perf/fix: assume variable as zero constant when subtracting from itse…

…lf (#1089)

* test: add test case for no-op subtraction

* fix: mark value as constant when coefficient zero

* test: add expected constant equality assertion

* perf: direct equality check for constants

* fix: return constant zero if coeff zero

* fix: test assert to zero

frontend/cs/r1cs/api_assertions.go

@@ -28,6 +28,15 @@ import (
 
 // AssertIsEqual adds an assertion in the constraint builder (i1 == i2)
 func (builder *builder) AssertIsEqual(i1, i2 frontend.Variable) {
+	c1, i1Constant := builder.constantValue(i1)
+	c2, i2Constant := builder.constantValue(i2)
+
+	if i1Constant && i2Constant {
+		if c1 != c2 {
+			panic("non-equal constant values")
+		}
+		return
+	}
 	// encoded 1 * i1 == i2
 	r := builder.getLinearExpression(builder.toVariable(i1))
 	o := builder.getLinearExpression(builder.toVariable(i2))

frontend/cs/r1cs/builder.go

@@ -308,6 +308,9 @@ func (builder *builder) constantValue(v frontend.Variable) (constraint.Element,
 			// and are always reduced to one element. may not always be true?
 			return constraint.Element{}, false
 		}
+		if _v[0].Coeff.IsZero() {
+			return constraint.Element{}, true
+		}
 		if !(_v[0].WireID() == 0) { // public ONE WIRE
 			return constraint.Element{}, false
 		}

frontend/cs/r1cs/r1cs_test.go

@@ -160,3 +160,23 @@ func TestPreCompileHook(t *testing.T) {
 		t.Error("callback not called")
 	}
 }
+
+type subSameNoConstraintCircuit struct {
+	A frontend.Variable
+}
+
+func (c *subSameNoConstraintCircuit) Define(api frontend.API) error {
+	r := api.Sub(c.A, c.A)
+	api.AssertIsEqual(r, 0)
+	return nil
+}
+
+func TestSubSameNoConstraint(t *testing.T) {
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), NewBuilder, &subSameNoConstraintCircuit{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ccs.GetNbConstraints() != 0 {
+		t.Fatal("expected 0 constraints")
+	}
+}

frontend/cs/scs/api_test.go

@@ -238,3 +238,22 @@ func TestMulAccFastTrack(t *testing.T) {
 	assert.NoError(err)
 	_ = solution
 }
+
+type subSameNoConstraintCircuit struct {
+	A frontend.Variable
+}
+
+func (c *subSameNoConstraintCircuit) Define(api frontend.API) error {
+	r := api.Sub(c.A, c.A)
+	api.AssertIsEqual(r, 0)
+	return nil
+}
+
+func TestSubSameNoConstraint(t *testing.T) {
+	assert := test.NewAssert(t)
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &subSameNoConstraintCircuit{})
+	assert.NoError(err)
+	if ccs.GetNbConstraints() != 0 {
+		t.Fatal("expected 0 constraints")
+	}
+}

frontend/cs/scs/builder.go

@@ -312,7 +312,10 @@ func (builder *builder) ConstantValue(v frontend.Variable) (*big.Int, bool) {
 }
 
 func (builder *builder) constantValue(v frontend.Variable) (constraint.Element, bool) {
-	if _, ok := v.(expr.Term); ok {
+	if vv, ok := v.(expr.Term); ok {
+		if vv.Coeff.IsZero() {
+			return constraint.Element{}, true
+		}
 		return constraint.Element{}, false
 	}
 	return builder.cs.FromInterface(v), true