C++: support impure binary operations in HashCons

Author: Robert Marsh

cpp/ql/src/semmle/code/cpp/valuenumbering/HashCons.qll

@@ -288,7 +288,6 @@ private predicate mk_Conversion(Type t, HC child, Conversion conv) {
 }
 
 private predicate analyzableBinaryOp(BinaryOperation op) {
-  op.isPure() and
   strictcount (op.getLeftOperand().getFullyConverted()) = 1 and
   strictcount (op.getRightOperand().getFullyConverted()) = 1 and
   strictcount (op.getOperator()) = 1

cpp/ql/test/library-tests/valuenumbering/HashCons/HashCons.expected

@@ -63,3 +63,6 @@
 | test.cpp:141:12:141:17 | call to getInt | 141:c12-c17 141:c29-c34 |
 | test.cpp:146:10:146:11 | ih | 146:c10-c11 146:c31-c32 |
 | test.cpp:146:13:146:25 | call to getDoubledInt | 146:c13-c25 146:c34-c46 |
+| test.cpp:150:3:150:3 | x | 150:c3-c3 150:c9-c9 151:c3-c3 151:c9-c9 152:c12-c12 |
+| test.cpp:150:3:150:5 | ... ++ | 150:c3-c5 150:c9-c11 151:c3-c5 151:c9-c11 152:c10-c12 |
+| test.cpp:150:3:150:11 | ... + ... | 150:c3-c11 151:c3-c11 |

cpp/ql/test/library-tests/valuenumbering/HashCons/test.cpp

@@ -138,10 +138,16 @@ class IntHolder {
 
 public:
   int getDoubledInt() {
-    return getInt() + this->getInt();
+    return getInt() + this->getInt(); // getInt() and this->getInt() should be the same
   }
 };
 
-int quadrupleInt(IntHolder ih) {
+int test09(IntHolder ih) {
   return ih.getDoubledInt() + ih.getDoubledInt();
 }
+
+int test10(int x) {
+  x++ + x++;
+  x++ + x++; // same as above
+  return ++x; // ++x is not the same as x++
+}