Repository: ConsenSys/mythril-classic (python)

Issue:
Independence optimization
## Description
Add independence-optimization for constraint solving. 
When a state with constraint set C needs to add a new constraint c’, instead of passing C ∧ c’ to the constraint solver to test satisfiability, pass R(C, c’) ∧ c’ where the expression R(C, c’)  returns only the constraints in C that have symbolic variables also in c’.

### Benefits
This optimization reduces the difficulty of the query and also increases constraint cache hits.

Pull request description:
Independence optimization
closes #232 

Patch:
diff --git a/mythril/laser/smt/__init__.py b/mythril/laser/smt/__init__.py
index 9cb81244..98f52d7e 100644
--- a/mythril/laser/smt/__init__.py
+++ b/mythril/laser/smt/__init__.py
@@ -18,6 +18,7 @@ from mythril.laser.smt.expression import Expression, simplify
 from mythril.laser.smt.bool import Bool, is_true, is_false, Or, Not, And
 from mythril.laser.smt.array import K, Array, BaseArray
 from mythril.laser.smt.solver import Solver, Optimize
+from mythril.laser.smt.model import Model
 
 from typing import Union, Any, Optional, List, TypeVar, Generic
 import z3
diff --git a/mythril/laser/smt/independence_solver.py b/mythril/laser/smt/independence_solver.py
new file mode 100644
index 00000000..781a26fd
--- /dev/null
+++ b/mythril/laser/smt/independence_solver.py
@@ -0,0 +1,150 @@
+import z3
+
+from mythril.laser.smt.model import Model
+from mythril.laser.smt.bool import Bool
+from typing import Set, Tuple, Dict, List, cast
+
+
+def _get_expr_variables(expression: z3.ExprRef) -> List[z3.ExprRef]:
+    """
+    Gets the variables that make up the current expression
+    :param expression:
+    :return:
+    """
+    result = []
+    if not expression.children() and not isinstance(expression, z3.BitVecNumRef):
+        result.append(expression)
+    for child in expression.children():
+        c_children = _get_expr_variables(child)
+        result.extend(c_children)
+    return result
+
+
+class DependenceBucket:
+    """ Bucket object to contain a set of conditions that are dependent on each other """
+
+    def __init__(self, variables=None, conditions=None):
+        """
+        Initializes a DependenceBucket object
+        :param variables: Variables contained in the conditions
+        :param conditions: The conditions that are dependent on each other
+        """
+        self.variables = variables or []  # type: List[z3.ExprRef]
+        self.conditions = conditions or []  # type: List[z3.ExprRef]
+
+
+class DependenceMap:
+    """ DependenceMap object that maintains a set of dependence buckets, used to separate independent smt queries """
+
+    def __init__(self):
+        """ Initializes a DependenceMap object """
+        self.buckets = []  # type: List[DependenceBucket]
+        self.variable_map = {}  # type: Dict[str, DependenceBucket]
+
+    def add_condition(self, condition: z3.BoolRef) -> None:
+        """
+        Add condition to the dependence map
+        :param condition: The condition that is to be added to the dependence map
+        """
+        variables = set(_get_expr_variables(condition))
+        relevant_buckets = set()
+        for variable in variables:
+            try:
+                bucket = self.variable_map[str(variable)]
+                relevant_buckets.add(bucket)
+            except KeyError:
+                continue
+
+        new_bucket = DependenceBucket(variables, [condition])
+        self.buckets.append(new_bucket)
+
+        if relevant_buckets:
+            # Merge buckets, and rewrite variable map accordingly
+            relevant_buckets.add(new_bucket)
+            new_bucket = self._merge_buckets(relevant_buckets)
+
+        for variable in variables:
+            self.variable_map[str(variable)] = new_bucket
+
+    def _merge_buckets(self, bucket_list: Set[DependenceBucket]) -> DependenceBucket:
+        """ Merges the buckets in bucket list """
+        variables = []  # type: List[str]
+        conditions = []  # type: List[z3.BoolRef]
+        for bucket in bucket_list:
+            self.buckets.remove(bucket)
+            variables += bucket.variables
+            conditions += bucket.conditions
+
+        new_bucket = DependenceBucket(variables, conditions)
+        self.buckets.append(new_bucket)
+
+        return new_bucket
+
+
+class IndependenceSolver:
+    """An SMT solver object that uses independence optimization"""
+
+    def __init__(self):
+        """"""
+        self.raw = z3.Solver()
+        self.constraints = []
+        self.models = []
+
+    def set_timeout(self, timeout: int) -> None:
+        """Sets the timeout that will be used by this solver, timeout is in
+        milliseconds.
+
+        :param timeout:
+        """
+        self.raw.set(timeout=timeout)
