From 6c2923f2541caaca2be85cf99641412be8fb7bdd Mon Sep 17 00:00:00 2001
From: Josselin <josselin@trailofbits.com>
Date: Wed, 6 Feb 2019 09:01:57 -0500
Subject: [PATCH] Add constant folding visitor (fix #144)

---
 slither/core/solidity_types/array_type.py     |   4 +
 .../visitors/expression/constants_folding.py  | 104 ++++++++++++++++++
 2 files changed, 108 insertions(+)
 create mode 100644 slither/visitors/expression/constants_folding.py

diff --git a/slither/core/solidity_types/array_type.py b/slither/core/solidity_types/array_type.py
index 7bcc9c664..a396c9af0 100644
--- a/slither/core/solidity_types/array_type.py
+++ b/slither/core/solidity_types/array_type.py
@@ -2,6 +2,7 @@ from slither.core.variables.variable import Variable
 from slither.core.solidity_types.type import Type
 from slither.core.expressions.expression import Expression
 from slither.core.expressions import Literal
+from slither.visitors.expression.constants_folding import ConstantFolding
 
 class ArrayType(Type):
 
@@ -11,6 +12,9 @@ class ArrayType(Type):
             if isinstance(length, int):
                 length = Literal(length)
             assert isinstance(length, Expression)
+            if not isinstance(length, Literal):
+                cf = ConstantFolding(length)
+                length = cf.result()
         super(ArrayType, self).__init__()
         self._type = t
         self._length = length
diff --git a/slither/visitors/expression/constants_folding.py b/slither/visitors/expression/constants_folding.py
new file mode 100644
index 000000000..bca1bfe04
--- /dev/null
+++ b/slither/visitors/expression/constants_folding.py
@@ -0,0 +1,104 @@
+import logging
+
+from .expression import ExpressionVisitor
+from slither.core.expressions import BinaryOperationType, Literal
+
+class NotConstant(Exception):
+    pass
+
+
+KEY = 'ConstantFolding'
+
+def get_val(expression):
+    val = expression.context[KEY]
+    # we delete the item to reduce memory use
+    del expression.context[KEY]
+    return val
+
+def set_val(expression, val):
+    expression.context[KEY] = val
+
+class ConstantFolding(ExpressionVisitor):
+
+    def result(self):
+        return Literal(int(get_val(self._expression)))
+
+    def _post_identifier(self, expression):
+        if not expression.value.is_constant:
+            raise NotConstant
+        expr = expression.value.expression
+        # assumption that we won't have infinite loop
+        if not isinstance(expr, Literal):
+            cf = ConstantFolding(expr)
+            expr = cf.result()
+        set_val(expression, int(expr.value))
+
+    def _post_binary_operation(self, expression):
+        left = get_val(expression.expression_left)
+        right = get_val(expression.expression_right)
+        if expression.type  == BinaryOperationType.POWER:
+            set_val(expression, left ** right)
+        elif expression.type  == BinaryOperationType.MULTIPLICATION:
+            set_val(expression, left * right)
+        elif expression.type  == BinaryOperationType.DIVISION:
+            set_val(expression, left / right)
+        elif expression.type  == BinaryOperationType.MODULO:
+            set_val(expression, left % right)
+        elif expression.type  == BinaryOperationType.ADDITION:
+            set_val(expression, left + right)
+        elif expression.type  == BinaryOperationType.SUBTRACTION:
+            if(left-right) <0:
+                # Could trigger underflow
+                raise NotConstant
+            set_val(expression, left - right)
+        elif expression.type  == BinaryOperationType.LEFT_SHIFT:
+            set_val(expression, left << right)
+        elif expression.type  == BinaryOperationType.RIGHT_SHIFT:
+            set_val(expression, left >> right)
+        else:
+            raise NotConstant
+
+    def _post_unary_operation(self, expression):
+        raise NotConstant
+
+    def _post_literal(self, expression):
+        if expression.value.isdigit():
+            set_val(expression, int(expression.value))
+        else:
+            raise NotConstant
+
+    def _post_assignement_operation(self, expression):
+        raise NotConstant
+
+    def _post_call_expression(self, expression):
+        raise NotConstant
+
+    def _post_conditional_expression(self, expression):
+        raise NotConstant
+
+    def _post_elementary_type_name_expression(self, expression):
+        raise NotConstant
+
+    def _post_index_access(self, expression):
+        raise NotConstant
+
+    def _post_member_access(self, expression):
+        raise NotConstant
+
+    def _post_new_array(self, expression):
+        raise NotConstant
+
+    def _post_new_contract(self, expression):
+        raise NotConstant
+
+    def _post_new_elementary_type(self, expression):
+        raise NotConstant
+
+    def _post_tuple_expression(self, expression):
+        raise NotConstant
+
+    def _post_type_conversion(self, expression):
+        raise NotConstant
+
+
+