9 files changed, 1421 insertions, 0 deletions

diff --git a/src/futex.rs b/src/futex.rs
new file mode 100755
index 0000000..ad74f11
--- /dev/null
+++ b/src/futex.rs
@@ -0,0 +1,535 @@
+use core::sync::atomic::Ordering;
+
+use cfg_if::cfg_if;
+
+cfg_if! {
+	if #[cfg(any(
+		target_os = "linux",
+		target_os = "android",
+		//all(target_os = "emscripten", target_feature = "atomics"),
+		//target_os = "freebsd",
+		//target_os = "openbsd",
+		//target_os = "dragonfly",
+		//target_os = "fuchsia",
+	))] {
+		mod linux;
+		use linux::{Futex, Primitive, SmallFutex, SmallPrimitive};
+	} else if #[cfg(all(target_os = "windows", not(target_vendor = "win7")))] {
+		mod windows;
+		use windows::Futex;
+	}
+}
+
+const SPIN_COUNT: u32 = 100;
+
+const UNLOCKED: SmallPrimitive = 0;
+const LOCKED: SmallPrimitive = 1; // locked, no other threads waiting
+const CONTENDED: SmallPrimitive = 2; // locked, and other threads waiting (contended)
+
+pub struct Mutex(SmallFutex);
+
+impl Mutex {
+	#[inline]
+	pub const fn new() -> Self {
+		Self(Futex::new(UNLOCKED))
+	}
+
+	/// Locks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already locked by the current thread and the
+	/// `unsafe_lock` feature is enabled.
+	#[inline]
+	pub unsafe fn lock(&self) {
+		if self
+			.0
+			.compare_exchange(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed)
+			.is_err()
+		{
+			self.lock_contended();
+		}
+	}
+
+	/// If the mutex is unlocked, it is locked, and this function returns
+	/// `true'. Otherwise, `false` is returned.
+	#[inline]
+	pub unsafe fn try_lock(&self) -> bool {
+		self.0
+			.compare_exchange(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed)
+			.is_ok()
+	}
+
+	/// Unlocks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already unlocked or if it has been locked on
+	/// a different thread.
+	#[inline]
+	pub unsafe fn unlock(&self) {
+		if self.0.swap(UNLOCKED, Ordering::Release) == CONTENDED {
+			// We only wake up one thread. When that thread locks the mutex, it
+			// will mark the mutex as CONTENDED (see lock_contended above),
+			// which makes sure that any other waiting threads will also be
+			// woken up eventually.
+			self.0.wake();
+		}
+	}
+
+	#[inline]
+	pub unsafe fn is_locked(&self) -> bool {
+		self.0.load(Ordering::Relaxed) == UNLOCKED
+	}
+
+	#[cold]
+	fn lock_contended(&self) {
+		// Spin first to speed things up if the lock is released quickly.
+		let mut state = self.spin();
+
+		// If it's unlocked now, attempt to take the lock
+		// without marking it as contended.
+		if state == UNLOCKED {
+			match self
+				.0
+				.compare_exchange(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed)
+			{
+				Ok(_) => return, // Locked!
+				Err(s) => state = s,
+			}
+		}
+
+		loop {
+			// Put the lock in contended state.
+			// We avoid an unnecessary write if it as already set to CONTENDED,
+			// to be friendlier for the caches.
+			if state != CONTENDED && self.0.swap(CONTENDED, Ordering::Acquire) == UNLOCKED {
+				// We changed it from UNLOCKED to CONTENDED, so we just successfully locked it.
+				return;
+			}
+
+			// Wait for the futex to change state, assuming it is still CONTENDED.
+			self.0.wait(CONTENDED);
+
+			// Spin again after waking up.
+			state = self.spin();
+		}
+	}
+
+	fn spin(&self) -> SmallPrimitive {
+		let mut spin = SPIN_COUNT;
+		loop {
+			// We only use `load` (and not `swap` or `compare_exchange`)
+			// while spinning, to be easier on the caches.
+			let state = self.0.load(Ordering::Acquire);
+
+			// We stop spinning when the mutex is UNLOCKED,
+			// but also when it's CONTENDED.
