Rewrite the handling of threads==1, so it's a little more readable, a…

…nd gets compiled with -O3 in a way that puts this case at the start of the function instead of at the end, which for a mysterious reason results in more stable performance.

PiperOrigin-RevId: 262565366

tensorflow/lite/experimental/ruy/thread_pool.cc

@@ -155,23 +155,26 @@ class Thread {
 
 void ThreadPool::ExecuteImpl(int task_count, int stride, Task* tasks) {
   RUY_DCHECK_GE(task_count, 1);
-  if (task_count > 1) {
-    // Task #0 will be run on the current thread.
-    CreateThreads(task_count - 1);
-    counter_to_decrement_when_ready_.Reset(task_count - 1);
-    for (int i = 1; i < task_count; i++) {
-      auto task_address = reinterpret_cast<std::uintptr_t>(tasks) + i * stride;
-      threads_[i - 1]->StartWork(reinterpret_cast<Task*>(task_address));
-    }
+
+  // Case of 1 thread: just run the single task on the current thread.
+  if (task_count == 1) {
+    (tasks + 0)->Run();
+    return;
+  }
+
+  // Task #0 will be run on the current thread.
+  CreateThreads(task_count - 1);
+  counter_to_decrement_when_ready_.Reset(task_count - 1);
+  for (int i = 1; i < task_count; i++) {
+    auto task_address = reinterpret_cast<std::uintptr_t>(tasks) + i * stride;
+    threads_[i - 1]->StartWork(reinterpret_cast<Task*>(task_address));
   }
 
   // Execute task #0 immediately on the current thread.
   (tasks + 0)->Run();
 
-  if (task_count > 1) {
-    // Wait for the threads submitted above to finish.
-    counter_to_decrement_when_ready_.Wait();
-  }
+  // Wait for the threads submitted above to finish.
+  counter_to_decrement_when_ready_.Wait();
 }
 
 // Ensures that the pool has at least the given count of threads.