From 77154750c42b25d6265dc4fae6bda681334b28a8 Mon Sep 17 00:00:00 2001
From: Tom French <tom@tomfren.ch>
Date: Fri, 22 Sep 2023 01:12:48 +0100
Subject: [PATCH] chore: remove redundant predicate from brillig quotients

---
 .../brillig/brillig_gen/brillig_directive.rs  | 37 ++++++-------------
 .../ssa/acir_gen/acir_ir/generated_acir.rs    |  6 +--
 2 files changed, 13 insertions(+), 30 deletions(-)

diff --git a/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_directive.rs b/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_directive.rs
index ee7bbeed46e..ea4ef0aa8cb 100644
--- a/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_directive.rs
+++ b/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_directive.rs
@@ -39,40 +39,37 @@ pub(crate) fn directive_invert() -> GeneratedBrillig {
 }
 
 /// Generates brillig bytecode which computes `a / b` and returns the quotient and remainder.
-/// It returns `(0,0)` if the predicate is null.
-///
 ///
 /// This is equivalent to the Noir (psuedo)code
 ///
 /// ```ignore
-/// fn quotient<T>(a: T, b: T, predicate: bool) -> (T,T) {
-///    if predicate != 0 {
-///      (a/b, a-a/b*b)
-///    } else {
-///      (0,0)
-///    }
+/// fn quotient<T>(a: T, b: T) -> (T,T) {
+///    (a/b, a-a/b*b)
 /// }
 /// ```
+///
+/// This bytecode explicitly does not handle the case where `b == 0`. This is because the [brillig opcode][brillig]
+/// itself contains a predicate which can handle this case. If `b == 0` then this predicate should be set to zero,
+/// which will result in the return values being set to (0, 0).
+///
+/// [brillig]: acvm::acir::circuit::brillig::Brillig
 pub(crate) fn directive_quotient(bit_size: u32) -> GeneratedBrillig {
     // `a` is (0) (i.e register index 0)
     // `b` is (1)
-    // `predicate` is (2)
     GeneratedBrillig {
         byte_code: vec![
-            // If the predicate is zero, we jump to the exit segment
-            BrilligOpcode::JumpIfNot { condition: RegisterIndex::from(2), location: 6 },
-            //q = a/b is set into register (3)
+            //q = a/b is set into register (2)
             BrilligOpcode::BinaryIntOp {
                 op: BinaryIntOp::UnsignedDiv,
                 lhs: RegisterIndex::from(0),
                 rhs: RegisterIndex::from(1),
-                destination: RegisterIndex::from(3),
+                destination: RegisterIndex::from(2),
                 bit_size,
             },
             //(1)= q*b
             BrilligOpcode::BinaryIntOp {
                 op: BinaryIntOp::Mul,
-                lhs: RegisterIndex::from(3),
+                lhs: RegisterIndex::from(2),
                 rhs: RegisterIndex::from(1),
                 destination: RegisterIndex::from(1),
                 bit_size,
@@ -88,17 +85,7 @@ pub(crate) fn directive_quotient(bit_size: u32) -> GeneratedBrillig {
             //(0) = q
             BrilligOpcode::Mov {
                 destination: RegisterIndex::from(0),
-                source: RegisterIndex::from(3),
-            },
-            BrilligOpcode::Stop,
-            // Exit segment: we return 0,0
-            BrilligOpcode::Const {
-                destination: RegisterIndex::from(0),
-                value: Value::from(0_usize),
-            },
-            BrilligOpcode::Const {
-                destination: RegisterIndex::from(1),
-                value: Value::from(0_usize),
+                source: RegisterIndex::from(2),
             },
             BrilligOpcode::Stop,
         ],
diff --git a/compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs b/compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs
index 0adb14246b7..98180b51635 100644
--- a/compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs
+++ b/compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs
@@ -544,11 +544,7 @@ impl GeneratedAcir {
         let r_witness = self.next_witness_index();
 
         let quotient_code = brillig_directive::directive_quotient(max_bit_size);
-        let inputs = vec![
-            BrilligInputs::Single(lhs),
-            BrilligInputs::Single(rhs),
-            BrilligInputs::Single(predicate.clone()),
-        ];
+        let inputs = vec![BrilligInputs::Single(lhs), BrilligInputs::Single(rhs)];
         let outputs = vec![BrilligOutputs::Simple(q_witness), BrilligOutputs::Simple(r_witness)];
         self.brillig(Some(predicate), quotient_code, inputs, outputs);