[clang][dataflow] Add synthetic fields to `RecordStorageLocation` (#73860)

Synthetic fields are intended to model the internal state of a class
(e.g. the value stored in a `std::optional`) without having to depend on
that class's implementation details.

Today, this is typically done with properties on `RecordValue`s, but
these have several drawbacks:

* Care must be taken to call `refreshRecordValue()` before modifying a
property so that the modified property values aren’t seen by other
environments that may have access to the same `RecordValue`.

* Properties aren’t associated with a storage location. If an analysis
needs to associate a location with the value stored in a property (e.g.
to model the reference returned by `std::optional::value()`), it needs
to manually add an indirection using a `PointerValue`. (See for example
the way this is done in UncheckedOptionalAccessModel.cpp, specifically
in `maybeInitializeOptionalValueMember()`.)

* Properties don’t participate in the builtin compare, join, and widen
operations. If an analysis needs to apply these operations to
properties, it needs to override the corresponding methods of
`ValueModel`.

* Longer-term, we plan to eliminate `RecordValue`, as by-value
operations on records aren’t really “a thing” in C++ (see
https://discourse.llvm.org/t/70086#changed-structvalue-api-14). This
would obviously eliminate the ability to set properties on
`RecordValue`s.

To demonstrate the advantages of synthetic fields, this patch converts
UncheckedOptionalAccessModel.cpp to synthetic fields. This greatly
simplifies the implementation of the check.

This PR is pretty big; to make it easier to review, I have broken it
down into a stack of three commits, each of which contains a set of
logically related changes. I considered submitting each of these as a
separate PR, but the commits only really make sense when taken together.

To review, I suggest first looking at the changes in
UncheckedOptionalAccessModel.cpp. This gives a flavor for how the
various API changes work together in the context of an analysis. Then,
review the rest of the changes.

1 files changed, 39 insertions, 1 deletions

diff --git a/clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp b/clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp
index 0a2fcd4..fa11497 100644
--- a/clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp
+++ b/clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp
@@ -68,11 +68,38 @@ StorageLocation &DataflowAnalysisContext::createStorageLocation(QualType Type) {
       else
         FieldLocs.insert({Field, &createStorageLocation(
                                      Field->getType().getNonReferenceType())});
-    return arena().create<RecordStorageLocation>(Type, std::move(FieldLocs));
+
+    RecordStorageLocation::SyntheticFieldMap SyntheticFields;
+    for (const auto &Entry : getSyntheticFields(Type))
+      SyntheticFields.insert(
+          {Entry.getKey(),
+           &createStorageLocation(Entry.getValue().getNonReferenceType())});
+
+    return createRecordStorageLocation(Type, std::move(FieldLocs),
+                                       std::move(SyntheticFields));
   }
   return arena().create<ScalarStorageLocation>(Type);
 }
 
+// Returns the keys for a given `StringMap`.
+// Can't use `StringSet` as the return type as it doesn't support `operator==`.
+template <typename T>
+static llvm::DenseSet<llvm::StringRef> getKeys(const llvm::StringMap<T> &Map) {
+  return llvm::DenseSet<llvm::StringRef>(Map.keys().begin(), Map.keys().end());
+}
+
+RecordStorageLocation &DataflowAnalysisContext::createRecordStorageLocation(
+    QualType Type, RecordStorageLocation::FieldToLoc FieldLocs,
+    RecordStorageLocation::SyntheticFieldMap SyntheticFields) {
+  assert(Type->isRecordType());
+  assert(containsSameFields(getModeledFields(Type), FieldLocs));
+  assert(getKeys(getSyntheticFields(Type)) == getKeys(SyntheticFields));
+
+  RecordStorageLocationCreated = true;
+  return arena().create<RecordStorageLocation>(Type, std::move(FieldLocs),
+                                               std::move(SyntheticFields));
+}
+
 StorageLocation &
 DataflowAnalysisContext::getStableStorageLocation(const ValueDecl &D) {
   if (auto *Loc = DeclToLoc.lookup(&D))
@@ -367,3 +394,14 @@ clang::dataflow::FieldSet clang::dataflow::getObjectFields(QualType Type) {
   getFieldsFromClassHierarchy(Type, Fields);
   return Fields;
 }
+
+bool clang::dataflow::containsSameFields(
+    const clang::dataflow::FieldSet &Fields,
+    const clang::dataflow::RecordStorageLocation::FieldToLoc &FieldLocs) {
+  if (Fields.size() != FieldLocs.size())
+    return false;
+  for ([[maybe_unused]] auto [Field, Loc] : FieldLocs)
+    if (!Fields.contains(cast_or_null<FieldDecl>(Field)))
+      return false;
+  return true;
+}