Why Does My Multithreaded Program Stall in Optimized Mode But Work in -O0?

Multithreading Program Stalls in Optimized Mode but Runs Normally in -O0

This issue arises from a multithreading program where shared variables are not atomic. Specifically, the problem lies with the boolean variable finished that is accessed by two threads without any synchronization or atomicity.

In the optimized mode, the compiler assumes that shared variables accessed by multiple threads are protected. However, in this program, finished is declared as a regular bool, making it non-atomic. This allows the compiler to optimize out the synchronization mechanisms that are essential for multithreaded operation.

To resolve this issue, finished should be declared as std::atomic. This type provides atomic read and write operations, ensuring that concurrent accesses to the variable remain well-defined and consistent.

Fixed Code:

#include <iostream>
#include <future>
#include <atomic>

static std::atomic<bool> finished = false;

int func()
{
    size_t i = 0;
    while (!finished)
        ++i;
    return i;
}

int main()
{
    auto result = std::async(std::launch::async, func);
    std::this_thread::sleep_for(std::chrono::seconds(1));
    finished = true;
    std::cout </bool></atomic></future></iostream>

Live Demo: coliru

Additional Considerations:

• Volatile variables alone cannot guarantee atomicity. While they prevent the compiler from optimizing away read/write operations, they do not provide memory barriers or synchronization primitives.
• Modern compilers are highly aggressive in optimization. It is important to explicitly indicate shared memory regions and resource synchronization requirements to prevent unexpected behavior.

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