Implement computeHostNumHardwareThreads() for FreeBSD

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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D92271

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Support/Unix/Threading.inc b/llvm/lib/Support/Unix/Threading.inc
index 2d0aaca..667d023 100644
--- a/llvm/lib/Support/Unix/Threading.inc
+++ b/llvm/lib/Support/Unix/Threading.inc
@@ -28,6 +28,7 @@
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
 #include <errno.h>
+#include <sys/cpuset.h>
 #include <sys/sysctl.h>
 #include <sys/user.h>
 #include <unistd.h>
@@ -282,7 +283,13 @@ SetThreadPriorityResult llvm::set_thread_priority(ThreadPriority Priority) {
 #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);