+			if state != LOCKED || spin == 0 {
+				return state;
+			}
+
+			core::hint::spin_loop();
+			spin -= 1;
+		}
+	}
+}
+
+pub struct RwLock {
+	// The state consists of a 30-bit reader counter, a 'readers waiting' flag, and a 'writers waiting' flag.
+	// Bits 0.30:
+	//   0: Unlocked
+	//   1.=0x3FFF_FFFE: Locked by N readers
+	//   0x3FFF_FFFF: Write locked
+	// Bit 30: Readers are waiting on this futex.
+	// Bit 31: Writers are waiting on the writer_notify futex.
+	state: Futex,
+	// The 'condition variable' to notify writers through.
+	// Incremented on every signal.
+	writer_notify: Futex,
+}
+
+const READ_LOCKED: Primitive = 1;
+const MASK: Primitive = (1 << 30) - 1;
+const WRITE_LOCKED: Primitive = MASK;
+const DOWNGRADE: Primitive = READ_LOCKED.wrapping_sub(WRITE_LOCKED); // READ_LOCKED - WRITE_LOCKED
+const MAX_READERS: Primitive = MASK - 1;
+const READERS_WAITING: Primitive = 1 << 30;
+const WRITERS_WAITING: Primitive = 1 << 31;
+
+#[inline]
+fn is_unlocked(state: Primitive) -> bool {
+	state & MASK == 0
+}
+
+#[inline]
+fn is_write_locked(state: Primitive) -> bool {
+	state & MASK == WRITE_LOCKED
+}
+
+#[inline]
+fn has_readers_waiting(state: Primitive) -> bool {
+	state & READERS_WAITING != 0
+}
+
+#[inline]
+fn has_writers_waiting(state: Primitive) -> bool {
+	state & WRITERS_WAITING != 0
+}
+
+#[inline]
+fn is_read_lockable(state: Primitive) -> bool {
+	// This also returns false if the counter could overflow if we tried to read lock it.
+	//
+	// We don't allow read-locking if there's readers waiting, even if the lock is unlocked
+	// and there's no writers waiting. The only situation when this happens is after unlocking,
+	// at which point the unlocking thread might be waking up writers, which have priority over readers.
+	// The unlocking thread will clear the readers waiting bit and wake up readers, if necessary.
+	state & MASK < MAX_READERS && !has_readers_waiting(state) && !has_writers_waiting(state)
+}
+
+#[inline]
+fn is_read_lockable_after_wakeup(state: Primitive) -> bool {
+	// We make a special case for checking if we can read-lock _after_ a reader thread that went to
+	// sleep has been woken up by a call to `downgrade`.
+	//
+	// `downgrade` will wake up all readers and place the lock in read mode. Thus, there should be
+	// no readers waiting and the lock should be read-locked (not write-locked or unlocked).
+	//
+	// Note that we do not check if any writers are waiting. This is because a call to `downgrade`
+	// implies that the caller wants other readers to read the value protected by the lock. If we
+	// did not allow readers to acquire the lock before writers after a `downgrade`, then only the
+	// original writer would be able to read the value, thus defeating the purpose of `downgrade`.
+	state & MASK < MAX_READERS
+		&& !has_readers_waiting(state)
+		&& !is_write_locked(state)
+		&& !is_unlocked(state)
+}
+
+#[inline]
+fn has_reached_max_readers(state: Primitive) -> bool {
+	state & MASK == MAX_READERS
+}
+
+impl RwLock {
+	#[inline]
+	pub const fn new() -> Self {
+		Self {
+			state: Futex::new(0),
+			writer_notify: Futex::new(0),
+		}
+	}
+
+	#[inline]
+	pub fn try_read(&self) -> bool {
+		self.state
+			.fetch_update(Ordering::Acquire, Ordering::Relaxed, |s| {
+				is_read_lockable(s).then(|| s + READ_LOCKED)
+			})
+			.is_ok()
+	}
+
+	#[inline]
+	pub fn read(&self) {
+		let state = self.state.load(Ordering::Relaxed);
+		if !is_read_lockable(state)
+			|| self
+				.state
+				.compare_exchange_weak(
+					state,
+					state + READ_LOCKED,
+					Ordering::Acquire,
+					Ordering::Relaxed,
+				)
+				.is_err()
+		{
+			self.read_contended();
+		}
+	}
+
+	/// # Safety
+	///
+	/// The `RwLock` must be read-locked (N readers) in order to call this.