+
+    def add(self, *constraints: Tuple[Bool]) -> None:
+        """Adds the constraints to this solver.
+
+        :param constraints: constraints to add
+        """
+        raw_constraints = [
+            c.raw for c in cast(Tuple[Bool], constraints)
+        ]  # type: List[z3.BoolRef]
+        self.constraints.extend(raw_constraints)
+
+    def append(self, *constraints: Tuple[Bool]) -> None:
+        """Adds the constraints to this solver.
+
+        :param constraints: constraints to add
+        """
+        raw_constraints = [
+            c.raw for c in cast(Tuple[Bool], constraints)
+        ]  # type: List[z3.BoolRef]
+        self.constraints.extend(raw_constraints)
+
+    def check(self) -> z3.CheckSatResult:
+        """Returns z3 smt check result. """
+        dependence_map = DependenceMap()
+        for constraint in self.constraints:
+            dependence_map.add_condition(constraint)
+
+        self.models = []
+        for bucket in dependence_map.buckets:
+            self.raw.reset()
+            self.raw.append(*bucket.conditions)
+            check_result = self.raw.check()
+            if check_result == z3.sat:
+                self.models.append(self.raw.model())
+            else:
+                return check_result
+
+        return z3.sat
+
+    def model(self) -> Model:
+        """Returns z3 model for a solution. """
+        return Model(self.models)
+
+    def reset(self) -> None:
+        """Reset this solver."""
+        self.constraints = []
+
+    def pop(self, num) -> None:
+        """Pop num constraints from this solver.
+        """
+        self.constraints.pop(num)
diff --git a/mythril/laser/smt/model.py b/mythril/laser/smt/model.py
new file mode 100644
index 00000000..524683e9
--- /dev/null
+++ b/mythril/laser/smt/model.py
@@ -0,0 +1,59 @@
+import z3
+
+from typing import Union, List
+
+
+class Model:
+    """ The model class wraps a z3 model
+
+    This implementation allows for multiple internal models, this is required for the use of an independence solver
+    which has models for multiple queries which need an uniform output.
+    """
+
+    def __init__(self, models: List[z3.ModelRef] = None):
+        """
+        Initialize a model object
+        :param models: the internal z3 models that this model should use
+        """
+        self.raw = models or []
+
+    def decls(self) -> List[z3.ExprRef]:
+        """Get the declarations for this model"""
+        result = []  # type: List[z3.ExprRef]
+        for internal_model in self.raw:
+            result.extend(internal_model.decls())
+        return result
+
+    def __getitem__(self, item) -> Union[None, z3.ExprRef]:
+        """ Get declaration from model
+         If item is an int, then the declaration at offset item is returned
+         If item is a declaration, then the interpretation is returned
+         """
+        for internal_model in self.raw:
+            is_last_model = self.raw.index(internal_model) == len(self.raw) - 1
+
+            try:
+                result = internal_model[item]
+                if result is not None:
+                    return result
+            except IndexError:
+                if is_last_model:
+                    raise
+                continue
+        return None
+
+    def eval(
+        self, expression: z3.ExprRef, model_completion: bool = False
+    ) -> Union[None, z3.ExprRef]:
+        """ Evaluate the expression using this model
+
+        :param expression: The expression to evaluate
+        :param model_completion: Use the default value if the model has no interpretation of the given expression
+        :return: The evaluated expression
+        """
+        for internal_model in self.raw:
+            is_last_model = self.raw.index(internal_model) == len(self.raw) - 1
+            is_relevant_model = expression.decl() in list(internal_model.decls())
+            if is_relevant_model or is_last_model:
+                return internal_model.eval(expression, model_completion)
+        return None
diff --git a/mythril/laser/smt/solver.py b/mythril/laser/smt/solver.py
index 21c18195..b3754e94 100644
--- a/mythril/laser/smt/solver.py
+++ b/mythril/laser/smt/solver.py
@@ -3,6 +3,7 @@ import z3
 from typing import Union, cast, TypeVar, Generic, List, Sequence
 
 from mythril.laser.smt.expression import Expression
+from mythril.laser.smt.model import Model
 from mythril.laser.smt.bool import Bool
 
 
@@ -20,28 +21,26 @@ class BaseSolver(Generic[T]):
 
         :param timeout:
         """
-        assert timeout > 0  # timeout <= 0 isn't supported by z3
         self.raw.set(timeout=timeout)
 
-    def add(self, constraints: Union[Bool, List[Bool]]) -> None:
+    def add(self, *constraints: List[Bool]) -> None:
         """Adds the constraints to this solver.
 
