C++: Use value numbering to better detect whether a write is certain.

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

@@ -6,6 +6,7 @@ private import DataFlowImplCommon as DataFlowImplCommon
 private import DataFlowUtil
 private import semmle.code.cpp.models.interfaces.PointerWrapper
 private import DataFlowPrivate
+private import semmle.code.cpp.ir.ValueNumbering
 
 /**
  * Holds if `operand` is an operand that is not used by the dataflow library.
@@ -864,7 +865,7 @@ private module Cached {
    * to a specific address.
    */
   private predicate isCertainAddress(Operand operand) {
-    operand.getDef() instanceof VariableAddressInstruction
+    valueNumberOfOperand(operand).getAnInstruction() instanceof VariableAddressInstruction
     or
     operand.getType() instanceof Cpp::ReferenceType
   }

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

@@ -732,7 +732,7 @@ void test_does_not_write_source_to_dereference()
 {
   int x;
   does_not_write_source_to_dereference(&x);
-  sink(x); // $ ast,ir=733:7 SPURIOUS: ast,ir=726:11
+  sink(x); // $ ast=733:7 ir SPURIOUS: ast=726:11
 }
 
 void sometimes_calls_sink_eq(int x, int n) {

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

@@ -134,7 +134,7 @@ void pointer_test() {
 	sink(*p3); // $ ast,ir
 
 	*p3 = 0;
-	sink(*p3); // $ SPURIOUS: ast,ir
+	sink(*p3); // $ SPURIOUS: ast
 }
 
 // --- return values ---