Optimize thread_queue_hack with std::mutex

[lucifer4330k]

Replaced the manual spinlock implementation (using __sync_bool_compare_and_swap and usleep(0)) in thread_queue_hack with std::mutex.

This change:

• Replaces busy-waiting with efficient OS-level blocking.
• Fixes a potential lock leak in push_back when the queue overflows (previously threw without unlocking).
• Avoids using memset on non-trivial types (which was unsafe if member types were changed).
• Standardizes synchronization using C++17 primitives.
• 4 threads: throughput improvement
• 8 threads: Comparable throughput, but with better CPU scheduling behavior.
  #gsoc

[Copilot]

Pull request overview

This PR modernizes the thread_queue_hack synchronization mechanism by replacing a custom spinlock implementation with std::mutex.

Changes:

• Replaced manual spinlock (__sync_bool_compare_and_swap with busy-waiting) with std::mutex for more efficient OS-level blocking
• Fixed lock leak in push_back when queue overflows by unlocking before throwing exception
• Replaced unsafe memset calls with std::fill to properly handle the now-non-trivial type containing std::mutex

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[Copilot]

The placement of std::mutex as the first member may affect cacheline alignment strategy. The original code used a simple int m_lock which is 4 bytes, while std::mutex is typically much larger (often 40+ bytes on Linux). This could cause the m_queue array to span additional cachelines, potentially impacting the performance benefits of the lt_cacheline_aligned attribute on this class. Consider placing m_mutex after m_queue to keep the frequently-accessed queue data at the beginning of the cacheline, or verify that this new layout doesn't negatively impact false sharing patterns.

Suggested change

	std::mutex m_mutex;
	value_type m_queue[max_size + 1];
	value_type m_queue[max_size + 1];
	std::mutex m_mutex;

[Copilot]

The <cstring> header is no longer needed after removing the memset calls. Consider removing this unused include.

Suggested change

#include <cstring>

[Copilot]

The PR description mentions "C++17 primitives", but std::mutex has been available since C++11. This is a minor inaccuracy in the description, though the change is still correct and beneficial.