+	#[inline]
+	pub unsafe fn read_unlock(&self) {
+		let state = self.state.fetch_sub(READ_LOCKED, Ordering::Release) - READ_LOCKED;
+
+		// It's impossible for a reader to be waiting on a read-locked RwLock,
+		// except if there is also a writer waiting.
+		debug_assert!(!has_readers_waiting(state) || has_writers_waiting(state));
+
+		// Wake up a writer if we were the last reader and there's a writer waiting.
+		if is_unlocked(state) && has_writers_waiting(state) {
+			self.wake_writer_or_readers(state);
+		}
+	}
+
+	#[cold]
+	fn read_contended(&self) {
+		let mut has_slept = false;
+		let mut state = self.spin_read();
+
+		loop {
+			// If we have just been woken up, first check for a `downgrade` call.
+			// Otherwise, if we can read-lock it, lock it.
+			if (has_slept && is_read_lockable_after_wakeup(state)) || is_read_lockable(state) {
+				match self.state.compare_exchange_weak(
+					state,
+					state + READ_LOCKED,
+					Ordering::Acquire,
+					Ordering::Relaxed,
+				) {
+					Ok(_) => return, // Locked!
+					Err(s) => {
+						state = s;
+						continue;
+					}
+				}
+			}
+
+			// Check for overflow.
+			assert!(
+				!has_reached_max_readers(state),
+				"too many active read locks on RwLock"
+			);
+
+			// Make sure the readers waiting bit is set before we go to sleep.
+			if !has_readers_waiting(state) {
+				if let Err(s) = self.state.compare_exchange(
+					state,
+					state | READERS_WAITING,
+					Ordering::Relaxed,
+					Ordering::Relaxed,
+				) {
+					state = s;
+					continue;
+				}
+			}
+
+			// Wait for the state to change.
+			self.state.wait(state | READERS_WAITING);
+			has_slept = true;
+
+			// Spin again after waking up.
+			state = self.spin_read();
+		}
+	}
+
+	#[inline]
+	pub fn try_write(&self) -> bool {
+		self.state
+			.fetch_update(Ordering::Acquire, Ordering::Relaxed, |s| {
+				is_unlocked(s).then(|| s + WRITE_LOCKED)
+			})
+			.is_ok()
+	}
+
+	#[inline]
+	pub fn write(&self) {
+		if self
+			.state
+			.compare_exchange_weak(0, WRITE_LOCKED, Ordering::Acquire, Ordering::Relaxed)
+			.is_err()
+		{
+			self.write_contended();
+		}
+	}
+
+	/// # Safety
+	///
+	/// The `RwLock` must be write-locked (single writer) in order to call this.
+	#[inline]
+	pub unsafe fn write_unlock(&self) {
+		let state = self.state.fetch_sub(WRITE_LOCKED, Ordering::Release) - WRITE_LOCKED;
+
+		debug_assert!(is_unlocked(state));
+
+		if has_writers_waiting(state) || has_readers_waiting(state) {
+			self.wake_writer_or_readers(state);
+		}
+	}
+
+	/// # Safety
+	///
+	/// The `RwLock` must be write-locked (single writer) in order to call this.
+	#[inline]
+	pub unsafe fn downgrade(&self) {
+		// Removes all write bits and adds a single read bit.
+		let state = self.state.fetch_add(DOWNGRADE, Ordering::Release);
+		debug_assert!(
+			is_write_locked(state),
+			"RwLock must be write locked to call `downgrade`"
+		);
+
+		if has_readers_waiting(state) {
+			// Since we had the exclusive lock, nobody else can unset this bit.
+			self.state.fetch_sub(READERS_WAITING, Ordering::Relaxed);
+			self.state.wake_all();
+		}
+	}
+
+	#[cold]
+	fn write_contended(&self) {
+		let mut state = self.spin_write();
+
+		let mut other_writers_waiting = 0;
+
+		loop {
+			// If it's unlocked, we try to lock it.
