IRGen: Write .dwo files when -split-dwarf-file is used together with -fthinlto-index.

[tejohnson]

LGTM thanks!

clang/lib/CodeGen/BackendUtil.cpp

static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
   Conf.RelocModel = CGOpts.RelocationModel;
   Conf.CGOptLevel = getCGOptLevel(CGOpts);
   initTargetOptions(Conf.Options, CGOpts, TOpts, LOpts, HeaderOpts);
   Conf.SampleProfile = std::move(SampleProfile);
   Conf.UseNewPM = CGOpts.ExperimentalNewPassManager;
   Conf.DebugPassManager = CGOpts.DebugPassManager;
   Conf.RemarksWithHotness = CGOpts.DiagnosticsWithHotness;
   Conf.RemarksFilename = CGOpts.OptRecordFile;
+  Conf.DwoPath = CGOpts.SplitDwarfFile;
   switch (Action) {
   case Backend_EmitNothing:
     Conf.PreCodeGenModuleHook = [](size_t Task, const Module &Mod) {
       return false;
     };
     break;
   case Backend_EmitLL:
     Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {

clang/test/CodeGen/thinlto-split-dwarf.c

+// REQUIRES: x86-registered-target
+
+// RUN: %clang_cc1 -debug-info-kind=limited -triple x86_64-unknown-linux-gnu \
+// RUN:   -flto=thin -emit-llvm-bc \
+// RUN:   -o %t.o %s
+
+// RUN: llvm-lto2 run -thinlto-distributed-indexes %t.o \
+// RUN:   -o %t2.index \
+// RUN:   -r=%t.o,main,px
+
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu \
+// RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc \
+// RUN:   -o %t.native.o -split-dwarf-file %t.native.dwo -x ir %t.o
+
+// RUN: llvm-readobj -sections %t.native.o | FileCheck --check-prefix=O %s
+// RUN: llvm-readobj -sections %t.native.dwo | FileCheck --check-prefix=DWO %s
+
+// O-NOT: .dwo
+// DWO: .dwo
+
+int main() {}

llvm/include/llvm/LTO/Config.h

struct Config {
   std::string DefaultTriple;
 
   /// Sample PGO profile path.
   std::string SampleProfile;
 
   /// The directory to store .dwo files.
   std::string DwoDir;
 
+  /// The path to write a .dwo file to. This should generally only be used when
+  /// running an individual backend directly via thinBackend(), as otherwise
+  /// all .dwo files will be written to the same path.
+  std::string DwoPath;
+
   /// Optimization remarks file path.
   std::string RemarksFilename = "";
 
   /// Whether to emit optimization remarks with hotness informations.
   bool RemarksWithHotness = false;
 
   /// Whether to emit the pass manager debuggging informations.
   bool DebugPassManager = false;

llvm/lib/LTO/LTOBackend.cpp

 }
 
 void codegen(Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
              unsigned Task, Module &Mod) {
   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
     return;
 
   std::unique_ptr<ToolOutputFile> DwoOut;
+  SmallString<1024> DwoFile(Conf.DwoPath);
   if (!Conf.DwoDir.empty()) {
     std::error_code EC;
     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
       report_fatal_error("Failed to create directory " + Conf.DwoDir + ": " +
                          EC.message());
 
-    SmallString<1024> DwoFile(Conf.DwoDir);
+    DwoFile = Conf.DwoDir;
     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
+  }
+
+  if (!DwoFile.empty()) {
+    std::error_code EC;
     TM->Options.MCOptions.SplitDwarfFile = DwoFile.str().str();
     DwoOut = llvm::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::F_None);
     if (EC)
       report_fatal_error("Failed to open " + DwoFile + ": " + EC.message());
   }
 
   auto Stream = AddStream(Task);
   legacy::PassManager CodeGenPasses;