chore(ssa refactoring): Support basic recursive functions (#1387)

* Support basic recursive functions

* Add comment

crates/noirc_evaluator/src/ssa_refactor/opt/inlining.rs

@@ -404,9 +404,25 @@ impl<'function> PerFunctionContext<'function> {
             }
             TerminatorInstruction::JmpIf { condition, then_destination, else_destination } => {
                 let condition = self.translate_value(*condition);
-                let then_block = self.translate_block(*then_destination, block_queue);
-                let else_block = self.translate_block(*else_destination, block_queue);
-                self.context.builder.terminate_with_jmpif(condition, then_block, else_block);
+
+                // See if the value of the condition is known, and if so only inline the reachable
+                // branch. This lets us inline some recursive functions without recurring forever.
+                let dfg = &mut self.context.builder.current_function.dfg;
+                match dfg.get_numeric_constant(condition) {
+                    Some(constant) => {
+                        let next_block =
+                            if constant.is_zero() { *else_destination } else { *then_destination };
+                        let next_block = self.translate_block(next_block, block_queue);
+                        self.context.builder.terminate_with_jmp(next_block, vec![]);
+                    }
+                    None => {
+                        let then_block = self.translate_block(*then_destination, block_queue);
+                        let else_block = self.translate_block(*else_destination, block_queue);
+                        self.context
+                            .builder
+                            .terminate_with_jmpif(condition, then_block, else_block);
+                    }
+                }
                 None
             }
             TerminatorInstruction::Return { return_values } => {
@@ -424,7 +440,11 @@ impl<'function> PerFunctionContext<'function> {
 #[cfg(test)]
 mod test {
     use crate::ssa_refactor::{
-        ir::{instruction::BinaryOp, map::Id, types::Type},
+        ir::{
+            instruction::{BinaryOp, TerminatorInstruction},
+            map::Id,
+            types::Type,
+        },
         ssa_builder::FunctionBuilder,
     };
 
@@ -524,4 +544,84 @@ mod test {
         let inlined = ssa.inline_functions();
         assert_eq!(inlined.functions.len(), 1);
     }
+
+    #[test]
+    fn recursive_functions() {
+        // fn main f0 {
+        //   b0():
+        //     v0 = call factorial(Field 5)
+        //     return v0
+        // }
+        // fn factorial f1 {
+        //   b0(v0: Field):
+        //     v1 = lt v0, Field 1
+        //     jmpif v1, then: b1, else: b2
+        //   b1():
+        //     return Field 1
+        //   b2():
+        //     v2 = sub v0, Field 1
+        //     v3 = call factorial(v2)
+        //     v4 = mul v0, v3
+        //     return v4
+        // }
+        let main_id = Id::test_new(0);
+        let mut builder = FunctionBuilder::new("main".into(), main_id);
+
+        let factorial_id = Id::test_new(1);
+        let factorial = builder.import_function(factorial_id);
+
+        let five = builder.field_constant(5u128);
+        let results = builder.insert_call(factorial, vec![five], vec![Type::field()]).to_vec();
+        builder.terminate_with_return(results);
+
+        builder.new_function("factorial".into(), factorial_id);
+        let b1 = builder.insert_block();
+        let b2 = builder.insert_block();
+
+        let one = builder.field_constant(1u128);
+
+        let v0 = builder.add_parameter(Type::field());
+        let v1 = builder.insert_binary(v0, BinaryOp::Lt, one);
+        builder.terminate_with_jmpif(v1, b1, b2);
+
+        builder.switch_to_block(b1);
+        builder.terminate_with_return(vec![one]);
+
+        builder.switch_to_block(b2);
+        let factorial_id = builder.import_function(factorial_id);
+        let v2 = builder.insert_binary(v0, BinaryOp::Sub, one);
+        let v3 = builder.insert_call(factorial_id, vec![v2], vec![Type::field()])[0];
+        let v4 = builder.insert_binary(v0, BinaryOp::Mul, v3);
+        builder.terminate_with_return(vec![v4]);
+
+        let ssa = builder.finish();
+        assert_eq!(ssa.functions.len(), 2);
+
+        // Expected SSA:
+        //
+        // fn main f2 {
+        //   b0():
+        //     jmp b1()
+        //   b1():
+        //     return Field 120
+        // }
+        let inlined = ssa.inline_functions();
+        assert_eq!(inlined.functions.len(), 1);
+
+        let main = inlined.main();
+        let b1 = &main.dfg[b1];
+
+        match b1.terminator() {
+            Some(TerminatorInstruction::Return { return_values }) => {
+                assert_eq!(return_values.len(), 1);
+                let value = main
+                    .dfg
+                    .get_numeric_constant(return_values[0])
+                    .expect("Expected a constant for the return value")
+                    .to_u128();
+                assert_eq!(value, 120);
+            }
+            other => unreachable!("Unexpected terminator {other:?}"),
+        }
+    }
 }