+			if is_unlocked(state) {
+				match self.state.compare_exchange_weak(
+					state,
+					state | WRITE_LOCKED | other_writers_waiting,
+					Ordering::Acquire,
+					Ordering::Relaxed,
+				) {
+					Ok(_) => return, // Locked!
+					Err(s) => {
+						state = s;
+						continue;
+					}
+				}
+			}
+
+			// Set the waiting bit indicating that we're waiting on it.
+			if !has_writers_waiting(state) {
+				if let Err(s) = self.state.compare_exchange(
+					state,
+					state | WRITERS_WAITING,
+					Ordering::Relaxed,
+					Ordering::Relaxed,
+				) {
+					state = s;
+					continue;
+				}
+			}
+
+			// Other writers might be waiting now too, so we should make sure
+			// we keep that bit on once we manage lock it.
+			other_writers_waiting = WRITERS_WAITING;
+
+			// Examine the notification counter before we check if `state` has changed,
+			// to make sure we don't miss any notifications.
+			let seq = self.writer_notify.load(Ordering::Acquire);
+
+			// Don't go to sleep if the lock has become available,
+			// or if the writers waiting bit is no longer set.
+			state = self.state.load(Ordering::Relaxed);
+			if is_unlocked(state) || !has_writers_waiting(state) {
+				continue;
+			}
+
+			// Wait for the state to change.
+			self.writer_notify.wait(seq);
+
+			// Spin again after waking up.
+			state = self.spin_write();
+		}
+	}
+
+	/// Wakes up waiting threads after unlocking.
+	///
+	/// If both are waiting, this will wake up only one writer, but will fall
+	/// back to waking up readers if there was no writer to wake up.
+	#[cold]
+	fn wake_writer_or_readers(&self, mut state: Primitive) {
+		assert!(is_unlocked(state));
+
+		// The readers waiting bit might be turned on at any point now,
+		// since readers will block when there's anything waiting.
+		// Writers will just lock the lock though, regardless of the waiting bits,
+		// so we don't have to worry about the writer waiting bit.
+		//
+		// If the lock gets locked in the meantime, we don't have to do
+		// anything, because then the thread that locked the lock will take
+		// care of waking up waiters when it unlocks.
+
+		// If only writers are waiting, wake one of them up.
+		if state == WRITERS_WAITING {
+			match self
+				.state
+				.compare_exchange(state, 0, Ordering::Relaxed, Ordering::Relaxed)
+			{
+				Ok(_) => {
+					self.wake_writer();
+					return;
+				}
+				Err(s) => {
+					// Maybe some readers are now waiting too. So, continue to the next `if`.
+					state = s;
+				}
+			}
+		}
+
+		// If both writers and readers are waiting, leave the readers waiting
+		// and only wake up one writer.
+		if state == READERS_WAITING + WRITERS_WAITING {
+			if self
+				.state
+				.compare_exchange(state, READERS_WAITING, Ordering::Relaxed, Ordering::Relaxed)
+				.is_err()
+			{
+				// The lock got locked. Not our problem anymore.
+				return;
+			}
+			if self.wake_writer() {
+				return;
+			}
+			// No writers were actually blocked on futex_wait, so we continue
+			// to wake up readers instead, since we can't be sure if we notified a writer.
+			state = READERS_WAITING;
+		}
+
+		// If readers are waiting, wake them all up.
+		if state == READERS_WAITING
+			&& self
+				.state
+				.compare_exchange(state, 0, Ordering::Relaxed, Ordering::Relaxed)
+				.is_ok()
+		{
+			self.state.wake_all();
+		}
+	}
+
+	/// This wakes one writer and returns true if we woke up a writer that was
+	/// blocked on futex_wait.
+	///
+	/// If this returns false, it might still be the case that we notified a
+	/// writer that was about to go to sleep.
+	fn wake_writer(&self) -> bool {
+		self.writer_notify.fetch_add(1, Ordering::Release);
+		self.writer_notify.wake()
+		// Note that FreeBSD and DragonFlyBSD don't tell us whether they woke
+		// up any threads or not, and always return `false` here. That still
+		// results in correct behavior: it just means readers get woken up as
+		// well in case both readers and writers were waiting.
+	}
+
+	/// Spin for a while, but stop directly at the given condition.
