[lto] Add beginning of -save-temps option.

[silvas]

Summary

This is useful for debugging issues with LTO.
The option follows the analogous option in ld64 and the gold plugin (per
Rafael's suggestion).

For starters, this only dumps the combined bitcode file.
In a future patch I will add dumping for the .o file.

The naming of the output follows ld64's convention which is slightly more
consistent IMO (consistent .lto.<extension> for all the files).

[ruiu]

We generally don't add much code for debugging, but since the amount of code for this is small, this should be okay.

ELF/SymbolTable.cpp

150

Add a comment that this function is for debugging.

167

Instead of creating a file name here, I'd do that in saveBCFile.

[ruiu]

Did you miss my comments?

[silvas]

Oops, sorry, will fix up.

[silvas]

r263057, sorry about that.

ELF/Config.h

struct Configuration {
   bool GcSections;
   bool GnuHash = false;
   bool ICF;
   bool Mips64EL = false;
   bool NoUndefined;
   bool NoinhibitExec;
   bool PrintGcSections;
   bool Relocatable;
+  bool SaveTemps;
   bool Shared;
   bool Static = false;
   bool StripAll;
   bool SysvHash = true;
   bool Verbose;
   bool ZExecStack;
   bool ZNodelete;
   bool ZNow;

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->GcSections = Args.hasArg(OPT_gc_sections);
   Config->ICF = Args.hasArg(OPT_icf);
   Config->NoUndefined = Args.hasArg(OPT_no_undefined);
   Config->NoinhibitExec = Args.hasArg(OPT_noinhibit_exec);
   Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
+  Config->SaveTemps = Args.hasArg(OPT_save_temps);
   Config->Shared = Args.hasArg(OPT_shared);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->Verbose = Args.hasArg(OPT_verbose);
 
   Config->DynamicLinker = getString(Args, OPT_dynamic_linker);
   Config->Entry = getString(Args, OPT_entry);
   Config->Fini = getString(Args, OPT_fini, "_fini");
   Config->Init = getString(Args, OPT_init, "_init");

ELF/Options.td

 def no_warn_mismatch : Flag<["--"], "no-warn-mismatch">;
 def version_script : Separate<["--"], "version-script">;
 def warn_common : Flag<["--"], "warn-common">;
 def warn_shared_textrel : Flag<["--"], "warn-shared-textrel">;
 def G : Separate<["-"], "G">;
 
 // Aliases for ignored options
 def alias_version_script_version_script : Joined<["--"], "version-script=">, Alias<version_script>;
+
+// Debugging options
+def save_temps : Flag<["-"], "save-temps">;

ELF/SymbolTable.cpp

if (!DB)
       continue;
     GlobalValue *GV = M->getNamedValue(B->getName());
     assert(GV);
     Keep.push_back(GV);
   }
   Mover.move(std::move(M), Keep, [](GlobalValue &, IRMover::ValueAdder) {});
 }
 
+static void saveBCFile(std::string Path, Module &M) {
+  std::error_code EC;
+  raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
+  check(EC);
+  WriteBitcodeToFile(&M, OS, /* ShouldPreserveUseListOrder */ true);
+}
+
 // Merge all the bitcode files we have seen, codegen the result and return
 // the resulting ObjectFile.
 template <class ELFT>
 ObjectFile<ELFT> *SymbolTable<ELFT>::createCombinedLtoObject() {
   LLVMContext Context;
   Module Combined("ld-temp.o", Context);
   IRMover Mover(Combined);
   for (const std::unique_ptr<BitcodeFile> &F : BitcodeFiles)
     addBitcodeFile(Mover, *F, Context);
+  if (Config->SaveTemps)
+    saveBCFile(Config->OutputFile.str() + ".lto.bc", Combined);
   std::unique_ptr<InputFile> F = codegen(Combined);
   ObjectFiles.emplace_back(cast<ObjectFile<ELFT>>(F.release()));
   return &*ObjectFiles.back();
 }
 
 template <class ELFT> void SymbolTable<ELFT>::addCombinedLtoObject() {
   if (BitcodeFiles.empty())
     return;

test/ELF/lto/Inputs/save-temps.ll

+target triple = "x86_64-unknown-linux-gnu"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @bar() {
+  ret void
+}

test/ELF/lto/save-temps.ll

+; REQUIRES: x86
+; RUN: rm -f %t.so %t.so.lto.bc
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-as %p/Inputs/save-temps.ll -o %t2.o
+; RUN: ld.lld -shared -m elf_x86_64 %t.o %t2.o -o %t.so -save-temps
+; RUN: llvm-nm %t.so | FileCheck %s
+; RUN: llvm-nm %t.so.lto.bc | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @foo() {
+  ret void
+}
+
+; CHECK-DAG: T bar
+; CHECK-DAG: T foo