[lld/COFF] Add /dwodir to enable DWARF fission with LTO

[HaohaiWen]

Summary

This patch added /dwodir to lld/COFF which is equivalent to lld/ELF
option -plugin-opt=dwo_dir=. This option tells LTO backend to create
dwo directory and files and all dwo files will be in it. Otherwise all
dwarf sections will be embeded into image even if -gsplit-dwarf is
specified when using LTO.

[MaskRay]

Normally clang and lld patches are separate. You can git log -- lld/COFF to find who usually reviews lld/COFF code and add these folks as reviewers.
Not many people set subscription rules but some may appreciate if you let know that there is such a change.
Though, I have reviewed some lld/COFF patches and am happy to stamp this change.

[MaskRay]

I'll be away for a few days and back on July 5. Clicked "Accept" if you keep just the lld/COFF part for this patch and create another patch for clang/lib/Driver.

I guess @mstorsjo may have some opinions.

[HaohaiWen]

Remove driver changes

[HaohaiWen]

In D154070#4461674, @MaskRay wrote:

I'll be away for a few days and back on July 5. Clicked "Accept" if you keep just the lld/COFF part for this patch and create another patch for clang/lib/Driver.

I guess @mstorsjo may have some opinions.

Thank you.

Patch has been split to:
D154176

[mstorsjo]

No objection from me here, this seems straightforward. We should probably add a corresponding option in the lld mingw frontend too (as a separate patch later).

lld/COFF/Config.h

struct Configuration {
   int manifestID = 1;
   llvm::SetVector<StringRef> manifestDependencies;
   bool manifestUAC = true;
   std::vector<std::string> manifestInput;
   StringRef manifestLevel = "'asInvoker'";
   StringRef manifestUIAccess = "'false'";
   StringRef manifestFile;
 
+  // used for /dwodir
+  StringRef dwoDir;
+
   // Used for /aligncomm.
   std::map<std::string, int> alignComm;
 
   // Used for /failifmismatch.
   std::map<StringRef, std::pair<StringRef, InputFile *>> mustMatch;
 
   // Used for /alternatename.
   std::map<StringRef, StringRef> alternateNames;

lld/COFF/Driver.cpp

void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   for (auto *arg : args.filtered(OPT_manifestinput))
     config->manifestInput.push_back(arg->getValue());
 
   if (!config->manifestInput.empty() &&
       config->manifest != Configuration::Embed) {
     fatal("/manifestinput: requires /manifest:embed");
   }
 
+  // Handle /dwodir
+  config->dwoDir = args.getLastArgValue(OPT_dwodir);
+
   config->thinLTOEmitImportsFiles = args.hasArg(OPT_thinlto_emit_imports_files);
   config->thinLTOIndexOnly = args.hasArg(OPT_thinlto_index_only) ||
                              args.hasArg(OPT_thinlto_index_only_arg);
   config->thinLTOIndexOnlyArg =
       args.getLastArgValue(OPT_thinlto_index_only_arg);
   std::tie(config->thinLTOPrefixReplaceOld, config->thinLTOPrefixReplaceNew,
            config->thinLTOPrefixReplaceNativeObject) =
       getOldNewOptionsExtra(args, OPT_thinlto_prefix_replace);

lld/COFF/LTO.cpp

lto::Config BitcodeCompiler::createConfig() {
   else
     c.RelocModel = Reloc::PIC_;
 #ifndef NDEBUG
   c.DisableVerify = false;
 #else
   c.DisableVerify = true;
 #endif
   c.DiagHandler = diagnosticHandler;
+  c.DwoDir = ctx.config.dwoDir.str();
   c.OptLevel = ctx.config.ltoo;
   c.CPU = getCPUStr();
   c.MAttrs = getMAttrs();
   std::optional<CodeGenOpt::Level> optLevelOrNone = CodeGenOpt::getLevel(
       ctx.config.ltoCgo.value_or(args::getCGOptLevel(ctx.config.ltoo)));
   assert(optLevelOrNone && "Invalid optimization level!");
   c.CGOptLevel = *optLevelOrNone;
   c.AlwaysEmitRegularLTOObj = !ctx.config.ltoObjPath.empty();

lld/COFF/Options.td

 def no_color_diagnostics: Flag<["--"], "no-color-diagnostics">,
     HelpText<"Alias for --color-diagnostics=never">;
 def color_diagnostics_eq: Joined<["--"], "color-diagnostics=">,
     HelpText<"Use colors in diagnostics (default: auto)">,
     MetaVarName<"[auto,always,never]">;
 def defaultlib : P<"defaultlib", "Add the library to the list of input files">;
 def delayload : P<"delayload", "Delay loaded DLL name">;
 def diasdkdir : P<"diasdkdir", "Set the location of the DIA SDK">;
+def dwodir : P<"dwodir",
+    "Directory to store .dwo files when LTO and debug fission are used">;
 def entry   : P<"entry", "Name of entry point symbol">;
 def errorlimit : P<"errorlimit",
     "Maximum number of errors to emit before stopping (0 = no limit)">;
 def exclude_symbols  : P<"exclude-symbols", "Exclude symbols from automatic export">,
     MetaVarName<"<symbol[,symbol,...]>">;
 def export  : P<"export", "Export a function">;
 // No help text because /failifmismatch is not intended to be used by the user.
 def failifmismatch : P<"failifmismatch", "">;

lld/test/COFF/lto-debug-fission.ll

+; REQUIRES: x86
+
+; RUN: opt %s -o %t1.o
+; RUN: rm -rf %t.dir
+
+; Test to ensure that -dwodir:$DIR creates .dwo files under $DIR
+; RUN: lld-link -dwodir:%t.dir -noentry -dll %t1.o -out:%t.dll
+; RUN: llvm-readobj -h %t.dir/0.dwo | FileCheck %s
+
+; CHECK: Format: COFF-x86-64
+
+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-unknown-windows-msvc"
+
+define void @f() {
+entry:
+  ret void
+}