Merge pull request #4110 from yoff/SharedDataflow_ParsimoniousFlowNodes

Python: Shared dataflow, parsimonious flow nodes

python/ql/src/experimental/dataflow/internal/DataFlowPrivate.qll

@@ -7,6 +7,13 @@ private import DataFlowPublic
 //--------
 // Nodes
 //--------
+predicate isExpressionNode(ControlFlowNode node) { node.getNode() instanceof Expr }
+
+/** A control flow node which is also a dataflow node */
+class DataFlowCfgNode extends ControlFlowNode {
+  DataFlowCfgNode() { isExpressionNode(this) }
+}
+
 /**
  * A node associated with an object after an operation that might have
  * changed its state.

python/ql/src/experimental/dataflow/internal/DataFlowPublic.qll

@@ -20,7 +20,7 @@ newtype TNode =
   /** A node corresponding to an SSA variable. */
   TEssaNode(EssaVariable var) or
   /** A node corresponding to a control flow node. */
-  TCfgNode(ControlFlowNode node)
+  TCfgNode(DataFlowCfgNode node)
 
 /**
  * An element, viewed as a node in a data flow graph. Either an SSA variable
@@ -58,6 +58,15 @@ class Node extends TNode {
   ) {
     this.getLocation().hasLocationInfo(filepath, startline, startcolumn, endline, endcolumn)
   }
+
+  /** Convenience method for casting to EssaNode and calling getVar. */
+  EssaVariable asVar() { none() }
+
+  /** Convenience method for casting to CfgNode and calling getNode. */
+  ControlFlowNode asCfgNode() { none() }
+
+  /** Convenience method for casting to ExprNode and calling getNode and getNode again. */
+  Expr asExpr() { none() }
 }
 
 class EssaNode extends Node, TEssaNode {
@@ -67,6 +76,8 @@ class EssaNode extends Node, TEssaNode {
 
   EssaVariable getVar() { result = var }
 
+  override EssaVariable asVar() { result = var }
+
   /** Gets a textual representation of this element. */
   override string toString() { result = var.toString() }
 
@@ -76,12 +87,14 @@ class EssaNode extends Node, TEssaNode {
 }
 
 class CfgNode extends Node, TCfgNode {
-  ControlFlowNode node;
+  DataFlowCfgNode node;
 
   CfgNode() { this = TCfgNode(node) }
 
   ControlFlowNode getNode() { result = node }
 
+  override ControlFlowNode asCfgNode() { result = node }
+
   /** Gets a textual representation of this element. */
   override string toString() { result = node.toString() }
 
@@ -97,10 +110,14 @@ class CfgNode extends Node, TCfgNode {
  * to multiple `ExprNode`s, just like it may correspond to multiple
  * `ControlFlow::Node`s.
  */
-class ExprNode extends Node { }
+class ExprNode extends CfgNode {
+  ExprNode() { isExpressionNode(node) }
+
+  override Expr asExpr() { result = node.getNode() }
+}
 
 /** Gets a node corresponding to expression `e`. */
-ExprNode exprNode(DataFlowExpr e) { none() }
+ExprNode exprNode(DataFlowExpr e) { result.getNode().getNode() = e }
 
 /**
  * The value of a parameter at function entry, viewed as a node in a data