SILOptimizer: Compute ProtocolConformanceAnalysis cache lazily.

When serializing modules with `-experimental-skip-all-function-bodies`, this
analysis was eagerly populating a cache that would go unused since there is no
optimization to do. With `-experimental-lazy-typecheck`, this work would also
trigger unnecessary typechecking requests. NFC.

Author: tshortli

include/swift/SILOptimizer/Analysis/ProtocolConformanceAnalysis.h

@@ -39,9 +39,7 @@ class ProtocolConformanceAnalysis : public SILAnalysis {
       SoleConformingTypeMap;
 
   ProtocolConformanceAnalysis(SILModule *Mod)
-      : SILAnalysis(SILAnalysisKind::ProtocolConformance), M(Mod) {
-    init();
-  }
+      : SILAnalysis(SILAnalysisKind::ProtocolConformance), M(Mod) {}
 
   ~ProtocolConformanceAnalysis();
 
@@ -66,14 +64,15 @@ class ProtocolConformanceAnalysis : public SILAnalysis {
   virtual void invalidateFunctionTables() override {}
 
   /// Get the nominal types that implement a protocol.
-  ArrayRef<NominalTypeDecl *> getConformances(const ProtocolDecl *P) const {
-    auto ConformsListIt = ProtocolConformanceCache.find(P);
-    return ConformsListIt != ProtocolConformanceCache.end()
+  ArrayRef<NominalTypeDecl *> getConformances(const ProtocolDecl *P) {
+    populateConformanceCacheIfNecessary();
+    auto ConformsListIt = ProtocolConformanceCache->find(P);
+    return ConformsListIt != ProtocolConformanceCache->end()
                ? ArrayRef<NominalTypeDecl *>(ConformsListIt->second.begin(),
                                              ConformsListIt->second.end())
                : ArrayRef<NominalTypeDecl *>();
   }
-  
+
   /// Traverse ProtocolConformanceMapCache recursively to determine sole
   /// conforming concrete type. 
   NominalTypeDecl *findSoleConformingType(ProtocolDecl *Protocol);
@@ -83,17 +82,17 @@ class ProtocolConformanceAnalysis : public SILAnalysis {
   bool getSoleConformingType(ProtocolDecl *Protocol, ClassHierarchyAnalysis *CHA, CanType &ConcreteType);
 
 private:
-  /// Compute inheritance properties.
-  void init();
-
   /// The module.
   SILModule *M;
 
   /// A cache that maps a protocol to its conformances.
-  ProtocolConformanceMap ProtocolConformanceCache;
+  llvm::Optional<ProtocolConformanceMap> ProtocolConformanceCache;
 
   /// A cache that holds SoleConformingType for protocols.
   SoleConformingTypeMap SoleConformingTypeCache;
+
+  /// Populates `ProtocolConformanceCache` if necessary.
+  void populateConformanceCacheIfNecessary();
 };
 
 } // namespace swift

lib/SILOptimizer/Analysis/ProtocolConformanceAnalysis.cpp

@@ -70,7 +70,11 @@ class NominalTypeWalker : public ASTWalker {
 };
 } // end anonymous namespace
 
-void ProtocolConformanceAnalysis::init() {
+void ProtocolConformanceAnalysis::populateConformanceCacheIfNecessary() {
+  if (ProtocolConformanceCache.has_value())
+    return;
+
+  ProtocolConformanceCache.emplace();
 
   // We only do this in Whole-Module compilation mode.
   if (!M->isWholeModule())
@@ -84,7 +88,7 @@ void ProtocolConformanceAnalysis::init() {
 
   /// This operation is quadratic and should only be performed
   /// in whole module compilation!
-  NominalTypeWalker Walker(ProtocolConformanceCache);
+  NominalTypeWalker Walker(*ProtocolConformanceCache);
   for (auto *D : Decls) {
     D->walk(Walker);
   }