[LTO] Support LLVM level link time optimization on Darwin

This commit adds -llvm-lto flag for driver to enable LTO at LLVM level.
When -llvm-lto given, compiler emits LLVM bitcode file instead of object
file and link them using libLTO.dylib plugin.

Author: kateinoigakukun

include/swift/Option/Options.td

@@ -500,6 +500,10 @@ def disable_bridging_pch : Flag<["-"], "disable-bridging-pch">,
   Flags<[HelpHidden]>,
   HelpText<"Disable automatic generation of bridging PCH files">;
 
+def llvm_lto : Flag<["-"], "llvm-lto">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Enable LLVM LTO">;
+
 // Experimental feature options
 
 // Note: this flag will be removed when JVP/differential generation in the

lib/Driver/DarwinToolChains.cpp

@@ -234,16 +234,19 @@ void
 toolchains::Darwin::addLinkerInputArgs(InvocationInfo &II,
                                        const JobContext &context) const {
   ArgStringList &Arguments = II.Arguments;
+  auto InputType = context.Args.hasArg(options::OPT_llvm_lto) ?
+                      file_types::TY_LLVM_BC :
+                      file_types::TY_Object;
   if (context.shouldUseInputFileList()) {
     Arguments.push_back("-filelist");
     Arguments.push_back(context.getTemporaryFilePath("inputs", "LinkFileList"));
     II.FilelistInfos.push_back(
-        {Arguments.back(), file_types::TY_Object,
+        {Arguments.back(), InputType,
          FilelistInfo::WhichFiles::InputJobsAndSourceInputActions});
   } else {
     addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,
-                           file_types::TY_Object);
-    addInputsOfType(Arguments, context.InputActions, file_types::TY_Object);
+                           InputType);
+    addInputsOfType(Arguments, context.InputActions, InputType);
   }
 
 
@@ -306,6 +309,20 @@ toolchains::Darwin::addArgsToLinkARCLite(ArgStringList &Arguments,
   }
 }
 
+void
+toolchains::Darwin::addLTOLibArgs(ArgStringList &Arguments,
+                                  const JobContext &context) const {
+  llvm::SmallString<128> LTOLibPath;
+  if (findXcodeClangPath(LTOLibPath)) {
+    llvm::sys::path::remove_filename(LTOLibPath); // 'clang'
+    llvm::sys::path::remove_filename(LTOLibPath); // 'bin'
+    llvm::sys::path::append(LTOLibPath, "lib", "libLTO.dylib");
+
+    Arguments.push_back("-lto_library");
+    Arguments.push_back(context.Args.MakeArgString(LTOLibPath));
+  }
+}
+
 void
 toolchains::Darwin::addSanitizerArgs(ArgStringList &Arguments,
                                      const DynamicLinkJobAction &job,
@@ -723,6 +740,10 @@ toolchains::Darwin::constructInvocation(const DynamicLinkJobAction &job,
 
   addArgsToLinkARCLite(Arguments, context);
 
+  if (context.Args.hasArg(options::OPT_llvm_lto)) {
+    addLTOLibArgs(Arguments, context);
+  }
+
   for (const Arg *arg :
        context.Args.filtered(options::OPT_F, options::OPT_Fsystem)) {
     Arguments.push_back("-F");
@@ -786,18 +807,20 @@ toolchains::Darwin::constructInvocation(const StaticLinkJobAction &job,
   ArgStringList &Arguments = II.Arguments;
 
   Arguments.push_back("-static");
-
+  auto InputType = context.Args.hasArg(options::OPT_llvm_lto) ?
+                      file_types::TY_LLVM_BC :
+                      file_types::TY_Object;
   if (context.shouldUseInputFileList()) {
     Arguments.push_back("-filelist");
     Arguments.push_back(context.getTemporaryFilePath("inputs", "LinkFileList"));
-    II.FilelistInfos.push_back({Arguments.back(), file_types::TY_Object,
+    II.FilelistInfos.push_back({Arguments.back(), InputType,
                                 FilelistInfo::WhichFiles::InputJobs});
   } else {
     addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,
-                           file_types::TY_Object);
+                           InputType);
   }
 
-  addInputsOfType(Arguments, context.InputActions, file_types::TY_Object);
+  addInputsOfType(Arguments, context.InputActions, InputType);
 
   Arguments.push_back("-o");
 

lib/Driver/Driver.cpp

@@ -1434,12 +1434,15 @@ static bool isSDKTooOld(StringRef sdkPath, const llvm::Triple &target) {
 void Driver::buildOutputInfo(const ToolChain &TC, const DerivedArgList &Args,
                              const bool BatchMode, const InputFileList &Inputs,
                              OutputInfo &OI) const {
+  auto LinkerInputType = Args.hasArg(options::OPT_llvm_lto)
+                            ? file_types::TY_LLVM_BC
+                            : file_types::TY_Object;
   // By default, the driver does not link its output; this will be updated
   // appropriately below if linking is required.
 
   OI.CompilerOutputType = driverKind == DriverKind::Interactive
                               ? file_types::TY_Nothing
-                              : file_types::TY_Object;
+                              : LinkerInputType;
 
   if (const Arg *A = Args.getLastArg(options::OPT_num_threads)) {
     if (BatchMode) {
@@ -1469,14 +1472,14 @@ void Driver::buildOutputInfo(const ToolChain &TC, const DerivedArgList &Args,
                        diag::error_static_emit_executable_disallowed);
 
       OI.LinkAction = LinkKind::Executable;
-      OI.CompilerOutputType = file_types::TY_Object;
+      OI.CompilerOutputType = LinkerInputType;
       break;
 
     case options::OPT_emit_library:
       OI.LinkAction = Args.hasArg(options::OPT_static) ?
                       LinkKind::StaticLibrary :
                       LinkKind::DynamicLibrary;
-      OI.CompilerOutputType = file_types::TY_Object;
+      OI.CompilerOutputType = LinkerInputType;
       break;
 
     case options::OPT_static:

lib/Driver/ToolChains.h

@@ -48,6 +48,9 @@ class LLVM_LIBRARY_VISIBILITY Darwin : public ToolChain {
   void addDeploymentTargetArgs(llvm::opt::ArgStringList &Arguments,
                                const JobContext &context) const;
 
+  void addLTOLibArgs(llvm::opt::ArgStringList &Arguments,
+                     const JobContext &context) const;
+
   void addCommonFrontendArgs(
       const OutputInfo &OI, const CommandOutput &output,
       const llvm::opt::ArgList &inputArgs,

test/Driver/Inputs/lto/lib.swift

@@ -0,0 +1,3 @@
+public func libraryFunction() {
+    print("Hello")
+}

test/Driver/Inputs/lto/main.swift

@@ -0,0 +1,3 @@
+import A
+
+libraryFunction()

test/Driver/link-time-opt-darwin.swift

@@ -0,0 +1,7 @@
+// REQUIRES: OS=macosx
+// RUN: %target-swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -llvm-lto | %FileCheck %s
+// CHECK: bin/swift -frontend -emit-bc
+// CHECK-NEXT: bin/ld {{.+}} -lto_library {{.+}}/lib/libLTO.dylib
+
+// RUN: %target-swiftc_driver %S/Inputs/lto/lib.swift -llvm-lto -emit-library -emit-module -module-name A
+// RUN: %target-swiftc_driver %S/Inputs/lto/main.swift -L. -I. -lA -llvm-lto