HashCons: Make some functionality apparent.

The user knows that an expression functionally determines its
hashCons value, and that an expression functionally determines
its number of children, but this is not provable from the
definitions, and so not usable by the optimiser. By storing
the result of those known-functional calls in a variable,
rather than repeating the call, we enable better join orders.

Author: pavgust

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

@@ -766,11 +766,13 @@ private predicate mk_ClassAggregateLiteral(Class c, HC_Fields hcf, ClassAggregat
   analyzableClassAggregateLiteral(cal) and
   c = cal.getUnspecifiedType() and
   (
-    exists(HC_Fields tail, Expr e, Field f |
+    exists(HC_Fields tail, Expr e, Field f, int numChildren, HashCons eCons |
       f.getInitializationOrder() = cal.getNumChild() - 1 and
       e = cal.getFieldExpr(f).getFullyConverted() and
-      hcf = HC_FieldCons(c, cal.getNumChild() - 1, f, hashCons(e), tail) and
-      mk_FieldCons(c, cal.getNumChild() - 1, f, hashCons(e), tail, cal)
+      eCons = hashCons(e) and
+      numChildren = cal.getNumChild() and
+      hcf = HC_FieldCons(c, numChildren - 1, f, eCons, tail) and
+      mk_FieldCons(c, numChildren - 1, f, eCons, tail, cal)
     )
     or
     cal.getNumChild() = 0 and
@@ -801,9 +803,10 @@ private predicate mk_ArrayCons(Type t, int i, HashCons hc, HC_Array hca, ArrayAg
 private predicate mk_ArrayAggregateLiteral(Type t, HC_Array hca, ArrayAggregateLiteral aal) {
   t = aal.getUnspecifiedType() and
   (
-    exists(HashCons head, HC_Array tail |
-      hca = HC_ArrayCons(t, aal.getNumChild() - 1, head, tail) and
-      mk_ArrayCons(t, aal.getNumChild() - 1, head, tail, aal)
+    exists(HashCons head, HC_Array tail, int numElements |
+      numElements = aal.getNumChild() and
+      hca = HC_ArrayCons(t, numElements - 1, head, tail) and
+      mk_ArrayCons(t, numElements - 1, head, tail, aal)
     )
     or
     aal.getNumChild() = 0 and