C#: Take phi nodes into account in Steps::getARead()

Author: hvitved

csharp/ql/src/semmle/code/csharp/dataflow/internal/Steps.qll

@@ -6,18 +6,17 @@ import csharp
  * Provides functionality for performing simple data flow analysis.
  * This library is used by the dispatch library, which in turn is used by the
  * SSA library, so we cannot make use of the SSA library in this library.
- * Instead, this library relies on a self-contained, minimalistic SSA-like
- * implementation.
+ * Instead, this library relies on the `BaseSsa` library.
  */
 module Steps {
   private import semmle.code.csharp.dataflow.internal.BaseSSA
 
   /**
-   * Gets a read that is guaranteed to read the value assigned at definition `def`.
+   * Gets a read that may read the value assigned at definition `def`.
    */
   private AssignableRead getARead(AssignableDefinition def) {
     exists(BaseSsa::Definition ssaDef |
-      ssaDef.getDefinition() = def and
+      ssaDef.getAnUltimateDefinition().getDefinition() = def and
       result = ssaDef.getARead()
     )
     or

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.cs

@@ -278,7 +278,7 @@ void M9()
 
     void M10(bool b)
     {
-        var x = ""; // GOOD [FALSE POSITIVE]
+        var x = ""; // GOOD
         Action action;
         if (b)
             action = () => System.Console.WriteLine(x);

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.expected

@@ -9,7 +9,6 @@
 | DeadStoreOfLocal.cs:142:26:142:27 | Exception ex | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:142:26:142:27 | ex | ex |
 | DeadStoreOfLocal.cs:246:17:246:24 | Int32 y = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:246:17:246:17 | y | y |
 | DeadStoreOfLocal.cs:261:17:261:21 | Int32 x = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:261:17:261:17 | x | x |
-| DeadStoreOfLocal.cs:281:13:281:18 | String x = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:281:13:281:13 | x | x |
 | DeadStoreOfLocal.cs:300:23:300:28 | Object v1 | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:300:27:300:28 | v1 | v1 |
 | DeadStoreOfLocal.cs:314:18:314:23 | Object v2 | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:314:22:314:23 | v2 | v2 |
 | DeadStoreOfLocal.cs:331:9:331:32 | ... = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:327:23:327:23 | b | b |