Improve the reporting of the number of CPUs on Mac OS

runtime/bin/platform_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include <mach-o/dyld.h>
+#include <sys/types.h>
+#include <sys/sysctl.h>
 
 #include "bin/file.h"
 #include "bin/platform.h"
 
 #include <crt_externs.h>  // NOLINT
 #include <signal.h>  // NOLINT
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
 
 #include "bin/fdutils.h"
@@ skipping 11 matching lines @@
   act.sa_handler = SIG_IGN;
   if (sigaction(SIGPIPE, &act, 0) != 0) {
     perror("Setting signal handler failed");
     return false;
   }
   return true;
 }
 
 
 int Platform::NumberOfProcessors() {
-  return sysconf(_SC_NPROCESSORS_ONLN);
+  int32_t cpus = -1;
+  size_t cpus_length = sizeof(cpus);
+  if (sysctlbyname("hw.logicalcpu", &cpus, &cpus_length, NULL, 0) == 0) {
+    return cpus;
+  } else {
+    // Failed, fallback to using sysconf.
+    return sysconf(_SC_NPROCESSORS_ONLN);
+  }
 }
 
 
 const char* Platform::OperatingSystem() {
   return "macos";
 }
 
 
 const char* Platform::LibraryExtension() {
   return "dylib";
@@ skipping 55 matching lines @@
   // Return the canonical path as the returned path might contain symlinks.
   char* canon_path = File::GetCanonicalPath(path);
   free(path);
   return canon_path;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)