Honor CPU affinity in ThreadPool::getNumCores

This reports that number of CPUs that are actually usable by the current
process.

This means that I can do something like `taskset -c 0 emcc hello.c -O2`
and the internal call to `wasm-opt` within emcc will only use a single
core.

Ideally this would work on macOS and windows too, but I'm not even sure
how easy it is to control affinity on those OSes.

Author: sbc100

src/support/threads.cpp

@@ -20,6 +20,10 @@
 #include <iostream>
 #include <string>
 
+#ifdef __linux__
+#include <sched.h> // For sched_getaffinity
+#endif
+
 #include "compiler-support.h"
 #include "threads.h"
 #include "utilities.h"
@@ -145,6 +149,18 @@ size_t ThreadPool::getNumCores() {
   return 1;
 #else
   size_t num = std::max(1U, std::thread::hardware_concurrency());
+#ifdef __linux__
+  // On linux we can do better since we can get the number of CPU that are
+  // actually usable by the current process.
+  cpu_set_t cpu_set;
+  CPU_ZERO(&cpu_set);
+  if (sched_getaffinity(0, sizeof(cpu_set), &cpu_set) == 0) {
+    num = 0;
+    for (int i = 0; i < CPU_SETSIZE; i++) {
+      if (CPU_ISSET(i, &cpu_set)) num++;
+    }
+  }
+#endif
   if (getenv("BINARYEN_CORES")) {
     num = std::stoi(getenv("BINARYEN_CORES"));
   }