+	#[inline]
+	fn spin_until(&self, f: impl Fn(Primitive) -> bool) -> Primitive {
+		let mut spin = SPIN_COUNT; // Chosen by fair dice roll.
+		loop {
+			let state = self.state.load(Ordering::Relaxed);
+			if f(state) || spin == 0 {
+				return state;
+			}
+			core::hint::spin_loop();
+			spin -= 1;
+		}
+	}
+
+	#[inline]
+	fn spin_write(&self) -> Primitive {
+		// Stop spinning when it's unlocked or when there's waiting writers, to keep things somewhat fair.
+		self.spin_until(|state| is_unlocked(state) || has_writers_waiting(state))
+	}
+
+	#[inline]
+	fn spin_read(&self) -> Primitive {
+		// Stop spinning when it's unlocked or read locked, or when there's waiting threads.
+		self.spin_until(|state| {
+			!is_write_locked(state) || has_readers_waiting(state) || has_writers_waiting(state)
+		})
+	}
+}
diff --git a/src/futex/linux.rs b/src/futex/linux.rs
new file mode 100755
index 0000000..80ca5a6
--- /dev/null
+++ b/src/futex/linux.rs
@@ -0,0 +1,61 @@
+use core::ops::Deref;
+use core::sync::atomic::{AtomicU32, Ordering};
+
+use libc::{syscall, SYS_futex};
+
+#[repr(C)]
+pub struct Futex(AtomicU32);
+
+pub type Atomic = AtomicU32;
+pub type Primitive = u32;
+
+pub type SmallFutex = Futex;
+pub type SmallAtomic = Atomic;
+pub type SmallPrimitive = Primitive;
+
+impl Futex {
+	#[inline]
+	pub const fn new(initial_value: u32) -> Self {
+		Self(AtomicU32::new(initial_value))
+	}
+
+	#[inline]
+	pub fn wait(&self, expected_start_value: u32) {
+		unsafe {
+			syscall(
+				SYS_futex,
+				core::ptr::from_ref(&self.0),
+				libc::FUTEX_WAIT_BITSET | libc::FUTEX_PRIVATE_FLAG,
+				expected_start_value,
+				core::ptr::null::<()>(),
+				core::ptr::null::<u32>(), // This argument is unused for FUTEX_WAIT_BITSET.
+				!0u32,                    // A full bitmask, to make it behave like a regular FUTEX_WAIT.
+			);
+		}
+	}
+
+	#[inline]
+	pub fn wake(&self) -> bool {
+		let ptr = &self.0 as *const AtomicU32;
+		const OP: libc::c_int = libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG;
+		unsafe { libc::syscall(libc::SYS_futex, ptr, OP, 1) > 0 }
+	}
+
+	#[inline]
+	pub fn wake_all(&self) {
+		let ptr = &raw const self.0;
+		let op = libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG;
+		unsafe {
+			syscall(libc::SYS_futex, ptr, op, i32::MAX);
+		}
+	}
+}
+
+impl Deref for Futex {
+	type Target = Atomic;
+
+	#[inline]
+	fn deref(&self) -> &Self::Target {
+		&self.0
+	}
+}
diff --git a/src/futex/windows.rs b/src/futex/windows.rs
new file mode 100755
index 0000000..153d9ef
--- /dev/null
+++ b/src/futex/windows.rs
@@ -0,0 +1,114 @@
+use core::ffi::c_void;
+
+#[cfg(not(target_vendor = "win7"))]
+// Use raw-dylib to import synchronization functions to workaround issues with the older mingw import library.
