Add Real Enable/Disable Flags for Cross-Module Incremental Builds

In the legacy driver, these flags will merely be propagated to the
frontends to indicate that they should disable serialization of
incremental information in swift module files.

In the new driver, these flags control whether the Swift driver performs
an incremental build that is aware of metadata embedded in the module.

Kudos to David for coming up with our new marketing name: Incremental
Imports.

rdar://74363450

Author: CodaFi

include/swift/Frontend/FrontendOptions.h

@@ -298,7 +298,7 @@ class FrontendOptions {
   ///
   /// This flag is currently only propagated from the driver to
   /// any merge-modules jobs.
-  bool EnableExperimentalCrossModuleIncrementalBuild = false;
+  bool DisableCrossModuleIncrementalBuild = false;
 
   /// Best effort to output a .swiftmodule regardless of any compilation
   /// errors. SIL generation and serialization is skipped entirely when there

include/swift/Option/Options.td

@@ -1111,6 +1111,18 @@ def emit_supported_features : Flag<["-"], "emit-supported-features">,
   HelpText<"Emit a JSON file including all supported compiler features">, ModeOpt,
   Flags<[FrontendOption, NoInteractiveOption, DoesNotAffectIncrementalBuild]>;
 
+def enable_incremental_imports :
+  Flag<["-"], "enable-incremental-imports">,
+  Flags<[FrontendOption]>,
+  HelpText<"Enable cross-module incremental build metadata and "
+           "driver scheduling for Swift modules">;
+
+def disable_incremental_imports :
+  Flag<["-"], "disable-incremental-imports">,
+  Flags<[FrontendOption]>,
+  HelpText<"Disable cross-module incremental build metadata and "
+           "driver scheduling for Swift modules">;
+
 def index_file : Flag<["-"], "index-file">,
   HelpText<"Produce index data for a source file">, ModeOpt,
   Flags<[NoInteractiveOption, DoesNotAffectIncrementalBuild]>;

include/swift/Serialization/SerializationOptions.h

@@ -132,7 +132,7 @@ namespace swift {
     bool SerializeAllSIL = false;
     bool SerializeOptionsForDebugging = false;
     bool IsSIB = false;
-    bool ExperimentalCrossModuleIncrementalInfo = false;
+    bool DisableCrossModuleIncrementalInfo = false;
   };
 
 } // end namespace swift

lib/Driver/Driver.cpp

@@ -1008,8 +1008,9 @@ Driver::buildCompilation(const ToolChain &TC,
     const bool EmitFineGrainedDependencyDotFileAfterEveryImport = ArgList->hasArg(
         options::
             OPT_driver_emit_fine_grained_dependency_dot_file_after_every_import);
-    const bool EnableCrossModuleDependencies = ArgList->hasArg(
-        options::OPT_enable_experimental_cross_module_incremental_build);
+    const bool EnableCrossModuleDependencies
+        = ArgList->hasArg(options::OPT_enable_incremental_imports,
+                          options::OPT_disable_incremental_imports, true);
 
     // clang-format off
     C = std::make_unique<Compilation>(

lib/Driver/ToolChains.cpp

@@ -1050,9 +1050,7 @@ ToolChain::constructInvocation(const MergeModuleJobAction &job,
 
   context.Args.AddLastArg(Arguments, options::OPT_import_objc_header);
 
-  context.Args.AddLastArg(
-      Arguments,
-      options::OPT_enable_experimental_cross_module_incremental_build);
+  context.Args.AddLastArg(Arguments, options::OPT_disable_incremental_imports);
 
   Arguments.push_back("-module-name");
   Arguments.push_back(context.Args.MakeArgString(context.OI.ModuleName));

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -103,8 +103,8 @@ bool ArgsToFrontendOptionsConverter::convert(
 
   Opts.ImportPrescan |= Args.hasArg(OPT_import_prescan);
 
-  Opts.EnableExperimentalCrossModuleIncrementalBuild |=
-      Args.hasArg(OPT_enable_experimental_cross_module_incremental_build);
+  Opts.DisableCrossModuleIncrementalBuild |=
+      Args.hasArg(OPT_disable_incremental_imports);
 
   // Always track system dependencies when scanning dependencies.
   if (const Arg *ModeArg = Args.getLastArg(OPT_modes_Group)) {

lib/Frontend/Frontend.cpp

@@ -178,8 +178,8 @@ SerializationOptions CompilerInvocation::computeSerializationOptions(
       opts.SerializeOptionsForDebugging.getValueOr(
           !isModuleExternallyConsumed(module));
 
-  serializationOpts.ExperimentalCrossModuleIncrementalInfo =
-      opts.EnableExperimentalCrossModuleIncrementalBuild;
+  serializationOpts.DisableCrossModuleIncrementalInfo =
+      opts.DisableCrossModuleIncrementalBuild;
 
   return serializationOpts;
 }

lib/FrontendTool/FrontendTool.cpp

@@ -1536,7 +1536,11 @@ static bool performCompileStepsPostSILGen(CompilerInstance &Instance,
 
     SerializationOptions serializationOpts =
         Invocation.computeSerializationOptions(outs, Instance.getMainModule());
-    if (serializationOpts.ExperimentalCrossModuleIncrementalInfo) {
+
+    const bool canEmitIncrementalInfoIntoModule =
+        !serializationOpts.DisableCrossModuleIncrementalInfo &&
+        (Action == FrontendOptions::ActionType::MergeModules);
+    if (canEmitIncrementalInfoIntoModule) {
       const auto alsoEmitDotFile =
           Instance.getInvocation()
               .getLangOptions()

lib/Serialization/Serialization.cpp

@@ -5482,7 +5482,7 @@ void Serializer::writeToStream(
     S.writeInputBlock(options);
     S.writeSIL(SILMod, options.SerializeAllSIL);
     S.writeAST(DC);
-    if (options.ExperimentalCrossModuleIncrementalInfo && DepGraph) {
+    if (!options.DisableCrossModuleIncrementalInfo && DepGraph) {
       fine_grained_dependencies::writeFineGrainedDependencyGraph(
           S.Out, *DepGraph, fine_grained_dependencies::Purpose::ForSwiftModule);
     }