Revert "Merge pull request github#3419 from MathiasVP/flat-structs"

There was unfortunately a semantic merge conflict between github#3419 and
 github#3587 that caused a performance regression on (at least) OpenJDK.

This reverts commit 982fb38, reversing
changes made to b841cac.

Author: jbj

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -184,24 +184,46 @@ private class ArrayContent extends Content, TArrayContent {
   override string toString() { result = "array" }
 }
 
+private predicate storeStepNoChi(Node node1, Content f, PostUpdateNode node2) {
+  exists(FieldAddressInstruction fa, StoreInstruction store |
+    store = node2.asInstruction() and
+    store.getDestinationAddress() = fa and
+    store.getSourceValue() = node1.asInstruction() and
+    f.(FieldContent).getField() = fa.getField()
+  )
+}
+
+private predicate storeStepChi(Node node1, Content f, PostUpdateNode node2) {
+  exists(FieldAddressInstruction fa, StoreInstruction store |
+    node1.asInstruction() = store and
+    store.getDestinationAddress() = fa and
+    node2.asInstruction().(ChiInstruction).getPartial() = store and
+    f.(FieldContent).getField() = fa.getField()
+  )
+}
+
 /**
  * Holds if data can flow from `node1` to `node2` via an assignment to `f`.
  * Thus, `node2` references an object with a field `f` that contains the
  * value of `node1`.
  */
-predicate storeStep(Node node1, Content f, StoreStepNode node2) {
-  node2.getStoredValue() = node1 and
-  f.(FieldContent).getField() = node2.getAField()
+predicate storeStep(Node node1, Content f, PostUpdateNode node2) {
+  storeStepNoChi(node1, f, node2) or
+  storeStepChi(node1, f, node2)
 }
 
 /**
  * Holds if data can flow from `node1` to `node2` via a read of `f`.
  * Thus, `node1` references an object with a field `f` whose value ends up in
  * `node2`.
  */
-predicate readStep(Node node1, Content f, ReadStepNode node2) {
-  node2.getReadValue() = node1 and
-  f.(FieldContent).getField() = node2.getAField()
+predicate readStep(Node node1, Content f, Node node2) {
+  exists(FieldAddressInstruction fa, LoadInstruction load |
+    load.getSourceAddress() = fa and
+    node1.asInstruction() = load.getSourceValueOperand().getAnyDef() and
+    fa.getField() = f.(FieldContent).getField() and
+    load = node2.asInstruction()
+  )
 }
 
 /**