prevent huge expressions during expression propagation (#412)

src/cwe_checker_lib/src/analysis/expression_propagation/mod.rs

@@ -55,9 +55,15 @@ impl<'a> crate::analysis::forward_interprocedural_fixpoint::Context<'a> for Cont
                 let mut extended_expression = expression.clone();
                 for input_var in expression.input_vars().into_iter() {
                     if let Some(expr) = insertable_expressions.get(input_var) {
+                        // We limit the complexity of expressions to insert.
+                        // This prevents extremely large expressions that can lead to extremely high RAM usage.
+                        // FIXME: Right now this limit is quite arbitrary. Maybe there is a better way to achieve the same result?
+                        if expr.recursion_depth() < 10 {
                             extended_expression.substitute_input_var(input_var, expr)
                         }
                     }
+                }
+                extended_expression.substitute_trivial_operations();
                 insertable_expressions.insert(var.clone(), extended_expression.clone());
                 // Expressions dependent on the assigned variable are no longer insertable.
                 insertable_expressions.retain(|_input_var, input_expr| {

src/cwe_checker_lib/src/intermediate_representation/expression.rs

@@ -208,6 +208,22 @@ impl Expression {
             }
         }
     }
+
+    /// Compute a recursion depth for the expression.
+    ///
+    /// Because of the recursive nature of the [Expression] type,
+    /// overly complex expressions are very costly to clone, which in turn can negatively affect some analyses.
+    /// The recursion depth measure can be used to detect and handle such cases.
+    pub fn recursion_depth(&self) -> u64 {
+        use Expression::*;
+        match self {
+            Const(_) | Unknown { .. } | Var(_) => 0,
+            Subpiece { arg, .. } | Cast { arg, .. } | UnOp { arg, .. } => arg.recursion_depth() + 1,
+            BinOp { lhs, rhs, .. } => {
+                std::cmp::max(lhs.recursion_depth(), rhs.recursion_depth()) + 1
+            }
+        }
+    }
 }
 
 impl fmt::Display for Expression {