For llvm's internal function which retrieves the number of available

"hardware threads", use cpuset_getaffinity(2) on FreeBSD, so it will
honor processor sets configured by the cpuset(1) command.

This should make it possible to avoid e.g. lld creating a huge number of
threads on a machine with many cores, even for linking simple programs.

This will also be submitted upstream.

Submitted by:	mjg
MFC after:	1 week

contrib/llvm-project/llvm/lib/Support/Unix/Threading.inc

@@ -26,6 +26,10 @@
 #include <pthread_np.h> // For pthread_getthreadid_np() / pthread_set_name_np()
 #endif
 
+#if defined(__FreeBSD__)
+#include <sys/cpuset.h>
+#endif
+
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
 #include <errno.h>
 #include <sys/sysctl.h>
@@ -282,6 +286,13 @@ SetThreadPriorityResult llvm::set_thread_priority(ThreadPriority Priority) {
 #include <thread>
 
 int computeHostNumHardwareThreads() {
+#ifdef __FreeBSD__
+  cpuset_t mask;
+  CPU_ZERO(&mask);
+  if (cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_TID, -1, sizeof(mask),
+                         &mask) == 0)
+    return CPU_COUNT(&mask);
+#endif
 #ifdef __linux__
   cpu_set_t Set;
   if (sched_getaffinity(0, sizeof(Set), &Set) == 0)