C#: Use Gvn comparison instead of StructuralComparisonConfiguration in SelfAssignment.

Author: michaelnebel

csharp/ql/src/Likely Bugs/SelfAssignment.ql

@@ -13,44 +13,32 @@
 import csharp
 import semmle.code.csharp.commons.StructuralComparison
 
-class StructuralComparisonConfig extends StructuralComparisonConfiguration {
-  StructuralComparisonConfig() { this = "SelfAssignment" }
-
-  override predicate candidate(ControlFlowElement x, ControlFlowElement y) {
-    exists(AssignExpr ae |
-      // Member initializers are never self-assignments, in particular
-      // not initializers such as `new C { F = F };`
-      not ae instanceof MemberInitializer and
-      // Enum field initializers are never self assignments. `enum E { A = 42 }`
-      not ae.getParent().(Field).getDeclaringType() instanceof Enum
-    |
-      ae.getLValue() = x and
-      ae.getRValue() = y
-    ) and
-    forall(Expr e | e = x.(Expr).getAChildExpr*() |
-      // Non-trivial property accesses may have side-effects,
-      // so these are not considered
-      e instanceof PropertyAccess implies e instanceof TrivialPropertyAccess
-    )
-  }
+private predicate candidate(AssignExpr ae) {
+  // Member initializers are never self-assignments, in particular
+  // not initializers such as `new C { F = F };`
+  not ae instanceof MemberInitializer and
+  // Enum field initializers are never self assignments. `enum E { A = 42 }`
+  not ae.getParent().(Field).getDeclaringType() instanceof Enum and
+  forall(Expr e | e = ae.getLValue().getAChildExpr*() |
+    // Non-trivial property accesses may have side-effects,
+    // so these are not considered
+    e instanceof PropertyAccess implies e instanceof TrivialPropertyAccess
+  )
+}
 
-  AssignExpr getSelfAssignExpr() {
-    exists(Expr x, Expr y |
-      same(x, y) and
-      result.getLValue() = x and
-      result.getRValue() = y
-    )
-  }
+private predicate selfAssignExpr(AssignExpr ae) {
+  candidate(ae) and
+  sameGvn(ae.getLValue(), ae.getRValue())
 }
 
-Declaration getDeclaration(Expr e) {
+private Declaration getDeclaration(Expr e) {
   result = e.(VariableAccess).getTarget()
   or
   result = e.(MemberAccess).getTarget()
   or
   result = getDeclaration(e.(ArrayAccess).getQualifier())
 }
 
-from StructuralComparisonConfig c, AssignExpr ae, Declaration target
-where ae = c.getSelfAssignExpr() and target = getDeclaration(ae.getLValue())
+from AssignExpr ae, Declaration target
+where selfAssignExpr(ae) and target = getDeclaration(ae.getLValue())
 select ae, "This assignment assigns $@ to itself.", target, target.getName()