Refine Naive Dependency Collection Algorithm

Split off the notion of "recording" dependencies from the notion of
"collecting" dependencies. This corrects an oversight in the previous
design where dependency replay and recording were actually not "free" in
WMO where we actually never track dependencies. This architecture also
lays the groundwork for the removal of the referenced name trackers.

The algorithm builds upon the infrastructure for dependency sources and
sinks laid down during the cut over to request-based dependency tracking
in swiftlang#30723.

The idea of the naive algorithm is this:

For a chain of requests A -> B* -> C -> D* -> ... -> L where L is a lookup
request and all starred requests are cached, once L writes into the
dependency collector, the active stack is walked and at each cache-point
the results of dependency collection are associated with the request
itself (in this example, B* and D* have all the names L found associated
with them). Subsequent evaluations of these cached requests (B* and D*
et al) will then *replay* the previous lookup results from L into the
active referenced name tracker. One complication is, suppose the
evaluation of a cached request involves multiple downstream name
lookups. More concretely, suppose we have the following request trace:

A* -> B -> L
      |
       -> C -> L
          |
           -> D -> L
              |
               -> ...

Then A* must see the union of the results of each L. If this reminds
anyone of a union-find, that is no accident! A persistent union-find
a la Conchon and Filliatre is probably in order to help bring down peak
heap usage...

Author: CodaFi

include/swift/AST/Evaluator.h

@@ -225,7 +225,7 @@ class Evaluator {
   /// so all clients must cope with cycles.
   llvm::DenseMap<AnyRequest, std::vector<AnyRequest>> dependencies;
 
-  evaluator::DependencyCollector collector;
+  evaluator::DependencyRecorder recorder;
 
   /// Retrieve the request function for the given zone and request IDs.
   AbstractRequestFunction *getAbstractRequestFunction(uint8_t zoneID,
@@ -268,8 +268,8 @@ class Evaluator {
            typename std::enable_if<Request::isEverCached>::type * = nullptr>
   llvm::Expected<typename Request::OutputType>
   operator()(const Request &request) {
-    evaluator::DependencyCollector::StackRAII<Request> incDeps{collector,
-                                                               request};
+    evaluator::DependencyRecorder::StackRAII<Request> incDeps{recorder,
+                                                              request};
     // The request can be cached, but check a predicate to determine
     // whether this particular instance is cached. This allows more
     // fine-grained control over which instances get cache.
@@ -285,8 +285,8 @@ class Evaluator {
            typename std::enable_if<!Request::isEverCached>::type * = nullptr>
   llvm::Expected<typename Request::OutputType>
   operator()(const Request &request) {
-    evaluator::DependencyCollector::StackRAII<Request> incDeps{collector,
-                                                               request};
+    evaluator::DependencyRecorder::StackRAII<Request> incDeps{recorder,
+                                                              request};
     return getResultUncached(request);
   }
 
@@ -436,15 +436,15 @@ class Evaluator {
                                   !Request::isDependencySink>::type * = nullptr>
   void reportEvaluatedResult(const Request &r,
                              const typename Request::OutputType &o) {
-    collector.replay(ActiveRequest(r));
+    recorder.replay(ActiveRequest(r));
   }
 
   // Report the result of evaluating a request that is a dependency sink.
   template <typename Request,
             typename std::enable_if<Request::isDependencySink>::type * = nullptr>
   void reportEvaluatedResult(const Request &r,
                              const typename Request::OutputType &o) {
-    return collector.record(activeRequests, [&r, &o](auto &c) {
+    return recorder.record(activeRequests, [&r, &o](auto &c) {
       return r.writeDependencySink(c, o);
     });
   }

include/swift/AST/EvaluatorDependencies.h

@@ -107,13 +107,9 @@ inline DependencyScope getScopeForAccessLevel(AccessLevel l) {
 // of individual contexts.
 using DependencySource = llvm::PointerIntPair<SourceFile *, 1, DependencyScope>;
 
