synth/arm/async.cc

#include "async.h"

#include <array>
#include <atomic>
#include <chrono>
#include <utility>

#include "lock.h"
#include "trace.h"

namespace async {
namespace {

using namespace std::literals::chrono_literals;

struct Stuff {
    std::coroutine_handle<> h;
    std::chrono::system_clock::time_point expiration;

    Stuff* next;
};

struct Notification {
    bool pending;  // can only be true if stuff is nullptr
    Stuff* stuff;
};

std::atomic<Stuff*> work = nullptr;
std::array<Notification, static_cast<size_t>(AwaitableType::kNumTypes)>
    notifications = {};

}  // namespace

void schedule(std::coroutine_handle<> h, int ms) {
    InterruptLock lock;
    TRACE(tracing::TraceEvent::kAsyncSchedule);
    std::chrono::system_clock::time_point exp =
        std::chrono::system_clock::now() + std::chrono::milliseconds(ms);
    Stuff* news = new Stuff{
        .h = h,
        .expiration = exp,
    };

    Stuff* stuff = work;

    if (!stuff || stuff->expiration > exp) {
        news->next = stuff;
        work = news;
        return;
    }

    Stuff* s = stuff;
    while (s->next && s->next->expiration <= exp) {
        s = s->next;
    }

    news->next = s->next;
    s->next = news;
}

void step() {
    Stuff* stuff;
    // ensure all previous side effects are visible
    {
        InterruptLock lock;
        stuff = work;
    };

    if (stuff == nullptr) {
        return;
    }

    auto now = std::chrono::system_clock::now();
    auto dt = stuff->expiration - now;

    if (dt > 0ms) {
        return;
    }

    int stuffinqueue = 0;
    for (Stuff* s = stuff; s; s = s->next) stuffinqueue++;

    TRACE(tracing::TraceEvent::kAsyncTask);
    stuff->h();
    TRACE(tracing::TraceEvent::kAsyncTaskDone);

    if (stuff->h.done()) {
        stuff->h.destroy();
    }

    {
        InterruptLock lock;
        work = stuff->next;
    }
    delete stuff;
}

void reset() {
    Stuff* stuff = work;
    while (stuff) {
        Stuff* byebye = stuff;
        stuff = stuff->next;

        delete byebye;
    }
    work = nullptr;
}

void main_loop(bool (*idle_function)()) {
    while (1) {
        if (idle_function != nullptr) {
            if (idle_function()) {
                reset();
                break;
            };
        }

        step();
    }
}

void enqueue(std::coroutine_handle<> h, AwaitableType type) {
    auto ttype = static_cast<size_t>(type);

    {
        InterruptLock lock;
        TRACE(tracing::TraceEvent::kAsyncEnqueue);

        const bool was_notified =
            std::exchange(notifications[ttype].pending, false);
        if (was_notified) {
            TRACE(tracing::TraceEvent::kAsyncAwaitWasNotified);
            schedule(h);
            return;
        }

        Stuff* item = new Stuff{.h = h};
        Stuff* stuff = notifications[ttype].stuff;
        if (stuff == nullptr) {
            notifications[ttype].stuff = item;
            return;
        }
        while (stuff->next != nullptr) {
            stuff = stuff->next;
        }
        stuff->next = item;
    }
}

void resume(AwaitableType type) {
    auto ttype = static_cast<size_t>(type);
    Stuff* stuff = nullptr;
    {
        InterruptLock lock;

        stuff = notifications[ttype].stuff;
        if (stuff == nullptr) {
            notifications[ttype].pending = true;
            return;
        }

        notifications[ttype].stuff = stuff->next;
        schedule(stuff->h);
    }
    delete stuff;
}

}  // namespace async