+#[cfg_attr(
+	target_arch = "x86",
+	link(
+		name = "api-ms-win-core-synch-l1-2-0",
+		kind = "raw-dylib",
+		import_name_type = "undecorated"
+	)
+)]
+#[cfg_attr(
+	not(target_arch = "x86"),
+	link(name = "api-ms-win-core-synch-l1-2-0", kind = "raw-dylib")
+)]
+extern "system" {
+	type BOOL = i32;
+
+	pub const TRUE: BOOL = 1;
+	pub const INFINITE: u32 = 4294967295;
+
+	pub fn WaitOnAddress(
+		address: *const c_void,
+		compareaddress: *const c_void,
+		addresssize: usize,
+		dwmilliseconds: u32,
+	) -> BOOL;
+	pub fn WakeByAddressSingle(address: *const c_void);
+	pub fn WakeByAddressAll(address: *const c_void);
+}
+
+pub struct Futex(AtomicU32);
+pub type Primitive = u32;
+
+impl Futex {
+	#[inline]
+	pub const fn new(initial_value: Primitive) -> Self {
+		Self(AtomicU32::new(initial_value))
+	}
+
+	#[inline]
+	pub fn wait(&self, expected_start_value: Primitive) {
+		const SIZE: u32 = core::mem::size_of::<Atomic>();
+		let addr = core::ptr::from_ref(&self.0).cast::<c_void>();
+		let compare_addr = ptr::addr_of!(compare).cast::<c_void>();
+		WaitOnAddress(addr, expected_start_value, SIZE, INFINITE);
+	}
+
+	#[inline]
+	pub fn wake(&self) -> bool {
+		WakeByAddressSingle(core::ptr::from_ref(&self.0).cast::<c_void>());
+		false
+	}
+
+	#[inline]
+	pub fn wake_all(&self) {
+		unsafe {
+			let addr = core::ptr::from_ref(address).cast::<c_void>();
+			WakeByAddressAll(addr);
+		}
+	}
+}
+
+impl Deref for Futex {
+	type Target = Atomic;
+
+	#[inline]
+	fn deref(&self) -> &Self::Target {
+		&self.0
+	}
+}
+
+pub struct SmallFutex(AtomicU8);
+pub type SmallAtomic = AtomicU8;
+pub type SmallPrimitive = u8;
+
+impl SmallFutex {
+	#[inline]
+	pub const fn new(initial_value: SmallPrimitive) -> Self {
+		Self(AtomicU32::new(initial_value))
+	}
+
+	#[inline]
+	pub fn wait(&self, expected_start_value: SmallPrimitive) {
+		const SIZE: u32 = core::mem::size_of::<SmallAtomic>();
+		let addr = core::ptr::from_ref(&self.0).cast::<c_void>();
+		let compare_addr = ptr::addr_of!(compare).cast::<c_void>();
+		WaitOnAddress(addr, expected_start_value, SIZE, INFINITE);
+	}
+
+	#[inline]
+	pub fn wake(&self) -> bool {
+		WakeByAddressSingle(core::ptr::from_ref(&self.0).cast::<c_void>());
+		false
+	}
+
+	#[inline]
+	pub fn wake_all(&self) {
+		unsafe {
+			let addr = core::ptr::from_ref(address).cast::<c_void>();
+			WakeByAddressAll(addr);
+		}
+	}
+}
+
+impl Deref for SmallFutex {
+	type Target = SmallAtomic;
+
+	#[inline]
+	fn deref(&self) -> &Self::Target {
+		&self.0
+	}
+}
diff --git a/src/lazy_box.rs b/src/lazy_box.rs
new file mode 100755
index 0000000..0fd7fd9
--- /dev/null
+++ b/src/lazy_box.rs
@@ -0,0 +1,97 @@
+// This is used to wrap pthread {Mutex, Condvar, RwLock} in.
+
+use core::marker::PhantomData;
+use core::ops::{Deref, DerefMut};
+use core::ptr::null_mut;
+use core::sync::atomic::{AtomicPtr, Ordering};
+
+use alloc::boxed::Box;
+
+pub struct LazyBox<T: LazyInit> {
+	ptr: AtomicPtr<T>,
+	_phantom: PhantomData<T>,
+}
+
+pub trait LazyInit {
+	/// This is called before the box is allocated, to provide the value to
+	/// move into the new box.
+	///
+	/// It might be called more than once per LazyBox, as multiple threads
+	/// might race to initialize it concurrently, each constructing and initializing
+	/// their own box. All but one of them will be passed to `cancel_init` right after.
+	fn init() -> Box<Self>;
+
+	/// Any surplus boxes from `init()` that lost the initialization race
+	/// are passed to this function for disposal.
+	///
+	/// The default implementation calls destroy().
+	fn cancel_init(x: Box<Self>) {
+		Self::destroy(x);
+	}
+
+	/// This is called to destroy a used box.
