From b3458485ba97a79b58e907b19837953b4f35d9e0 Mon Sep 17 00:00:00 2001
From: Drew Lewis <cannada@google.com>
Date: Sat, 6 Jun 2026 19:45:46 -0400
Subject: [PATCH] Dont spin main for queue (#443)

* Use DynamicLatch in DispatchQueue to avoid main thread spinning

Replace the busy-yielding pending_task_count_ loop in DispatchQueue::finishTasks
with a blocking DynamicLatch. This avoids having the main thread consume CPU
cycles while waiting for dispatched tasks to complete.

The DynamicLatch implementation uses C++20 std::atomic::wait/notify_all for
efficient blocking and wakeup, with proper release-acquire semantics to ensure
task results are visible to the waiting thread.

* Reformat DynamicLatch to match DispatchQueue style

* Update attribution headers in DispatchQueue files to note modifications
---
 include/sta/DispatchQueue.hh | 54 +++++++++++++++++++++++++++++++++---
 util/DispatchQueue.cc        | 20 ++++++-------
 2 files changed, 60 insertions(+), 14 deletions(-)

diff --git a/include/sta/DispatchQueue.hh b/include/sta/DispatchQueue.hh
index ceda2e78..e30f9d70 100644
--- a/include/sta/DispatchQueue.hh
+++ b/include/sta/DispatchQueue.hh
@@ -1,11 +1,14 @@
-// Author Phillip Johnston
+// Original Author: Phillip Johnston
 // Licensed under CC0 1.0 Universal
-// https://github.com/embeddedartistry/embedded-resources/blob/master/examples/cpp/dispatch.cpp
-// https://embeddedartistry.com/blog/2017/2/1/dispatch-queues?rq=dispatch
+// Original source: https://github.com/embeddedartistry/embedded-resources/blob/master/examples/cpp/dispatch.cpp
+// Original article: https://embeddedartistry.com/blog/2017/2/1/dispatch-queues?rq=dispatch
+//
+// Modified for OpenSTA to use C++20 non-spinning DynamicLatch for synchronization.
 
 #pragma once
 
 #include <atomic>
+#include <cstddef>
 #include <condition_variable>
 #include <cstdint>
 #include <functional>
@@ -16,6 +19,49 @@
 
 namespace sta {
 
+class DynamicLatch
+{
+public:
+  explicit DynamicLatch(std::ptrdiff_t initial_count = 0) :
+    count_(initial_count)
+  {
+  }
+
+  // Delete copy/move constructors to prevent accidental slicing/copying of atomics
+  DynamicLatch(const DynamicLatch&) = delete;
+  DynamicLatch& operator=(const DynamicLatch&) = delete;
+
+  // Increases the latch count (used when a new task is dispatched)
+  void
+  countUp()
+  {
+    count_.fetch_add(1, std::memory_order_release);
+  }
+
+  // Decreases the latch count and wakes waiting threads if it hits zero
+  void
+  countDown(std::ptrdiff_t n = 1)
+  {
+    if (count_.fetch_sub(n, std::memory_order_release) == n) {
+      count_.notify_all();
+    }
+  }
+
+  // Blocks until the count reaches zero
+  void
+  wait() const
+  {
+    std::ptrdiff_t current = count_.load(std::memory_order_acquire);
+    while (current != 0) {
+      count_.wait(current, std::memory_order_acquire);
+      current = count_.load(std::memory_order_acquire);
+    }
+  }
+
+private:
+  mutable std::atomic<std::ptrdiff_t> count_{0};
+};
+
 class DispatchQueue
 {
   using fp_t = std::function<void(int thread)>;
@@ -45,7 +91,7 @@ private:
   std::vector<std::thread> threads_;
   std::queue<fp_t> q_;
   std::condition_variable cv_;
-  std::atomic<size_t> pending_task_count_;
+  DynamicLatch pending_task_count_latch_;
   bool quit_ = false;
 };
 
diff --git a/util/DispatchQueue.cc b/util/DispatchQueue.cc
index 5347a07a..c1cd11f7 100644
--- a/util/DispatchQueue.cc
+++ b/util/DispatchQueue.cc
@@ -1,15 +1,16 @@
-// Author Phillip Johnston
+// Original Author: Phillip Johnston
 // Licensed under CC0 1.0 Universal
-// https://github.com/embeddedartistry/embedded-resources/blob/master/examples/cpp/dispatch.cpp
-// https://embeddedartistry.com/blog/2017/2/1/dispatch-queues?rq=dispatch
+// Original source: https://github.com/embeddedartistry/embedded-resources/blob/master/examples/cpp/dispatch.cpp
+// Original article: https://embeddedartistry.com/blog/2017/2/1/dispatch-queues?rq=dispatch
+//
+// Modified for OpenSTA to use C++20 non-spinning DynamicLatch for synchronization.
 
 #include "DispatchQueue.hh"
 
 namespace sta {
 
 DispatchQueue::DispatchQueue(size_t thread_count) :
-  threads_(thread_count),
-  pending_task_count_(0)
+  threads_(thread_count)
 {
   for(size_t i = 0; i < thread_count; i++)
     threads_[i] = std::thread(&DispatchQueue::dispatch_thread_handler, this, i);
@@ -58,8 +59,7 @@ DispatchQueue::getThreadCount() const
 void
 DispatchQueue::finishTasks()
 {
-  while (pending_task_count_.load(std::memory_order_acquire) != 0)
-    std::this_thread::yield();
+  pending_task_count_latch_.wait();
 }
 
 void
@@ -67,7 +67,7 @@ DispatchQueue::dispatch(const fp_t& op)
 {
   std::unique_lock<std::mutex> lock(lock_);
   q_.push(op);
-  pending_task_count_++;
+  pending_task_count_latch_.countUp();
 
   // Manual unlocking is done before notifying, to avoid waking up
   // the waiting thread only to block again (see notify_one for details)
@@ -80,7 +80,7 @@ DispatchQueue::dispatch(fp_t&& op)
 {
   std::unique_lock<std::mutex> lock(lock_);
   q_.push(std::move(op));
-  pending_task_count_++;
+  pending_task_count_latch_.countUp();
 
   // Manual unlocking is done before notifying, to avoid waking up
   // the waiting thread only to block again (see notify_one for details)
@@ -106,7 +106,7 @@ DispatchQueue::dispatch_thread_handler(size_t i)
 
       op(i);
 
-      pending_task_count_--;
+      pending_task_count_latch_.countDown();
       lock.lock();
     }
   } while (!quit_);