feat(acvm)!: Simplification pass for ACIR (#151)

* Simplification pass for ACIR

* update comments

* Apply simplification during fallback

* Add unit test

* Move simplifier to optimiser module

* Fix unit tests

* make simplify public

acir/src/circuit/directives.rs

@@ -6,6 +6,15 @@ use crate::{
 };
 use serde::{Deserialize, Serialize};
 
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct QuotientDirective {
+    pub a: Expression,
+    pub b: Expression,
+    pub q: Witness,
+    pub r: Witness,
+    pub predicate: Option<Expression>,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 /// Directives do not apply any constraints.
 /// You can think of them as opcodes that allow one to use non-determinism
@@ -18,13 +27,7 @@ pub enum Directive {
     },
 
     //Performs euclidian division of a / b (as integers) and stores the quotient in q and the rest in r
-    Quotient {
-        a: Expression,
-        b: Expression,
-        q: Witness,
-        r: Witness,
-        predicate: Option<Expression>,
-    },
+    Quotient(QuotientDirective),
 
     //decomposition of a: a=\sum b[i]*radix^i where b is an array of witnesses < radix in little endian form
     ToLeRadix {
@@ -48,7 +51,7 @@ impl Directive {
     pub fn name(&self) -> &str {
         match self {
             Directive::Invert { .. } => "invert",
-            Directive::Quotient { .. } => "quotient",
+            Directive::Quotient(_) => "quotient",
             Directive::ToLeRadix { .. } => "to_le_radix",
             Directive::PermutationSort { .. } => "permutation_sort",
             Directive::Log { .. } => "log",
@@ -71,7 +74,7 @@ impl Directive {
                 write_u32(&mut writer, x.witness_index())?;
                 write_u32(&mut writer, result.witness_index())?;
             }
-            Directive::Quotient { a, b, q, r, predicate } => {
+            Directive::Quotient(QuotientDirective { a, b, q, r, predicate }) => {
                 a.write(&mut writer)?;
                 b.write(&mut writer)?;
                 write_u32(&mut writer, q.witness_index())?;
@@ -151,7 +154,7 @@ impl Directive {
                     false => None,
                 };
 
-                Ok(Directive::Quotient { a, b, q, r, predicate })
+                Ok(Directive::Quotient(QuotientDirective { a, b, q, r, predicate }))
             }
             2 => {
                 let a = Expression::read(&mut reader)?;
@@ -249,20 +252,20 @@ fn serialization_roundtrip() {
     // TODO: Find a way to ensure that we include all of the variants
     let invert = Directive::Invert { x: Witness(10), result: Witness(10) };
 
-    let quotient_none = Directive::Quotient {
+    let quotient_none = Directive::Quotient(QuotientDirective {
         a: Expression::default(),
         b: Expression::default(),
         q: Witness(1u32),
         r: Witness(2u32),
         predicate: None,
-    };
-    let quotient_predicate = Directive::Quotient {
+    });
+    let quotient_predicate = Directive::Quotient(QuotientDirective {
         a: Expression::default(),
         b: Expression::default(),
         q: Witness(1u32),
         r: Witness(2u32),
         predicate: Some(Expression::default()),
-    };
+    });
 
     let to_le_radix = Directive::ToLeRadix {
         a: Expression::default(),

acir/src/circuit/opcodes.rs

@@ -1,6 +1,6 @@
 use std::io::{Read, Write};
 
-use super::directives::{Directive, LogInfo};
+use super::directives::{Directive, LogInfo, QuotientDirective};
 use crate::native_types::Expression;
 use crate::serialization::{read_n, write_bytes};
 
@@ -149,7 +149,7 @@ impl std::fmt::Display for Opcode {
                 write!(f, "DIR::INVERT ")?;
                 write!(f, "(_{}, out: _{}) ", x.witness_index(), r.witness_index())
             }
-            Opcode::Directive(Directive::Quotient { a, b, q, r, predicate }) => {
+            Opcode::Directive(Directive::Quotient(QuotientDirective { a, b, q, r, predicate })) => {
                 write!(f, "DIR::QUOTIENT ")?;
                 if let Some(pred) = predicate {
                     writeln!(f, "PREDICATE = {pred}")?;

acir/src/native_types/expression/mod.rs

@@ -187,6 +187,10 @@ impl Expression {
         self.is_linear() && self.linear_combinations.len() == 1
     }
 
+    pub fn is_zero(&self) -> bool {
+        *self == Self::zero()
+    }
+
     /// Returns a `FieldElement` if the expression represents a constant polynomial.
     /// Otherwise returns `None`.
     ///

acvm/src/compiler.rs

@@ -14,6 +14,7 @@ use thiserror::Error;
 use transformers::{CSatTransformer, FallbackTransformer, IsOpcodeSupported, R1CSTransformer};
 
 use self::optimizers::RangeOptimizer;
+use self::optimizers::Simplifier;
 
 #[derive(PartialEq, Eq, Debug, Error)]
 pub enum CompileError {
@@ -25,13 +26,14 @@ pub fn compile(
     acir: Circuit,
     np_language: Language,
     is_opcode_supported: IsOpcodeSupported,
+    simplifier: &Simplifier,
 ) -> Result<Circuit, CompileError> {
     // Instantiate the optimizer.
     // Currently the optimizer and reducer are one in the same
     // for CSAT
 
     // Fallback transformer pass
-    let acir = FallbackTransformer::transform(acir, is_opcode_supported)?;
+    let acir = FallbackTransformer::transform(acir, is_opcode_supported, simplifier)?;
 
     // General optimizer pass
     let mut opcodes: Vec<Opcode> = Vec::new();

acvm/src/compiler/optimizers/mod.rs

@@ -1,5 +1,7 @@
 mod general;
 mod redundant_range;
+pub mod simplify;
 
 pub(crate) use general::GeneralOpt as GeneralOptimizer;
 pub(crate) use redundant_range::RangeOptimizer;
+pub(crate) use simplify::CircuitSimplifier as Simplifier;