parallel_for_each.h

#ifndef SOURCEMETA_CORE_PARALLEL_FOR_EACH_H_
#define SOURCEMETA_CORE_PARALLEL_FOR_EACH_H_
 
#include <algorithm> // std::max
#include <climits>   // UINT_MAX
#include <concepts>  // std::copyable, std::invocable
#include <exception> // std::exception_ptr, std::current_exception, std::rethrow_exception
#include <functional> // std::function
#include <iterator>   // std::input_iterator, std::iter_reference_t
#include <mutex>      // std::mutex, std::scoped_lock
#include <queue>      // std::queue
#include <stdexcept>  // std::runtime_error
#include <thread>     // std::thread
#include <utility>    // std::forward
#include <vector>     // std::vector
 
#if defined(_WIN32)
#include <process.h> // _beginthreadex
#ifndef NOMINMAX
#define NOMINMAX
#endif
#include <windows.h>
#else
#include <pthread.h>
#endif
 
namespace sourcemeta::core {
 
#ifndef DOXYGEN
#if defined(_WIN32)
// See
// https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/beginthread-beginthreadex?view=msvc-170
inline unsigned __stdcall parallel_for_each_windows_thread_start(
    void *argument) {
  auto *function_ptr = static_cast<std::function<void()> *>(argument);
  (*function_ptr)();
  delete function_ptr;
  return 0;
}
#endif
#endif

template <typename Iterator, typename Callback>
  requires std::input_iterator<Iterator> && std::copyable<Iterator> &&
           std::invocable<Callback, std::iter_reference_t<Iterator>,
                          std::size_t, std::size_t>
auto parallel_for_each(
    Iterator first, Iterator last, Callback &&callback,
    const std::size_t parallelism = std::thread::hardware_concurrency(),
    const std::size_t stack_size_bytes = 0) -> void {
  const auto effective_parallelism{(std::max)(parallelism, 1uz)};
 
  // Empty list
  if (first == last) {
    return;
  }
 
  // If it is a single element or there is no parallelism,
  // just do a normal loop without dealing with threads
  if (effective_parallelism == 1 || std::next(first) == last) {
    std::size_t cursor{1};
    for (auto iterator = first; iterator != last; ++iterator, ++cursor) {
      callback(*iterator, effective_parallelism, cursor);
    }
 
    return;
  }
 
  std::queue<Iterator> tasks;
  for (auto iterator = first; iterator != last; ++iterator) {
    tasks.push(iterator);
  }
 
  std::mutex queue_mutex;
  std::mutex exception_mutex;
 
  auto effective_callback = std::forward<Callback>(callback);
 
  std::exception_ptr exception = nullptr;
  auto handle_exception = [&exception_mutex,
                           &exception](const std::exception_ptr &pointer) {
    std::scoped_lock lock{exception_mutex};
    if (!exception) {
      exception = pointer;
    }
  };
 
  std::vector<std::thread> workers;
  workers.reserve(effective_parallelism);
 
  const auto total{tasks.size()};
 
  // Worker function that runs the actual per-item work and captures the
  // environment by reference. It will be heap-copied into the native thread
  // API.
  auto worker_callable = [&tasks, &queue_mutex, &effective_callback,
                          &handle_exception, effective_parallelism, total] {
    try {
      while (true) {
        Iterator iterator;
        std::size_t cursor{0};
        {
          std::scoped_lock lock{queue_mutex};
          if (tasks.empty()) {
            return;
          }
          iterator = tasks.front();
          cursor = total - tasks.size() + 1;
          tasks.pop();
        }
        effective_callback(*iterator, effective_parallelism, cursor);
      }
    } catch (...) {
      handle_exception(std::current_exception());
    }
  };
 
  const char *creation_error{nullptr};
 
  // TODO: Replace std::function with std::move_only_function once
  // Apple Clang ships libc++ 19+ (__cpp_lib_move_only_function)
#if defined(_WIN32)
  for (std::size_t index = 0; index < effective_parallelism; ++index) {
    auto *heap_function = new std::function<void()>(worker_callable);
    if (stack_size_bytes > static_cast<std::size_t>(UINT_MAX)) {
      delete heap_function;
      creation_error =
          "The requested stack size is too large for this platform";
      break;
    }
 
    auto raw_handle = _beginthreadex(
        nullptr, static_cast<unsigned>(stack_size_bytes),
        &parallel_for_each_windows_thread_start, heap_function, 0, nullptr);
    if (raw_handle == 0) {
      delete heap_function;
      creation_error = "Could not create thread";
      break;
    }
 
    HANDLE thread_handle = reinterpret_cast<HANDLE>(raw_handle);
    workers.emplace_back([thread_handle] {
      WaitForSingleObject(thread_handle, INFINITE);
      CloseHandle(thread_handle);
    });
  }
#else
  for (std::size_t index = 0; index < effective_parallelism; ++index) {
    // We can't use std::thread, as it doesn't let us tweak the thread stack
    // size
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    // A stack size the platform cannot honour, such as one below its minimum
    // or one that is not a multiple of its page size, must be reported rather
    // than dropped in favour of the default, as the caller asked for it to
    // bound how deep the work it hands over may recurse
    if (stack_size_bytes > 0 &&
        pthread_attr_setstacksize(&attr, stack_size_bytes) != 0) {
      pthread_attr_destroy(&attr);
      creation_error = "The requested stack size is not supported by this "
                       "platform";
      break;
    }
 
    auto *heap_function = new std::function<void()>(worker_callable);
    pthread_t pthread_handle;
    auto raw_handle = pthread_create(
        &pthread_handle, &attr,
        [](void *arg) -> void * {
          auto *function_ptr = static_cast<std::function<void()> *>(arg);
          (*function_ptr)();
          delete function_ptr;
          return nullptr;
        },
        heap_function);
    if (raw_handle != 0) {
      pthread_attr_destroy(&attr);
      delete heap_function;
      creation_error = "Could not create thread";
      break;
    }
    workers.emplace_back(
        [pthread_handle] { pthread_join(pthread_handle, nullptr); });
    pthread_attr_destroy(&attr);
  }
#endif
 
  // If thread creation failed after some workers have already started, those
  // workers keep referencing the stack locals of this frame, so unwinding past
  // them must be avoided. Drain the remaining tasks so the running workers stop
  // pulling new work and exit, then join every already-created worker before
  // propagating the failure
  if (creation_error != nullptr) {
    std::scoped_lock lock{queue_mutex};
    std::queue<Iterator> empty;
    tasks.swap(empty);
  }
 
  for (auto &worker_thread : workers) {
    worker_thread.join();
  }
 
  if (creation_error != nullptr) {
    throw std::runtime_error(creation_error);
  }
 
  if (exception) {
    std::rethrow_exception(exception);
  }
}
 
} // namespace sourcemeta::core
 
#endif