[Driver][MSVC] Support DWARF fission when using LTO on Windows

[HaohaiWen]

Summary

D154070 has added /dwodir to lld/COFF to tells LTO backend to create dwo
directory and files. This patch makes clang to emit /dwodir to lld when
user specify -gsplit-dwarf with LTO. This behavior is simiar to DWARF
fission with LTO for ELF.

A simple use case:
$clang-cl -c -flto -gdwarf main.c -o main.o
$clang-cl -c -flto -gdwarf a.c -o a.o
$clang-cl -flto -fuse-ld=lld -gdwarf -gsplit-dwarf main.o a.o

This'll generate a dwo file: main.exe_dwo/0.dwo

[mstorsjo]

I think this looks reasonable, unless e.g. @hans or @rnk would disagree.

[hans]

Looks reasonable to me too.

It would be nice to have some documentation for this feature though.

[HaohaiWen]

Thanks everyone.

It would be nice to have some documentation for this feature though.

This feature is same as Linux -gsplit-dwarf.

[hans]

In D154295#4477165, @HaohaiWen wrote:

It would be nice to have some documentation for this feature though.

This feature is same as Linux -gsplit-dwarf.

Right, but it would be nice to document that clang-cl supports it (which is not obvious since it's mostly trying to be cl.exe compatible).

[HaohaiWen]

In D154295#4477203, @hans wrote:

In D154295#4477165, @HaohaiWen wrote:

It would be nice to have some documentation for this feature though.

This feature is same as Linux -gsplit-dwarf.

Right, but it would be nice to document that clang-cl supports it (which is not obvious since it's mostly trying to be cl.exe compatible).

Do you know which doc file should be updated? I haven't found a doc describe -gdwarf for Windows.

[hans]

In D154295#4482818, @HaohaiWen wrote:

In D154295#4477203, @hans wrote:

In D154295#4477165, @HaohaiWen wrote:

It would be nice to have some documentation for this feature though.

This feature is same as Linux -gsplit-dwarf.

Right, but it would be nice to document that clang-cl supports it (which is not obvious since it's mostly trying to be cl.exe compatible).

Do you know which doc file should be updated? I haven't found a doc describe -gdwarf for Windows.

I'd suggest putting it under https://clang.llvm.org/docs/UsersManual.html#clang-cl

But it seems we might not even have any docs for -gsplit-dwarf on non-Windows either, I suppose this is already as documented as it was before :)

clang/lib/Driver/ToolChains/MSVC.cpp

void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   StringRef Linker =
       Args.getLastArgValue(options::OPT_fuse_ld_EQ, CLANG_DEFAULT_LINKER);
   if (Linker.empty())
     Linker = "link";
   // We need to translate 'lld' into 'lld-link'.
   else if (Linker.equals_insensitive("lld"))
     Linker = "lld-link";
 
-  if (Linker == "lld-link")
+  if (Linker == "lld-link") {
     for (Arg *A : Args.filtered(options::OPT_vfsoverlay))
       CmdArgs.push_back(
           Args.MakeArgString(std::string("/vfsoverlay:") + A->getValue()));
 
+    if (C.getDriver().isUsingLTO() &&
+        Args.hasFlag(options::OPT_gsplit_dwarf, options::OPT_gno_split_dwarf,
+                     false))
+      CmdArgs.push_back(Args.MakeArgString(Twine("/dwodir:") +
+                                           Output.getFilename() + "_dwo"));
+  }
+
   // Add filenames, libraries, and other linker inputs.
   for (const auto &Input : Inputs) {
     if (Input.isFilename()) {
       CmdArgs.push_back(Input.getFilename());
       continue;
     }
 
     const Arg &A = Input.getInputArg();

clang/test/Driver/lto-dwo.c

 // Confirm that -gsplit-dwarf=DIR is passed to linker
 
 // RUN: %clang --target=x86_64-unknown-linux -### %s -flto=thin -gsplit-dwarf -o a.out 2> %t
-// RUN: FileCheck -check-prefix=CHECK-LINK-DWO-DIR-DEFAULT < %t %s
+// RUN: FileCheck -check-prefix=CHECK-LINK-ELF-DWO-DIR-DEFAULT < %t %s
+// RUN: %clang_cl --target=x86_64-unknown-windows-msvc -### -fuse-ld=lld -flto -gsplit-dwarf -o a.out -- %s 2> %t
+// RUN: FileCheck -check-prefix=CHECK-LINK-COFF-DWO-DIR-DEFAULT < %t %s
 //
-// CHECK-LINK-DWO-DIR-DEFAULT: "-plugin-opt=dwo_dir=a.out_dwo"
+// CHECK-LINK-ELF-DWO-DIR-DEFAULT:  "-plugin-opt=dwo_dir=a.out_dwo"
+// CHECK-LINK-COFF-DWO-DIR-DEFAULT: "/dwodir:a.out_dwo"