+	///
+	/// The default implementation just drops it.
+	fn destroy(_: Box<Self>) {}
+}
+
+impl<T: LazyInit> LazyBox<T> {
+	#[inline]
+	pub const fn new() -> Self {
+		Self {
+			ptr: AtomicPtr::new(null_mut()),
+			_phantom: PhantomData,
+		}
+	}
+
+	#[inline]
+	fn get_pointer(&self) -> *mut T {
+		let ptr = self.ptr.load(Ordering::Acquire);
+		if ptr.is_null() {
+			self.initialize()
+		} else {
+			ptr
+		}
+	}
+
+	#[cold]
+	fn initialize(&self) -> *mut T {
+		let new_ptr = Box::into_raw(T::init());
+		match self
+			.ptr
+			.compare_exchange(null_mut(), new_ptr, Ordering::AcqRel, Ordering::Acquire)
+		{
+			Ok(_) => new_ptr,
+			Err(ptr) => {
+				// Lost the race to another thread.
+				// Drop the box we created, and use the one from the other thread instead.
+				T::cancel_init(unsafe { Box::from_raw(new_ptr) });
+				ptr
+			}
+		}
+	}
+}
+
+impl<T: LazyInit> Deref for LazyBox<T> {
+	type Target = T;
+	#[inline]
+	fn deref(&self) -> &T {
+		unsafe { &*self.get_pointer() }
+	}
+}
+
+impl<T: LazyInit> DerefMut for LazyBox<T> {
+	#[inline]
+	fn deref_mut(&mut self) -> &mut T {
+		unsafe { &mut *self.get_pointer() }
+	}
+}
+
+impl<T: LazyInit> Drop for LazyBox<T> {
+	fn drop(&mut self) {
+		let ptr = *self.ptr.get_mut();
+		if !ptr.is_null() {
+			T::destroy(unsafe { Box::from_raw(ptr) });
+		}
+	}
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100755
index 0000000..b26d0b1
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,187 @@
+#![no_std]
+#![forbid(unsafe_op_in_unsafe_fn)]
+
+extern crate alloc;
+
+// tier 1: linux, windows, wasm w/o atomics
+// tier 2: mac os, android, ios, fuschia, illumos, freebsd, netbsd, solaris, redox, uefi, zkvm, embedded
+// not supported: dragonfly, wasm w/ atomics, hermit, teeos, sgx, solid, xous
+
+cfg_if::cfg_if! {
+	if #[cfg(any(
+		all(target_os = "windows", not(target_vendor = "win7")),
+		target_os = "linux",
+		target_os = "android",
+		//target_os = "freebsd",
+		//target_os = "openbsd",
+		//target_os = "dragonfly",
+		//all(target_family = "wasm", target_feature = "atomics"),
+		//target_os = "hermit",
+	))] {
+		mod futex;
+	} else if #[cfg(any(
+		target_os = "dragonfly",
+		all(target_family = "wasm", target_feature = "atomics"),
+		target_os = "hermit"
+	))] {
+		// merge with above when implemented
+	//} else if #[cfg(target_os = "fuchsia")] {
+	//	mod fuchsia;
+	//  mod unix;
+	} else if #[cfg(any(
+		target_family = "unix",
+	))] {
+		extern crate alloc;
+
+		mod lazy_box;
+
+		mod pthread;
+		//mod queue;
+	} else if #[cfg(target_os = "teeos")] {
+		extern crate alloc;
+
+		mod lazy_box;
+
+		mod pthread;
+		//mod teeos;
+	} else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+		mod windows7;
+		//mod queue;
+	} else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+		//mod sgx;
+		//mod queue;
+	} else if #[cfg(target_os = "solid_asp3")] {
+		//mod itron;
+		//mod solid;
+	} else if #[cfg(target_os = "xous")] {
+		//mod xous;
+		//mod queue;
+	} else if #[cfg(any(
+		target_family = "wasm",
+		target_os = "uefi",
+		target_os = "zkvm"
+	))] {
+		mod no_threads;
+	} else if #[cfg(target_has_atomic = "8")] {
+		mod spin;
+	}
+}
+
+cfg_if::cfg_if! {
+	if #[cfg(any(
+		all(target_os = "windows", not(target_vendor = "win7")),
+		target_os = "linux",