-/// A \c DependencyCollector is an aggregator of named references discovered in a
-/// particular \c DependencyScope during the course of request evaluation.
-struct DependencyCollector {
-private:
-  /// A stack of dependency sources in the order they were evaluated.
-  llvm::SmallVector<evaluator::DependencySource, 8> dependencySources;
+struct DependencyRecorder;
 
+struct DependencyCollector {
   struct Reference {
   public:
     enum class Kind {
@@ -178,36 +174,11 @@ struct DependencyCollector {
     };
   };
 
-public:
-  enum class Mode {
-    // Enables the current "status quo" behavior of the dependency collector.
-    //
-    // By default, the dependency collector moves to register dependencies in
-    // the referenced name trackers at the top of the active dependency stack.
-    StatusQuo,
-    // Enables an experimental mode to only register private dependencies.
-    //
-    // This mode restricts the dependency collector to ignore changes of
-    // scope. This has practical effect of charging all unqualified lookups to
-    // the primary file being acted upon instead of to the destination file.
-    ExperimentalPrivateDependencies,
-  };
-  Mode mode;
-  llvm::DenseMap<AnyRequest, llvm::DenseSet<Reference, Reference::Info>>
-      requestReferences;
+  DependencyRecorder &parent;
   llvm::DenseSet<Reference, Reference::Info> scratch;
-  bool isRecording;
-
-  explicit DependencyCollector(Mode mode)
-      : mode{mode}, requestReferences{}, scratch{}, isRecording{false} {};
-
-private:
-  void realizeOrRecord(const Reference &ref);
 
 public:
-  void replay(const swift::ActiveRequest &req);
-  void record(const llvm::SetVector<swift::ActiveRequest> &stack,
-              llvm::function_ref<void(DependencyCollector &)> rec);
+  explicit DependencyCollector(DependencyRecorder &parent) : parent(parent) {}
 
 public:
   /// Registers a named reference from the current dependency scope to a member
@@ -246,6 +217,50 @@ struct DependencyCollector {
   /// a name that is found by \c AnyObject lookup.
   void addDynamicLookupName(DeclBaseName name);
 
+public:
+  const DependencyRecorder &getRecorder() const { return parent; }
+  bool empty() const { return scratch.empty(); }
+};
+
+/// A \c DependencyCollector is an aggregator of named references discovered in a
+/// particular \c DependencyScope during the course of request evaluation.
+struct DependencyRecorder {
+  friend DependencyCollector;
+private:
+  /// A stack of dependency sources in the order they were evaluated.
+  llvm::SmallVector<evaluator::DependencySource, 8> dependencySources;
+
+public:
+  enum class Mode {
+    // Enables the current "status quo" behavior of the dependency collector.
+    //
+    // By default, the dependency collector moves to register dependencies in
+    // the referenced name trackers at the top of the active dependency stack.
+    StatusQuo,
+    // Enables an experimental mode to only register private dependencies.
+    //
+    // This mode restricts the dependency collector to ignore changes of
+    // scope. This has practical effect of charging all unqualified lookups to
+    // the primary file being acted upon instead of to the destination file.
+    ExperimentalPrivateDependencies,
+  };
+  Mode mode;
+  llvm::DenseMap<AnyRequest, llvm::DenseSet<DependencyCollector::Reference,
+                                            DependencyCollector::Reference::Info>>
+      requestReferences;
+  bool isRecording;
+
+  explicit DependencyRecorder(Mode mode)
+      : mode{mode}, requestReferences{}, isRecording{false} {};
+
+private:
+  void realize(const DependencyCollector::Reference &ref);
+
+public:
+  void replay(const swift::ActiveRequest &req);
+  void record(const llvm::SetVector<swift::ActiveRequest> &stack,
+              llvm::function_ref<void(DependencyCollector &)> rec);
+
 public:
   /// Returns the scope of the current active scope.
   ///
@@ -275,14 +290,14 @@ struct DependencyCollector {
   /// dependency source stack. It is specialized to be zero-cost for
   /// requests that are not dependency sources.
   template <typename Request, typename = detail::void_t<>> struct StackRAII {
-    StackRAII(DependencyCollector &DC, const Request &Req) {}
+    StackRAII(DependencyRecorder &DR, const Request &Req) {}
   };
 
