Why does -O2 or greater optimization in clang break this code?

Question

I checked similar questions on the site, but I couldn't find anything that matches my scenario here. This is the code I'm trying to run (requires C++14):

#include <iostream>
#include <chrono>
#include <thread>
using namespace std;

class countdownTimer {
public:
    using duration_t = chrono::high_resolution_clock::duration;

    countdownTimer(duration_t duration) : duration{ duration }, paused{ true } {}

    countdownTimer(const countdownTimer&)               = default;
    countdownTimer(countdownTimer&&)                    = default;
    countdownTimer& operator=(countdownTimer&&)         = default;
    countdownTimer& operator=(const countdownTimer&)    = default;

    void start() noexcept {
        if (started) return;
        startTime = chrono::high_resolution_clock::now();
        endTime = startTime + duration;
        started = true;
        paused = false;
    }

    void pause() noexcept {
        if (paused || !started) return;
        pauseBegin = chrono::high_resolution_clock::now();
        paused = true;
    }

    void resume() noexcept {
        if (!paused || !started) return;
        auto pauseDuration = chrono::high_resolution_clock::now() - pauseBegin;
        startTime += pauseDuration;
        endTime += pauseDuration;
        paused = false;
    }

    double remainingSeconds() const noexcept {
        auto ret = double{ 0.0 };
        if (!started) ret = chrono::duration_cast<chrono::duration<double>>(duration).count();
        else if (paused) ret = chrono::duration_cast<chrono::duration<double>>(duration - (pauseBegin - startTime)).count();
        else ret = chrono::duration_cast<chrono::duration<double>>(duration - (chrono::high_resolution_clock::now() - startTime)).count();
        return (ret < 0.0) ? 0.0 : ret;
    }

    duration_t remainingTime() const noexcept {
        auto ret = duration_t{ 0ms };
        if (!started) ret = chrono::duration_cast<duration_t>(duration);
        else if (paused) ret = chrono::duration_cast<duration_t>(duration - (pauseBegin - startTime));
        else ret = chrono::duration_cast<duration_t>(duration - (chrono::high_resolution_clock::now() - startTime));
        return (ret < 0ms) ? 0ms : ret;
    }

    bool isPaused() const noexcept { return paused; }

    bool hasFinished() const noexcept { return remainingTime() == 0s; }

    void reset() noexcept {
        started = false;
        paused = true;
    }

private:
    chrono::high_resolution_clock::time_point startTime;
    chrono::high_resolution_clock::time_point endTime;
    chrono::high_resolution_clock::time_point pauseBegin;
    duration_t duration;
    bool paused;
    bool started;
};

int main() {
    countdownTimer timer(10s);
    timer.start();

    while (!timer.hasFinished()) {
        cout << timer.remainingSeconds() << endl;
        this_thread::sleep_for(1s);
    }
}

It's a simple countdown timer class that I wrote for one of my projects. The client code in main() is pretty self-explanatory, it should output a countdown from 10 to 0, and then exit the program. With no optimization or -O/-O1, it does exactly that:

10
8.99495
7.98992
6.9849
5.97981
4.9748
3.96973
2.9687
1.9677
0.966752
Program ended with exit code: 0

But if I step up the optimization to >=-O2, the program just keeps outputting 10, and runs forever. The countdown simply doesn't work, it's stuck at the starting value.

I'm using the latest Xcode on OS X. clang --version says Apple LLVM version 7.3.0 (clang-703.0.31).

The strange part is that my code doesn't contain any weird self-written loops, undefined behavior, or anything like that, it's pretty much just standard library calls, so it's very strange that optimization breaks it.

Any ideas?

PS: I haven't tried it on other compilers, but I'm about to. I'll update the question with those results.

Answer 1

bool started is not initialized. If you initialize it to false, it works with -O2:

live example

You can find errors like this using the Undefined behavior sanitizer:

$ g++ -std=c++14 -O2 -g -fsanitize=undefined -fno-omit-frame-pointer main.cpp && ./a.out

main.cpp:18:9: runtime error: load of value 106, which is not a valid value for type 'bool'

Answer 2

The bug is in your constructor:

 countdownTimer(duration_t duration)
 : duration{ duration }, paused{ true } {}

You forgot to initialize started. This triggers undefined behavior when you call start().

No version of clang that I have convenient access to will diagnose this error, but GCC versions 5 and 6 (on Linux - I don't have GCC on my Mac anymore) will:

$ g++ -O2 -Wall -Wextra -std=c++14 test.cc
test.cc: In function ‘int main()’:
test.cc:18:13: warning: ‘*((void*)& timer +33)’ is used uninitialized in this function [-Wuninitialized]
         if (started) return;
             ^~~~~~~
test.cc:74:20: note: ‘*((void*)& timer +33)’ was declared here
     countdownTimer timer(10s);
                    ^~~~~

(My copy of Xcode seems to be a bit out of date, with Apple LLVM version 7.0.2 (clang-700.1.81); it does not change the behavior of the program at -O2. It's possible that your clang would diagnose this error if you turned on the warnings.)

(I have filed a bug report with GCC about the IR gobbledygook in the diagnostics.)