Merge pull request #1068 from Consensys/fix/recorded-scs

fix: scs add/mul when recorded constraint is 0
Consensys · Mar 5, 2024 · 22d2c33 · 22d2c33
2 parents 7cfcd5a + 833fd73
commit 22d2c33
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Showing 2 changed files with 71 additions and 5 deletions.
diff --git a/frontend/cs/scs/api.go b/frontend/cs/scs/api.go
@@ -138,6 +138,14 @@ func (builder *builder) Mul(i1, i2 frontend.Variable, in ...frontend.Variable) f
 	if len(vars) == 0 {
 		return builder.cs.ToBigInt(k)
 	}
+	if k.IsZero() {
+		return 0
+	}
+	for i := range vars {
+		if vars[i].Coeff.IsZero() {
+			return 0
+		}
+	}
 	l := builder.mulConstant(vars[0], k)
 
 	return builder.splitProd(l, vars[1:])

diff --git a/frontend/cs/scs/api_test.go b/frontend/cs/scs/api_test.go
@@ -135,9 +135,6 @@ func (c *IssueDiv0Circuit) Define(api frontend.API) error {
 func TestExistDiv0(t *testing.T) {
 	assert := test.NewAssert(t)
 	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &IssueDiv0Circuit{})
-	if err != nil {
-		t.Fatal(err)
-	}
 	assert.NoError(err)
 	w, err := frontend.NewWitness(&IssueDiv0Circuit{
 		A1: 11, B1: 21,
@@ -150,9 +147,70 @@ func TestExistDiv0(t *testing.T) {
 		Res7: 0, Res8: 55,
 	}, ecc.BN254.ScalarField())
 	assert.NoError(err)
-	solution, err := ccs.Solve(w)
+	_, err = ccs.Solve(w)
 	assert.NoError(err)
-	_ = solution
+}
+
+type IssueDiv0Circuit2 struct {
+	A1, B1 frontend.Variable
+
+	Res1, Res2 frontend.Variable
+}
+
+func (c *IssueDiv0Circuit2) Define(api frontend.API) error {
+	// case 1
+	b1 := api.Mul(0, c.A1)
+	b2 := api.Mul(4, c.B1)
+	t1 := api.Mul(b1, b2)
+
+	b3 := api.Mul(2, c.A1)
+	b4 := api.Mul(5, c.B1)
+	t2 := api.Mul(b3, b4)
+
+	// test solver
+	api.AssertIsEqual(t1, c.Res1)
+	api.AssertIsEqual(t2, c.Res2)
+	return nil
+}
+
+func TestExistDiv02(t *testing.T) {
+	assert := test.NewAssert(t)
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &IssueDiv0Circuit2{})
+	assert.NoError(err)
+	w, err := frontend.NewWitness(&IssueDiv0Circuit2{
+		A1: 11, B1: 21,
+		Res1: 0, Res2: 2310,
+	}, ecc.BN254.ScalarField())
+	assert.NoError(err)
+	_, err = ccs.Solve(w)
+	assert.NoError(err)
+}
+
+type TestZeroMulNoConstraintCircuit struct {
+	A, B frontend.Variable
+}
+
+func (c *TestZeroMulNoConstraintCircuit) Define(api frontend.API) error {
+	// case 1
+	t1 := api.Mul(0, c.A)
+	t2 := api.Mul(t1, c.B)
+
+	t3 := api.Sub(c.A, c.A)
+	t4 := api.Mul(3, t3)
+
+	// test solver
+	api.AssertIsEqual(t2, 0)
+	api.AssertIsEqual(t4, 0)
+	return nil
+}
+
+func TestZeroMulNoConstraint(t *testing.T) {
+	assert := test.NewAssert(t)
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &TestZeroMulNoConstraintCircuit{})
+	assert.NoError(err)
+	if ccs.GetNbConstraints() != 0 {
+		t.Fatal("expected 0 constraints")
+	}
 }
 
 type mulAccFastTrackCircuit struct {