[lld][COFF][LTO] Implement /lldemit:llvm option (#66964)

With this new option, bitcode will be emited instead of object code.
This is analogous to the `--plugin-opt=emit-llvm` option in the ELF
linker.

Author: mizvekov

lld/COFF/Config.h

@@ -48,6 +48,8 @@ enum class ExportSource {
   ModuleDefinition,
 };
 
+enum class EmitKind { Obj, LLVM };
+
 // Represents an /export option.
 struct Export {
   StringRef name;       // N in /export:N or /export:E=N
@@ -311,6 +313,7 @@ struct Configuration {
   bool pseudoRelocs = false;
   bool stdcallFixup = false;
   bool writeCheckSum = false;
+  EmitKind emit = EmitKind::Obj;
 };
 
 } // namespace lld::coff

lld/COFF/Driver.cpp

@@ -1852,6 +1852,17 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   if (args.hasArg(OPT_lldsavetemps))
     config->saveTemps = true;
 
+  // Handle /lldemit
+  if (auto *arg = args.getLastArg(OPT_lldemit)) {
+    StringRef s = arg->getValue();
+    if (s == "obj")
+      config->emit = EmitKind::Obj;
+    else if (s == "llvm")
+      config->emit = EmitKind::LLVM;
+    else
+      error("/lldemit: unknown option: " + s);
+  }
+
   // Handle /kill-at
   if (args.hasArg(OPT_kill_at))
     config->killAt = true;
@@ -2395,7 +2406,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   // If -thinlto-index-only is given, we should create only "index
   // files" and not object files. Index file creation is already done
   // in addCombinedLTOObject, so we are done if that's the case.
-  if (config->thinLTOIndexOnly)
+  // Likewise, don't emit object files for other /lldemit options.
+  if (config->emit != EmitKind::Obj || config->thinLTOIndexOnly)
     return;
 
   // If we generated native object files from bitcode files, this resolves

lld/COFF/LTO.cpp

@@ -87,6 +87,15 @@ lto::Config BitcodeCompiler::createConfig() {
   c.RunCSIRInstr = ctx.config.ltoCSProfileGenerate;
   c.PGOWarnMismatch = ctx.config.ltoPGOWarnMismatch;
 
+  if (ctx.config.emit == EmitKind::LLVM) {
+    c.PostInternalizeModuleHook = [this](size_t task, const Module &m) {
+      if (std::unique_ptr<raw_fd_ostream> os =
+              openLTOOutputFile(ctx.config.outputFile))
+        WriteBitcodeToFile(m, *os, false);
+      return false;
+    };
+  }
+
   if (ctx.config.saveTemps)
     checkError(c.addSaveTemps(std::string(ctx.config.outputFile) + ".",
                               /*UseInputModulePath*/ true));

lld/COFF/Options.td

@@ -232,6 +232,7 @@ defm demangle : B<"demangle",
 def include_optional : Joined<["/", "-", "/?", "-?"], "includeoptional:">,
     HelpText<"Add symbol as undefined, but allow it to remain undefined">;
 def kill_at : F<"kill-at">;
+def lldemit : P<"lldemit", "Specify output type">;
 def lldmingw : F<"lldmingw">;
 def noseh : F<"noseh">;
 def osversion : P_priv<"osversion">;

lld/test/COFF/lto-emit-llvm.ll

@@ -0,0 +1,14 @@
+; REQUIRES: x86
+; RUN: llvm-as -o %T/lto.obj %s
+
+; RUN: lld-link /lldemit:llvm /out:%T/lto.bc /entry:main /subsystem:console %T/lto.obj
+; RUN: llvm-dis %T/lto.bc -o - | FileCheck %s
+
+; CHECK: define void @main()
+
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+define void @main() {
+  ret void
+}