{CallStack, CallStackHelper, CallStackId},
-    function::FunctionId,
+    function::{FunctionId, RuntimeType},
     instruction::{
         Instruction, InstructionId, InstructionResultType, Intrinsic, TerminatorInstruction,
     },
@@ -26,8 +26,13 @@ use serde_with::DisplayFromStr;
 /// owning most data in a function and handing out Ids to this data that can be
 /// shared without worrying about ownership.
 #[serde_as]
-#[derive(Debug, Default, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub(crate) struct DataFlowGraph {
+    /// Runtime of the [Function] that owns this [DataFlowGraph].
+    /// This might change during the `runtime_separation` pass where
+    /// ACIR functions are cloned as Brillig functions.
+    runtime: RuntimeType,
+
     /// All of the instructions in a function
     instructions: DenseMap<Instruction>,
 
@@ -100,6 +105,16 @@ pub(crate) struct DataFlowGraph {
 }
 
 impl DataFlowGraph {
+    /// Runtime type of the function.
+    pub(crate) fn runtime(&self) -> RuntimeType {
+        self.runtime
+    }
+
+    /// Set runtime type of the function.
+    pub(crate) fn set_runtime(&mut self, runtime: RuntimeType) {
+        self.runtime = runtime;
+    }
+
     /// Creates a new basic block with no parameters.
     /// After being created, the block is unreachable in the current function
     /// until another block is made to jump to it.
@@ -164,6 +179,11 @@ impl DataFlowGraph {
         id
     }
 
+    /// Check if the function runtime would simply ignore this instruction.
+    pub(crate) fn is_handled_by_runtime(&self, instruction: &Instruction) -> bool {
+        !(self.runtime().is_acir() && instruction.is_brillig_only())
+    }
+
     fn insert_instruction_without_simplification(
         &mut self,
         instruction_data: Instruction,
@@ -184,6 +204,10 @@ impl DataFlowGraph {
         ctrl_typevars: Option<Vec<Type>>,
         call_stack: CallStackId,
     ) -> InsertInstructionResult {
+        if !self.is_handled_by_runtime(&instruction_data) {
+            return InsertInstructionResult::InstructionRemoved;
+        }
+
         let id = self.insert_instruction_without_simplification(
             instruction_data,
             block,
@@ -194,14 +218,18 @@ impl DataFlowGraph {
         InsertInstructionResult::Results(id, self.instruction_results(id))
     }
 
-    /// Inserts a new instruction at the end of the given block and returns its results
+    /// Simplifies a new instruction and inserts it at the end of the given block and returns its results.
+    /// If the instruction is not handled by the current runtime, `InstructionRemoved` is returned.
     pub(crate) fn insert_instruction_and_results(
         &mut self,
         instruction: Instruction,
         block: BasicBlockId,
         ctrl_typevars: Option<Vec<Type>>,
         call_stack: CallStackId,
     ) -> InsertInstructionResult {
+        if !self.is_handled_by_runtime(&instruction) {
+            return InsertInstructionResult::InstructionRemoved;
+        }
         match instruction.simplify(self, block, ctrl_typevars.clone(), call_stack) {
             SimplifyResult::SimplifiedTo(simplification) => {
                 InsertInstructionResult::SimplifiedTo(simplification)

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

@@ -56,6 +56,12 @@ impl RuntimeType {
     }
 }
 
+impl Default for RuntimeType {
+    fn default() -> Self {
+        RuntimeType::Acir(InlineType::default())
+    }
+}
+
 /// A function holds a list of instructions.
 /// These instructions are further grouped into Basic blocks
 ///
@@ -72,8 +78,6 @@ pub(crate) struct Function {
 
     id: FunctionId,
 
-    runtime: RuntimeType,
-
     /// The DataFlowGraph holds the majority of data pertaining to the function
     /// including its blocks, instructions, and values.
     pub(crate) dfg: DataFlowGraph,
@@ -86,20 +90,20 @@ impl Function {
     pub(crate) fn new(name: String, id: FunctionId) -> Self {
         let mut dfg = DataFlowGraph::default();
         let entry_block = dfg.make_block();
-        Self { name, id, entry_block, dfg, runtime: RuntimeType::Acir(InlineType::default()) }
+        Self { name, id, entry_block, dfg }
     }
 
     /// Creates a new function as a clone of the one passed in with the passed in id.
     pub(crate) fn clone_with_id(id: FunctionId, another: &Function) -> Self {
         let dfg = another.dfg.clone();
         let entry_block = another.entry_block;
-        Self { name: another.name.clone(), id, entry_block, dfg, runtime: another.runtime }
+        Self { name: another.name.clone(), id, entry_block, dfg }
     }
 