         :param constraints:
         :return:
         """
-        if not isinstance(constraints, list):
-            self.raw.add(constraints.raw)
-            return
-        z3_constraints = [c.raw for c in constraints]  # type: Sequence[z3.BoolRef]
+        z3_constraints = [
+            c.raw for c in cast(List[Bool], constraints)
+        ]  # type: Sequence[z3.BoolRef]
         self.raw.add(z3_constraints)
 
-    def append(self, constraints: Union[Bool, List[Bool]]) -> None:
+    def append(self, *constraints: List[Bool]) -> None:
         """Adds the constraints to this solver.
 
         :param constraints:
         :return:
         """
-        self.add(constraints)
+        self.add(*constraints)
 
     def check(self) -> z3.CheckSatResult:
         """Returns z3 smt check result.
@@ -50,12 +49,12 @@ class BaseSolver(Generic[T]):
         """
         return self.raw.check()
 
-    def model(self) -> z3.ModelRef:
+    def model(self) -> Model:
         """Returns z3 model for a solution.
 
         :return:
         """
-        return self.raw.model()
+        return Model([self.raw.model()])
 
 
 class Solver(BaseSolver[z3.Solver]):


Test patch:
diff --git a/tests/laser/smt/__init__.py b/tests/laser/smt/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/laser/smt/independece_solver_test.py b/tests/laser/smt/independece_solver_test.py
new file mode 100644
index 00000000..829b4707
--- /dev/null
+++ b/tests/laser/smt/independece_solver_test.py
@@ -0,0 +1,145 @@
+from mythril.laser.smt.independence_solver import (
+    _get_expr_variables,
+    DependenceBucket,
+    DependenceMap,
+    IndependenceSolver,
+)
+from mythril.laser.smt import symbol_factory
+
+import z3
+
+
+def test_get_expr_variables():
+    # Arrange
+    x = z3.Bool("x")
+    y = z3.BitVec("y", 256)
+    z = z3.BitVec("z", 256)
+    b = z3.BitVec("b", 256)
+    expression = z3.If(x, y, z + b)
+
+    # Act
+    variables = list(map(str, _get_expr_variables(expression)))
+
+    # Assert
+    assert str(x) in variables
+    assert str(y) in variables
+    assert str(z) in variables
+    assert str(b) in variables
+
+
+def test_get_expr_variables_num():
+    # Arrange
+    b = z3.BitVec("b", 256)
+    expression = b + z3.BitVecVal(2, 256)
+
+    # Act
+    variables = _get_expr_variables(expression)
+
+    # Assert
+    assert [b] == variables
+
+
+def test_create_bucket():
+    # Arrange
+    x = z3.Bool("x")
+
+    # Act
+    bucket = DependenceBucket([x], [x])
+
+    # Assert
+    assert [x] == bucket.variables
+    assert [x] == bucket.conditions
+
+
+def test_dependence_map():
+    # Arrange
+    x = z3.BitVec("x", 256)
+    y = z3.BitVec("y", 256)
+    z = z3.BitVec("z", 256)
+    a = z3.BitVec("a", 256)
+    b = z3.BitVec("b", 256)
+
+    conditions = [x > y, y == z, a == b]
+
+    dependence_map = DependenceMap()
+
+    # Act
+    for condition in conditions:
+        dependence_map.add_condition(condition)
+
+    # Assert
+    assert 2 == len(dependence_map.buckets)
+
+    assert x in dependence_map.buckets[0].variables
+    assert y in dependence_map.buckets[0].variables
+    assert z in dependence_map.buckets[0].variables
+    assert len(set(dependence_map.buckets[0].variables)) == 3
+
+    assert conditions[0] in dependence_map.buckets[0].conditions
+    assert conditions[1] in dependence_map.buckets[0].conditions
+
+    assert a in dependence_map.buckets[1].variables
+    assert b in dependence_map.buckets[1].variables
+    assert len(set(dependence_map.buckets[1].variables)) == 2
+
