[embedded] Add a frontend flag to drop all code from a module and emit an empty object file

include/swift/Option/FrontendOptions.td

@@ -580,6 +580,10 @@ def enable_single_module_llvm_emission: Flag<["-"], "enable-single-module-llvm-e
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Emit LLVM IR into a single LLVM module in multithreaded mode.">;
 
+def emit_empty_object_file : Flag<["-"], "emit-empty-object-file">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Produce a valid but dummy object file when building a library module">;
+
 def stack_promotion_checks : Flag<["-"], "emit-stack-promotion-checks">,
   HelpText<"Emit runtime checks for correct stack promotion of objects.">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -2185,6 +2185,10 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       FEOpts.RequestedAction == FrontendOptions::ActionType::CompileModuleFromInterface)
     Opts.StopOptimizationAfterSerialization = true;
 
+  if (Args.getLastArg(OPT_emit_empty_object_file)) {
+    Opts.StopOptimizationAfterSerialization = true;
+  }
+
   // Propagate the typechecker's understanding of
   // -experimental-skip-*-function-bodies to SIL.
   Opts.SkipFunctionBodies = TCOpts.SkipFunctionBodies;

lib/IRGen/GenDecl.cpp

@@ -454,6 +454,11 @@ class PrettySynthesizedFileUnitEmission : public llvm::PrettyStackTraceEntry {
 
 /// Emit all the top-level code in the source file.
 void IRGenModule::emitSourceFile(SourceFile &SF) {
+  if (getSILModule().getOptions().StopOptimizationAfterSerialization) {
+    // We're asked to emit an empty IR module
+    return;
+  }
+
   // Type-check the file if we haven't already (this may be necessary for .sil
   // files, which don't get fully type-checked by parsing).
   performTypeChecking(SF);
@@ -1259,6 +1264,11 @@ static bool isLazilyEmittedFunction(SILFunction &f, SILModule &m) {
 
 void IRGenerator::emitGlobalTopLevel(
     const std::vector<std::string> &linkerDirectives) {
+  if (PrimaryIGM->getSILModule().getOptions().StopOptimizationAfterSerialization) {
+    // We're asked to emit an empty IR module
+    return;
+  }
+
   // Generate order numbers for the functions in the SIL module that
   // correspond to definitions in the LLVM module.
   unsigned nextOrderNumber = 0;
@@ -2195,6 +2205,11 @@ void IRGenerator::emitObjCActorsNeedingSuperclassSwizzle() {
 /// from other modules. This happens e.g. if a public class contains a (dead)
 /// private method.
 void IRGenModule::emitVTableStubs() {
+  if (getSILModule().getOptions().StopOptimizationAfterSerialization) {
+    // We're asked to emit an empty IR module
+    return;
+  }
+
   llvm::Function *stub = nullptr;
   for (auto I = getSILModule().zombies_begin();
            I != getSILModule().zombies_end(); ++I) {

lib/IRGen/GenReflection.cpp

@@ -1667,6 +1667,11 @@ llvm::ArrayRef<CanType> IRGenModule::getOrCreateSpecialStlibBuiltinTypes() {
 }
 
 void IRGenModule::emitBuiltinReflectionMetadata() {
+  if (getSILModule().getOptions().StopOptimizationAfterSerialization) {
+    // We're asked to emit an empty IR module
+    return;
+  }
+
   if (getSwiftModule()->isStdlibModule()) {
     auto SpecialBuiltins = getOrCreateSpecialStlibBuiltinTypes();
     BuiltinTypes.insert(SpecialBuiltins.begin(), SpecialBuiltins.end());

lib/IRGen/IRGenModule.cpp

@@ -1912,7 +1912,8 @@ void IRGenModule::cleanupClangCodeGenMetadata() {
 bool IRGenModule::finalize() {
   const char *ModuleHashVarName = "llvm.swift_module_hash";
   if (IRGen.Opts.OutputKind == IRGenOutputKind::ObjectFile &&
-      !Module.getGlobalVariable(ModuleHashVarName)) {
+      !Module.getGlobalVariable(ModuleHashVarName) &&
+      !getSILModule().getOptions().StopOptimizationAfterSerialization) {
     // Create a global variable into which we will store the hash of the
     // module (used for incremental compilation).
     // We have to create the variable now (before we emit the global lists).
@@ -1969,6 +1970,14 @@ bool IRGenModule::finalize() {
     if (F.hasDLLImportStorageClass())
       F.setDSOLocal(false);
 
+  if (getSILModule().getOptions().StopOptimizationAfterSerialization) {
+    // We're asked to emit an empty IR module, check that that's actually true
+    if (Module.global_size() != 0 || Module.size() != 0) {
+      Module.dump();
+      llvm::report_fatal_error("Module is not empty");
+    }
+  }
+
   return true;
 }