     /// Takes the signature (function name & runtime) from a function but does not copy the body.
     pub(crate) fn clone_signature(id: FunctionId, another: &Function) -> Self {
         let mut new_function = Function::new(another.name.clone(), id);
-        new_function.runtime = another.runtime;
+        new_function.set_runtime(another.runtime());
         new_function
     }
 
@@ -116,12 +120,12 @@ impl Function {
 
     /// Runtime type of the function.
     pub(crate) fn runtime(&self) -> RuntimeType {
-        self.runtime
+        self.dfg.runtime()
     }
 
     /// Set runtime type of the function.
     pub(crate) fn set_runtime(&mut self, runtime: RuntimeType) {
-        self.runtime = runtime;
+        self.dfg.set_runtime(runtime);
     }
 
     pub(crate) fn is_no_predicates(&self) -> bool {

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

@@ -1056,6 +1056,12 @@ impl Instruction {
             Instruction::Noop => Remove,
         }
     }
+
+    /// Some instructions are only to be used in Brillig and should be eliminated
+    /// after runtime separation, never to be be reintroduced in an ACIR runtime.
+    pub(crate) fn is_brillig_only(&self) -> bool {
+        matches!(self, Instruction::IncrementRc { .. } | Instruction::DecrementRc { .. })
+    }
 }
 
 /// Given a chain of operations like:

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

@@ -20,10 +20,10 @@ pub(crate) type ValueId = Id<Value>;
 pub(crate) enum Value {
     /// This value was created due to an instruction
     ///
-    /// instruction -- This is the instruction which defined it
-    /// typ -- This is the `Type` of the instruction
-    /// position -- Returns the position in the results
-    /// vector that this `Value` is located.
+    /// * `instruction`: This is the instruction which defined it
+    /// * `typ`: This is the `Type` of the instruction
+    /// * `position`: Returns the position in the results vector that this `Value` is located.
+    ///
     /// Example, if you add two numbers together, then the resulting
     /// value would have position `0`, the typ would be the type
     /// of the operands, and the instruction would map to an add instruction.

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

@@ -830,9 +830,12 @@ fn simplify(dfg: &DataFlowGraph, lhs: ValueId, rhs: ValueId) -> Option<(ValueId,
 mod test {
     use std::sync::Arc;
 
+    use noirc_frontend::monomorphization::ast::InlineType;
+
     use crate::ssa::{
         function_builder::FunctionBuilder,
         ir::{
+            function::RuntimeType,
             map::Id,
             types::{NumericType, Type},
         },
@@ -1153,6 +1156,7 @@ mod test {
 
         // Compiling main
         let mut builder = FunctionBuilder::new("main".into(), main_id);
+        builder.set_runtime(RuntimeType::Brillig(InlineType::default()));
         let v0 = builder.add_parameter(Type::unsigned(64));
         let zero = builder.numeric_constant(0u128, NumericType::unsigned(64));
         let typ = Type::Array(Arc::new(vec![Type::unsigned(64)]), 25);

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

@@ -246,6 +246,7 @@ mod test {
         // }
         let main_id = Id::test_new(0);
         let mut builder = FunctionBuilder::new("mutator".into(), main_id);
+        builder.set_runtime(RuntimeType::Brillig(InlineType::default()));
 
         let array_type = Type::Array(Arc::new(vec![Type::field()]), 2);
         let v0 = builder.add_parameter(array_type.clone());
@@ -295,6 +296,7 @@ mod test {
         // }
         let main_id = Id::test_new(0);
         let mut builder = FunctionBuilder::new("mutator2".into(), main_id);
+        builder.set_runtime(RuntimeType::Brillig(InlineType::default()));
 
         let array_type = Type::Array(Arc::new(vec![Type::field()]), 2);
         let reference_type = Type::Reference(Arc::new(array_type.clone()));

compiler/noirc_evaluator/src/ssa/parser/into_ssa.rs

@@ -59,8 +59,8 @@ impl Translator {
         let main_function = parsed_ssa.functions.remove(0);
         let main_id = FunctionId::test_new(0);
         let mut builder = FunctionBuilder::new(main_function.external_name.clone(), main_id);
-        builder.simplify = simplify;
         builder.set_runtime(main_function.runtime_type);
+        builder.simplify = simplify;
 
         // Map function names to their IDs so calls can be resolved
         let mut function_id_counter = 1;