+    assert conditions[2] in dependence_map.buckets[1].conditions
+
+
+def test_Independence_solver_unsat():
+    # Arrange
+    x = symbol_factory.BitVecSym("x", 256)
+    y = symbol_factory.BitVecSym("y", 256)
+    z = symbol_factory.BitVecSym("z", 256)
+    a = symbol_factory.BitVecSym("a", 256)
+    b = symbol_factory.BitVecSym("b", 256)
+
+    conditions = [x > y, y == z, y != z, a == b]
+
+    solver = IndependenceSolver()
+
+    # Act
+    solver.add(*conditions)
+    result = solver.check()
+
+    # Assert
+    assert z3.unsat == result
+
+
+def test_independence_solver_unsat_in_second_bucket():
+    # Arrange
+    x = symbol_factory.BitVecSym("x", 256)
+    y = symbol_factory.BitVecSym("y", 256)
+    z = symbol_factory.BitVecSym("z", 256)
+    a = symbol_factory.BitVecSym("a", 256)
+    b = symbol_factory.BitVecSym("b", 256)
+
+    conditions = [x > y, y == z, a == b, a != b]
+
+    solver = IndependenceSolver()
+
+    # Act
+    solver.add(*conditions)
+    result = solver.check()
+
+    # Assert
+    assert z3.unsat == result
+
+
+def test_independence_solver_sat():
+    # Arrange
+    x = symbol_factory.BitVecSym("x", 256)
+    y = symbol_factory.BitVecSym("y", 256)
+    z = symbol_factory.BitVecSym("z", 256)
+    a = symbol_factory.BitVecSym("a", 256)
+    b = symbol_factory.BitVecSym("b", 256)
+
+    conditions = [x > y, y == z, a == b]
+
+    solver = IndependenceSolver()
+
+    # Act
+    solver.add(*conditions)
+    result = solver.check()
+
+    # Assert
+    assert z3.sat == result
diff --git a/tests/laser/smt/model_test.py b/tests/laser/smt/model_test.py
new file mode 100644
index 00000000..7f9ae1d3
--- /dev/null
+++ b/tests/laser/smt/model_test.py
@@ -0,0 +1,56 @@
+from mythril.laser.smt import Solver, symbol_factory
+import z3
+
+
+def test_decls():
+    # Arrange
+    solver = Solver()
+    x = symbol_factory.BitVecSym("x", 256)
+    expression = x == symbol_factory.BitVecVal(2, 256)
+
+    # Act
+    solver.add(expression)
+    result = solver.check()
+    model = solver.model()
+
+    decls = model.decls()
+
+    # Assert
+    assert z3.sat == result
+    assert x.raw.decl() in decls
+
+
+def test_get_item():
+    # Arrange
+    solver = Solver()
+    x = symbol_factory.BitVecSym("x", 256)
+    expression = x == symbol_factory.BitVecVal(2, 256)
+
+    # Act
+    solver.add(expression)
+    result = solver.check()
+    model = solver.model()
+
+    x_concrete = model[x.raw.decl()]
+
+    # Assert
+    assert z3.sat == result
+    assert 2 == x_concrete
+
+
+def test_as_long():
+    # Arrange
+    solver = Solver()
+    x = symbol_factory.BitVecSym("x", 256)
+    expression = x == symbol_factory.BitVecVal(2, 256)
+
+    # Act
+    solver.add(expression)
+    result = solver.check()
+    model = solver.model()
+
+    x_concrete = model.eval(x.raw).as_long()
+
+    # Assert
+    assert z3.sat == result
+    assert 2 == x_concrete
diff --git a/tests/laser/state/calldata_test.py b/tests/laser/state/calldata_test.py
index f32f297a..46c28585 100644
--- a/tests/laser/state/calldata_test.py
+++ b/tests/laser/state/calldata_test.py
@@ -48,7 +48,7 @@ def test_concrete_calldata_constrain_index():
     value = calldata[2]
     constraint = value == 3
 
-    solver.add([constraint])
+    solver.add(constraint)
     result = solver.check()
 
     # Assert
@@ -65,7 +65,7 @@ def test_symbolic_calldata_constrain_index():
 
     constraints = [value == 1, calldata.calldatasize == 50]
 
-    solver.add(constraints)
+    solver.add(*constraints)
 
     result = solver.check()