[AutoDiff] Lazily create synthesized file during differentiation.

Make `ADContext` lazily create a `SynthesizedFileUnit` instead of creating one
during `ADContext` construction. This avoids always creating a
`SynthesizedFileUnit` in every module, since differentiation is a mandatory
transform that always runs.

It was nonetheless useful to test always creating a `SynthesizedFileUnit` for
testing purposes.

Author: dan-zheng

include/swift/SILOptimizer/Utils/Differentiation/ADContext.h

@@ -68,7 +68,7 @@ class ADContext {
   SILPassManager &passManager;
 
   /// A synthesized file unit.
-  SynthesizedFileUnit &synthesizedFile;
+  SynthesizedFileUnit *synthesizedFile = nullptr;
 
   /// The worklist (stack) of `differentiable_function` instructions to be
   /// processed.
@@ -124,9 +124,12 @@ class ADContext {
   SILModule &getModule() const { return module; }
   ASTContext &getASTContext() const { return module.getASTContext(); }
   SILPassManager &getPassManager() const { return passManager; }
-  SynthesizedFileUnit &getSynthesizedFile() { return synthesizedFile; }
   Lowering::TypeConverter &getTypeConverter() { return module.Types; }
 
+  /// Get or create a synthesized file for adding generated linear map structs
+  /// and branching trace enums. Used by `LinearMapInfo`.
+  SynthesizedFileUnit &getOrCreateSynthesizedFile();
+
   /// Returns true if the `differentiable_function` instruction worklist is
   /// empty.
   bool isDifferentiableFunctionInstsWorklistEmpty() const {

lib/SILOptimizer/Utils/Differentiation/ADContext.cpp

@@ -54,17 +54,19 @@ static FuncDecl *findOperatorDeclInProtocol(DeclName operatorName,
 // ADContext methods
 //===----------------------------------------------------------------------===//
 
-static SynthesizedFileUnit *createNewSynthesizedFileUnit(ModuleDecl *module) {
-  auto &ctx = module->getASTContext();
-  auto *file = new (ctx) SynthesizedFileUnit(*module);
-  module->addFile(*file);
-  return file;
-}
-
 ADContext::ADContext(SILModuleTransform &transform)
     : transform(transform), module(*transform.getModule()),
-      passManager(*transform.getPassManager()),
-      synthesizedFile(*createNewSynthesizedFileUnit(module.getSwiftModule())) {}
+      passManager(*transform.getPassManager()) {}
+
+SynthesizedFileUnit &ADContext::getOrCreateSynthesizedFile() {
+  if (synthesizedFile)
+    return *synthesizedFile;
+  auto *moduleDecl = module.getSwiftModule();
+  auto &ctx = moduleDecl->getASTContext();
+  synthesizedFile = new (ctx) SynthesizedFileUnit(*moduleDecl);
+  moduleDecl->addFile(*synthesizedFile);
+  return *synthesizedFile;
+}
 
 FuncDecl *ADContext::getPlusDecl() const {
   if (!cachedPlusFn) {

lib/SILOptimizer/Utils/Differentiation/LinearMapInfo.cpp

@@ -58,7 +58,7 @@ LinearMapInfo::LinearMapInfo(ADContext &context, AutoDiffLinearMapKind kind,
                              const DifferentiableActivityInfo &activityInfo)
     : kind(kind), original(original), derivative(derivative),
       activityInfo(activityInfo), indices(indices),
-      synthesizedFile(context.getSynthesizedFile()),
+      synthesizedFile(context.getOrCreateSynthesizedFile()),
       typeConverter(context.getTypeConverter()) {
   generateDifferentiationDataStructures(context, derivative);
 }