[coff] default to multiple parallel ThinLTO jobs

[inglorion]

Summary

lld-link allows the number of parallel ThinLTO jobs to be specified
using /opt:lldltojobs=N. If left unspecified, the implementation
conservatively defaults to 1. This leads to very long link times. This
change makes it so that the default is to automatically set the
parallelism, as we do in the ELF linker.

[ruiu]

LGTM. Seems reasonable.

[hans]

Seems reasonable to me, but I'm not familiar with the difference between LTOJobs and LTOPartitions. Does it make sense to have multiple jobs but just one partition? I thought the point of ThinLTO was to process multiple partitions in parallel.

[pcc]

In D31986#725140, @hans wrote:

Seems reasonable to me, but I'm not familiar with the difference between LTOJobs and LTOPartitions. Does it make sense to have multiple jobs but just one partition? I thought the point of ThinLTO was to process multiple partitions in parallel.

LTOPartitions is used by parallel LTO code generation, which is a separate feature to ThinLTO.

[hans]

In D31986#725147, @pcc wrote:

In D31986#725140, @hans wrote:

Seems reasonable to me, but I'm not familiar with the difference between LTOJobs and LTOPartitions. Does it make sense to have multiple jobs but just one partition? I thought the point of ThinLTO was to process multiple partitions in parallel.

LTOPartitions is used by parallel LTO code generation, which is a separate feature to ThinLTO.

I see. LGTM then.

lld/trunk/COFF/Config.h

struct Configuration {
   // Used for SafeSEH.
   Symbol *SEHTable = nullptr;
   Symbol *SEHCount = nullptr;
 
   // Used for /opt:lldlto=N
   unsigned LTOOptLevel = 2;
 
   // Used for /opt:lldltojobs=N
-  unsigned LTOJobs = 1;
+  unsigned LTOJobs = 0;
   // Used for /opt:lldltopartitions=N
   unsigned LTOPartitions = 1;
 
   // Used for /merge:from=to (e.g. /merge:.rdata=.text)
   std::map<StringRef, StringRef> Merge;
 
   // Used for /section=.name,{DEKPRSW} to set section attributes.
   std::map<StringRef, uint32_t> Section;

lld/trunk/COFF/LTO.cpp

static std::unique_ptr<lto::LTO> createLTO() {
   Conf.RelocModel = Reloc::PIC_;
   Conf.DisableVerify = true;
   Conf.DiagHandler = diagnosticHandler;
   Conf.OptLevel = Config->LTOOptLevel;
   if (Config->SaveTemps)
     checkError(Conf.addSaveTemps(std::string(Config->OutputFile) + ".",
                                  /*UseInputModulePath*/ true));
   lto::ThinBackend Backend;
-  if (Config->LTOJobs != -1u)
+  if (Config->LTOJobs != 0)
     Backend = lto::createInProcessThinBackend(Config->LTOJobs);
   return llvm::make_unique<lto::LTO>(std::move(Conf), Backend,
                                      Config->LTOPartitions);
 }
 
 BitcodeCompiler::BitcodeCompiler() : LTOObj(createLTO()) {}
 
 BitcodeCompiler::~BitcodeCompiler() = default;