Implement computeHostNumHardwareThreads() for FreeBSD

[dim]

Summary

This retrieves CPU affinity via FreeBSD's cpuset(2) API, and makes LLVM
respect affinity settings configured by the user via the cpuset(1)
command.

In particular, this allows to reduce the number of threads used on
machines with high core counts, which can interact badly with
parallelized build systems. This is particularly noticable with lld,
which spawns lots of threads even for linking e.g. hello_world!

This fix is related to PR48193, but does not adress the more fundamental
problem, which is that LLVM by default grabs as many CPUs and/or threads
as possible.

[MaskRay]

llvm/lib/Support/Unix/Threading.inc

30

Should this be merged with the following block?

292

Use #elif

[dim]

Address comments from review:

• Move FreeBSD specific include under common block
• Use #if #elif #endif for OS specific implementations

[MaskRay]

Looks great!

[aganea]

It would be nice if this was covered by a test. I essentially did what you did, but for Windows and added a test in D92419. @dim do you think that test makes sense for Unix as well? (it would need the 'AffinityMask' to be implemented in llvm/lib/Support/Unix/Program.inc)

[cem]

@aganea That seems reasonable to me.

llvm/lib/Support/Unix/Threading.inc

 
 #include <pthread.h>
 
 #if defined(__FreeBSD__) || defined(__OpenBSD__)
 #include <pthread_np.h> // For pthread_getthreadid_np() / pthread_set_name_np()
 #endif
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
 #include <errno.h>
+#include <sys/cpuset.h>
 #include <sys/sysctl.h>
 #include <sys/user.h>
 #include <unistd.h>
 #endif
 
 #if defined(__NetBSD__)
 #include <lwp.h> // For _lwp_self()
 #endif
return !setpriority(PRIO_DARWIN_THREAD, 0,
              : SetThreadPriorityResult::FAILURE;
 #endif
   return SetThreadPriorityResult::FAILURE;
 }
 
 #include <thread>
 
 int computeHostNumHardwareThreads() {
-#ifdef __linux__
+#if defined(__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);
+#elif defined(__linux__)
   cpu_set_t Set;
   if (sched_getaffinity(0, sizeof(Set), &Set) == 0)
     return CPU_COUNT(&Set);
 #endif
   // Guard against std::thread::hardware_concurrency() returning 0.
   if (unsigned Val = std::thread::hardware_concurrency())
     return Val;
   return 1;