C++: Fix duplicate "final global value" nodes

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/SsaInternals.qll

@@ -149,11 +149,16 @@ private newtype TDefOrUseImpl =
 private predicate isGlobalUse(
   GlobalLikeVariable v, IRFunction f, int indirection, int indirectionIndex
 ) {
-  exists(VariableAddressInstruction vai |
-    vai.getEnclosingIRFunction() = f and
-    vai.getAstVariable() = v and
-    isDef(_, _, _, vai, indirection, indirectionIndex)
-  )
+  // Generate a "global use" at the end of the function body if there's a
+  // direct definition somewhere in the body of the function
+  indirection =
+    min(int cand, VariableAddressInstruction vai |
+      vai.getEnclosingIRFunction() = f and
+      vai.getAstVariable() = v and
+      isDef(_, _, _, vai, cand, indirectionIndex)
+    |
+      cand
+    )
 }
 
 private predicate isGlobalDefImpl(
@@ -447,6 +452,57 @@ class FinalParameterUse extends UseImpl, TFinalParameterUse {
   }
 }
 
+/**
+ * A use that models a synthetic "last use" of a global variable just before a
+ * function returns.
+ *
+ * We model global variable flow by:
+ * - Inserting a last use of any global variable that's modified by a function
+ * - Flowing from the last use to the `VariableNode` that represents the global
+ *   variable.
+ * - Flowing from the `VariableNode` to an "initial def" of the global variable
+ * in any function that may read the global variable.
+ * - Flowing from the initial definition to any subsequent uses of the global
+ *   variable in the function body.
+ *
+ * For example, consider the following pair of functions:
+ * ```cpp
+ * int global;
+ * int source();
+ * void sink(int);
+ *
+ * void set_global() {
+ *   global = source();
+ * }
+ *
+ * void read_global() {
+ *  sink(global);
+ * }
+ * ```
+ * we insert global uses and defs so that (from the point-of-view of dataflow)
+ * the above scenario looks like:
+ * ```cpp
+ * int global; // (1)
+ * int source();
+ * void sink(int);
+ *
+ * void set_global() {
+ *   global = source();
+ *   __global_use(global); // (2)
+ * }
+ *
+ * void read_global() {
+ *  global = __global_def; // (3)
+ *  sink(global); // (4)
+ * }
+ * ```
+ * and flow from `source()` to the argument of `sink` is then modeled as
+ * follows:
+ * 1. Flow from `source()` to `(2)` (via SSA).
+ * 2. Flow from `(2)` to `(1)` (via a `jumpStep`).
+ * 3. Flow from `(1)` to `(3)` (via a `jumpStep`).
+ * 4. Flow from `(3)` to `(4)` (via SSA).
+ */
 class GlobalUse extends UseImpl, TGlobalUse {
   GlobalLikeVariable global;
   IRFunction f;
@@ -494,6 +550,12 @@ class GlobalUse extends UseImpl, TGlobalUse {
   override BaseSourceVariableInstruction getBase() { none() }
 }
 
+/**
+ * A definition that models a synthetic "initial definition" of a global
+ * variable just after the function entry point.
+ *
+ * See the QLDoc for `GlobalUse` for how this is used.
+ */
 class GlobalDefImpl extends DefOrUseImpl, TGlobalDefImpl {
   GlobalLikeVariable global;
   IRFunction f;

cpp/ql/test/library-tests/dataflow/dataflow-tests/dataflow-consistency.expected

@@ -159,6 +159,9 @@ postWithInFlow
 | test.cpp:808:5:808:21 | * ... [post update] | PostUpdateNode should not be the target of local flow. |
 | test.cpp:808:6:808:21 | global_indirect1 [inner post update] | PostUpdateNode should not be the target of local flow. |
 | test.cpp:832:5:832:17 | global_direct [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:931:5:931:18 | global_pointer [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:932:5:932:19 | * ... [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:932:6:932:19 | global_pointer [inner post update] | PostUpdateNode should not be the target of local flow. |
 viableImplInCallContextTooLarge
 uniqueParameterNodeAtPosition
 uniqueParameterNodePosition

cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected

@@ -300,6 +300,7 @@ irFlow
 | test.cpp:902:56:902:75 | *indirect_source(2) | test.cpp:911:19:911:48 | *global_array_static_indirect_2 |
 | test.cpp:914:46:914:53 | source | test.cpp:919:10:919:30 | global_pointer_static |
 | test.cpp:915:57:915:76 | *indirect_source(1) | test.cpp:921:19:921:50 | *global_pointer_static_indirect_1 |
+| test.cpp:932:23:932:28 | call to source | test.cpp:937:10:937:24 | * ... |
 | true_upon_entry.cpp:9:11:9:16 | call to source | true_upon_entry.cpp:13:8:13:8 | x |
 | true_upon_entry.cpp:17:11:17:16 | call to source | true_upon_entry.cpp:21:8:21:8 | x |
 | true_upon_entry.cpp:27:9:27:14 | call to source | true_upon_entry.cpp:29:8:29:8 | x |

cpp/ql/test/library-tests/dataflow/dataflow-tests/test.cpp

@@ -922,4 +922,18 @@ namespace GlobalArrays {
     sink(global_pointer_static_indirect_2); // clean: global_pointer_static_indirect_2 does not have 2 indirections
     indirect_sink(global_pointer_static_indirect_2); // clean: global_pointer_static_indirect_2 does not have 2 indirections
   }
+}
+
+namespace global_variable_conflation_test {
+  int* global_pointer;
+
+  void def() {
+    global_pointer = nullptr;
+    *global_pointer = source();
+  }
+
+  void use() {
+    sink(global_pointer); // clean
+    sink(*global_pointer); // $ ir MISSING: ast
+  }
 }