[decompiler] performance improvements in extraction (#1309)

* small speedups to extractor

* faster extraction

common/util/SimpleThreadGroup.cpp

@@ -0,0 +1,41 @@
+#include "SimpleThreadGroup.h"
+#include "common/util/Assert.h"
+
+void SimpleThreadGroup::run(const std::function<void(int)>& func, int num_runs) {
+  int num_workers = std::min(num_runs, (int)std::thread::hardware_concurrency());
+  run(func, num_runs, num_workers);
+}
+
+void SimpleThreadGroup::run(const std::function<void(int)>& func, int num_runs, int num_workers) {
+  ASSERT(m_joined);
+
+  // copy the function, in case the user gives us a temporary function.
+  m_func = func;
+
+  int start_idx = 0;
+  for (int thread_idx = 0; thread_idx < num_workers; thread_idx++) {
+    int end_idx = ((thread_idx + 1) * num_runs) / num_workers;
+    if (thread_idx == (num_workers - 1)) {
+      end_idx = num_runs;
+    }
+
+    m_threads.emplace_back([&, start_idx, end_idx]() {
+      for (int i = start_idx; i < end_idx; i++) {
+        m_func(i);
+      }
+    });
+
+    start_idx = end_idx;
+  }
+
+  m_joined = false;
+}
+
+void SimpleThreadGroup::join() {
+  ASSERT(!m_joined);
+  for (auto& t : m_threads) {
+    t.join();
+  }
+  m_threads.clear();
+  m_joined = true;
+}