Fix computeHostNumPhysicalCores() for Linux on POWER and Linux on Z

ThinLTO is run using a single thread on Linux on Power. The
compute_thread_count() routine calls getHostNumPhysicalCores which
returns -1 by default, and so `MaxThreadCount is set to 1.

unsigned llvm::ThreadPoolStrategy::compute_thread_count() const {
    int MaxThreadCount = UseHyperThreads
          ? computeHostNumHardwareThreads()
          : sys::getHostNumPhysicalCores();
     if (MaxThreadCount <= 0)
        MaxThreadCount = 1;
   …
}
Fix: provide custom implementation of getHostNumPhysicalCores for
Linux on Power and Linux on Z.

Reviewed By: Kai, uweigand

Differential Revision: https://reviews.llvm.org/D84764

llvm/lib/Support/Host.cpp

@@ -1271,6 +1271,12 @@ int computeHostNumPhysicalCores() {
   }
   return CPU_COUNT(&Enabled);
 }
+#elif (defined(__linux__) &&                                                   \
+       (defined(__ppc__) || defined(__powerpc__) || defined(__s390x__)))
+#include <unistd.h>
+
+// Gets the number of *physical cores* on the machine.
+int computeHostNumPhysicalCores() { return sysconf(_SC_NPROCESSORS_ONLN); }
 #elif defined(__APPLE__) && defined(__x86_64__)
 #include <sys/param.h>
 #include <sys/sysctl.h>

llvm/unittests/Support/Host.cpp

@@ -40,7 +40,9 @@ class HostTest : public testing::Test {
     // x86_64 Linux and Darwin.
     return (Host.isOSWindows() && llvm_is_multithreaded()) ||
            (Host.isX86() &&
-            (Host.isOSDarwin() || Host.getOS() == Triple::Linux));
+            (Host.isOSDarwin() || Host.getOS() == Triple::Linux)) ||
+           (Host.getOS() == Triple::Linux &&
+            (Host.isPPC64() || Host.isSystemZ()));
   }
 
   HostTest() : Host(Triple::normalize(sys::getProcessTriple())) {}