[lld-macho] Add support for --threads

[int3]

Summary

Code and test are largely identical to the LLD-ELF equivalents.

lld/MachO/Driver.cpp

 #include "llvm/BinaryFormat/Magic.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/TarWriter.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/TimeProfiler.h"
 #include "llvm/TextAPI/MachO/PackedVersion.h"
 
 #include <algorithm>
 
if (errOrWriter) {
       error("--reproduce: " + toString(errOrWriter.takeError()));
     }
   }
 
   config = make<Configuration>();
   symtab = make<SymbolTable>();
   target = createTargetInfo(args);
 
+  if (auto *arg = args.getLastArg(OPT_threads_eq)) {
+    StringRef v(arg->getValue());
+    unsigned threads = 0;
+    if (!llvm::to_integer(v, threads, 0) || threads == 0)
+      error(arg->getSpelling() + ": expected a positive integer, but got '" +
+            arg->getValue() + "'");
+    parallel::strategy = hardware_concurrency(threads);
+    // FIXME: use this to configure ThinLTO concurrency too
+  }
+
   config->entry = symtab->addUndefined(args.getLastArgValue(OPT_e, "_main"),
                                        /*file=*/nullptr,
                                        /*isWeakRef=*/false);
   for (const Arg *arg : args.filtered(OPT_u)) {
     config->explicitUndefineds.push_back(symtab->addUndefined(
         arg->getValue(), /*file=*/nullptr, /*isWeakRef=*/false));
   }
 

lld/MachO/Options.td

def color_diagnostics: Flag<["--"], "color-diagnostics">,
     Group<grp_lld>;
 def no_color_diagnostics: Flag<["--"], "no-color-diagnostics">,
     HelpText<"Alias for --color-diagnostics=never">,
     Group<grp_lld>;
 def color_diagnostics_eq: Joined<["--"], "color-diagnostics=">,
     HelpText<"Use colors in diagnostics (default: auto)">,
     MetaVarName<"[auto,always,never]">,
     Group<grp_lld>;
+def threads_eq : Joined<["--"], "threads=">,
+    HelpText<"Number of threads. '1' disables multi-threading. By default all available hardware threads are used">,
+    Group<grp_lld>;
 def reproduce: Separate<["--"], "reproduce">,
     Group<grp_lld>;
 def reproduce_eq: Joined<["--"], "reproduce=">,
     Alias<!cast<Separate>(reproduce)>,
     HelpText<"Write tar file containing inputs and command to reproduce link">,
     Group<grp_lld>;
 def version: Flag<["--"], "version">,
     HelpText<"Display the version number and exit">,

lld/test/MachO/threads.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
+
+## A positive integer is allowed.
+# RUN: %lld --threads=1 %t.o -o /dev/null
+# RUN: %lld --threads=2 %t.o -o /dev/null
+
+# RUN: not %lld --threads=all %t.o -o /dev/null 2>&1 | FileCheck %s -DN=all
+# RUN: not %lld --threads=0 %t.o -o /dev/null 2>&1 | FileCheck %s -DN=0
+# RUN: not %lld --threads=-1 %t.o -o /dev/null 2>&1 | FileCheck %s -DN=-1
+
+# CHECK: error: --threads=: expected a positive integer, but got '[[N]]'
+
+.globl _main
+_main:
+  ret