[lld/LTO] Add a flag to use the new PM during LTO

[davide]

Summary

Depends on D28996 / D28997 , also needs a test.
Please hold on reviewing until the other two revisions go in.

[chandlerc]

FWIW, this looks good to me, but I don't know much about LLD so maybe wait for someone else to look... Or maybe you know enough to land this directly Davide, you've been working lots in this space.

[ruiu]

Is this flag intended to be a permanent flag, or is it going to be used during transition from old to new? If it is going to be used indefinitely, new in -lto-use-new-pm doesn't make much sense.

[davide]

In D28998#653589, @ruiu wrote:

Is this flag intended to be a permanent flag, or is it going to be used during transition from old to new? If it is going to be used indefinitely, new in -lto-use-new-pm doesn't make much sense.

The hope is that once the new pass manager will be the default & baked for a bit, this flag will go away.

[davide]

Peter suggested we can use a cl::opt and therefore avoid teaching linkers about this. I think it's a good idea, so abandoning this for now.

ELF/Config.h

struct Configuration {
   bool EhFrameHdr;
   bool EnableNewDtags;
   bool ExportDynamic;
   bool FatalWarnings;
   bool GcSections;
   bool GdbIndex;
   bool GnuHash = false;
   bool ICF;
+  bool LTOUseNewPM;
   bool Mips64EL = false;
   bool MipsN32Abi = false;
   bool NoGnuUnique;
   bool NoUndefinedVersion;
   bool Nostdlib;
   bool OFormatBinary;
   bool OMagic;
   bool Pic;

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic =
       getArg(Args, OPT_export_dynamic, OPT_no_export_dynamic, false);
   Config->FatalWarnings =
       getArg(Args, OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->GdbIndex = Args.hasArg(OPT_gdb_index);
   Config->ICF = Args.hasArg(OPT_icf);
+  Config->LTOUseNewPM = Args.hasArg(OPT_lto_use_new_pm);
   Config->NoGnuUnique = Args.hasArg(OPT_no_gnu_unique);
   Config->NoUndefinedVersion = Args.hasArg(OPT_no_undefined_version);
   Config->Nostdlib = Args.hasArg(OPT_nostdlib);
   Config->OMagic = Args.hasArg(OPT_omagic);
   Config->Pie = getArg(Args, OPT_pie, OPT_nopie, false);
   Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->Discard = getDiscardOption(Args);

ELF/LTO.cpp

static std::unique_ptr<lto::LTO> createLTO() {
   // LLD supports the new relocations.
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
   Conf.Options.RelaxELFRelocations = true;
 
   Conf.RelocModel = Config->Pic ? Reloc::PIC_ : Reloc::Static;
   Conf.DisableVerify = Config->DisableVerify;
   Conf.DiagHandler = diagnosticHandler;
   Conf.OptLevel = Config->LTOO;
+  Conf.OptUseNewPM = Config->LTOUseNewPM;
 
   // Set up a custom pipeline if we've been asked to.
   Conf.OptPipeline = Config->LTONewPmPasses;
   Conf.AAPipeline = Config->LTOAAPipeline;
 
   if (Config->SaveTemps)
     checkError(Conf.addSaveTemps(std::string(Config->OutputFile) + ".",
                                  /*UseInputModulePath*/ true));

ELF/Options.td

 def image_base : J<"image-base=">, HelpText<"Set the base address">;
 
 def init: S<"init">, MetaVarName<"<symbol>">,
   HelpText<"Specify an initializer function">;
 
 def l: JoinedOrSeparate<["-"], "l">, MetaVarName<"<libName>">,
   HelpText<"Root name of library to use">;
 
+def lto_use_new_pm: F<"lto-use-new-pm">,
+  HelpText<"Run the new pass manager as part of LTO">;
+
 def lto_O: J<"lto-O">, MetaVarName<"<opt-level>">,
   HelpText<"Optimization level for LTO">;
 
 def m: JoinedOrSeparate<["-"], "m">, HelpText<"Set target emulation">;
 
 def Map: JS<"Map">, HelpText<"Print a link map to the specified file">;
 
 def nostdlib: F<"nostdlib">,