   template <typename Request>
   struct StackRAII<Request,
                    typename std::enable_if<Request::isDependencySource>::type> {
-    NullablePtr<DependencyCollector> Coll;
-    StackRAII(DependencyCollector &coll, const Request &Req) {
+    NullablePtr<DependencyRecorder> Coll;
+    StackRAII(DependencyRecorder &coll, const Request &Req) {
       auto Source = Req.readDependencySource(coll);
       // If there is no source to introduce, bail. This can occur if
       // a request originates in the context of a module.

include/swift/AST/IRGenRequests.h

@@ -200,7 +200,7 @@ class IRGenSourceFileRequest
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 class IRGenWholeModuleRequest

include/swift/AST/NameLookupRequests.h

@@ -189,7 +189,7 @@ class InheritedProtocolsRequest
 public:
   // Incremental dependencies
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &e) const;
+  readDependencySource(const evaluator::DependencyRecorder &e) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            ArrayRef<ProtocolDecl *> result) const;
 };
@@ -330,7 +330,7 @@ class GetDestructorRequest
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 class GenericParamListRequest :
@@ -434,7 +434,7 @@ class UnqualifiedLookupRequest
 public:
   // Incremental dependencies
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            LookupResult res) const;
 };
@@ -506,7 +506,7 @@ class ModuleQualifiedLookupRequest
 public:
   // Incremental dependencies
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            QualifiedLookupResult lookupResult) const;
 };
@@ -533,7 +533,7 @@ class QualifiedLookupRequest
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 /// The input type for a direct lookup request.

include/swift/AST/ParseRequests.h

@@ -103,7 +103,7 @@ class ParseSourceFileRequest
 
 public:
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 void simple_display(llvm::raw_ostream &out,
@@ -125,7 +125,7 @@ class CodeCompletionSecondPassRequest
 
 public:
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 /// The zone number for the parser.

include/swift/AST/SILGenRequests.h

@@ -104,7 +104,7 @@ class SILGenerationRequest
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 /// Parses a .sil file into a SILModule.

include/swift/AST/SimpleRequest.h

@@ -232,7 +232,7 @@ SourceLoc extractNearestSourceLoc(const std::tuple<First, Rest...> &value) {
 /// the 3 caching kinds defined above.
 /// \code
 ///   evaluator::DependencySource
-///   readDependencySource(const evaluator::DependencyCollector &) const;
+///   readDependencySource(const evaluator::DependencyRecorder &) const;
 /// \endcode
 ///
 /// Requests that define dependency sinks should instead override

include/swift/AST/TypeCheckRequests.h

@@ -123,7 +123,7 @@ class SuperclassTypeRequest
 public:
   // Incremental dependencies
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &e) const;
+  readDependencySource(const evaluator::DependencyRecorder &e) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            Type t) const;
 };
@@ -893,7 +893,7 @@ class TypeCheckFunctionBodyUntilRequest :
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 /// Request to obtain a list of stored properties in a nominal type.
@@ -2034,7 +2034,7 @@ class TypeCheckSourceFileRequest
 public:
   // Incremental dependencies.
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &) const;
+  readDependencySource(const evaluator::DependencyRecorder &) const;
 };
 
 /// Computes whether the specified type or a super-class/super-protocol has the
@@ -2277,7 +2277,7 @@ class LookupAllConformancesInContextRequest
 public:
   // Incremental dependencies
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &eval) const;
+  readDependencySource(const evaluator::DependencyRecorder &eval) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            ProtocolConformanceLookupResult r) const;
 };
@@ -2305,7 +2305,7 @@ class CheckRedeclarationRequest
 
 public:
   evaluator::DependencySource
-  readDependencySource(const evaluator::DependencyCollector &eval) const;
+  readDependencySource(const evaluator::DependencyRecorder &eval) const;
   void writeDependencySink(evaluator::DependencyCollector &tracker,
                            evaluator::SideEffect) const;
 };