+		target_os = "android",
+		//target_os = "freebsd",
+		//target_os = "openbsd",
+		//target_os = "dragonfly",
+		//all(target_family = "wasm", target_feature = "atomics"),
+		//target_os = "hermit",
+	))] {
+		pub use futex::Mutex;
+	} else if #[cfg(any(
+		target_os = "dragonfly",
+		all(target_family = "wasm", target_feature = "atomics"),
+		target_os = "hermit"
+	))] {
+		// merge with above when implemented
+	//} else if #[cfg(target_os = "fuchsia")] {
+	//	mod fuchsia;
+	//	pub use fuchsia::Mutex;
+	} else if #[cfg(any(
+		target_family = "unix",
+		//target_os = "teeos",
+	))] {
+		pub use pthread::Mutex;
+	} else if #[cfg(target_os = "teeos")] {
+		// merge with above when implemented
+	} else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+		pub use windows7::Mutex;
+	} else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+		//pub use sgx::Mutex;
+	} else if #[cfg(target_os = "solid_asp3")] {
+	   //pub use itron::Mutex;
+	} else if #[cfg(target_os = "xous")] {
+		//pub use xous::Mutex;
+	} else if #[cfg(any(
+		target_family = "wasm",
+		target_os = "uefi",
+		target_os = "zkvm"
+	))] {
+		pub use no_threads::Mutex;
+	} else if #[cfg(all(target_os = "none", target_has_atomic = "8"))] {
+		pub use spin::Mutex;
+	}
+}
+
+cfg_if::cfg_if! {
+	if #[cfg(any(
+		all(target_os = "windows", not(target_vendor = "win7")),
+		target_os = "linux",
+		target_os = "android",
+		//target_os = "freebsd",
+		//target_os = "openbsd",
+		//target_os = "dragonfly",
+		//target_os = "fuchsia",
+		//all(target_family = "wasm", target_feature = "atomics"),
+		//target_os = "hermit",
+	))] {
+		pub use futex::RwLock;
+	} else if #[cfg(any(
+		all(target_family = "wasm", target_feature = "atomics"),
+		target_os = "hermit"
+	))] {
+		// merge with above when implemented
+	} else if #[cfg(any(
+		target_family = "unix",
+		all(target_os = "windows", target_vendor = "win7"),
+		//all(target_vendor = "fortanix", target_env = "sgx"),
+		//target_os = "xous",
+	))] {
+		//pub use queue::RwLock;
+	} else if #[cfg(any(
+		all(target_vendor = "fortanix", target_env = "sgx"),
+		target_os = "xous",)
+	)] {
+		// merge with above when implemented
+	} else if #[cfg(target_os = "solid_asp3")] {
+		//pub use solid::RwLock;
+	} else if #[cfg(target_os = "teeos")] {
+		//pub use teeos::RwLock;
+	} else if #[cfg(any(
+		target_family = "wasm",
+		target_os = "uefi",
+		target_os = "zkvm"
+	))] {
+		pub use no_threads::RwLock;
+	} else if #[cfg(all(target_os = "none", target_has_atomic = "8"))] {
+		//pub use spin::RwLock;
+	}
+}
+
+unsafe impl lock_api::RawMutex for Mutex {
+	#[allow(clippy::declare_interior_mutable_const)]
+	const INIT: Self = Self::new();
+
+	type GuardMarker = lock_api::GuardNoSend;
+
+	#[no_panic::no_panic]
+	fn lock(&self) {
+		// safety: unsafe_lock is disabled
+		unsafe { self.lock() }
+	}
+
+	#[no_panic::no_panic]
+	fn try_lock(&self) -> bool {
+		unsafe { self.try_lock() }
+	}
+
+	#[no_panic::no_panic]
+	unsafe fn unlock(&self) {
+		unsafe { self.unlock() }
+	}
+
+	fn is_locked(&self) -> bool {
+		unsafe { self.is_locked() }
+	}
+}
diff --git a/src/no_threads.rs b/src/no_threads.rs
new file mode 100755
index 0000000..f831f5f
--- /dev/null
+++ b/src/no_threads.rs
@@ -0,0 +1,122 @@
+use core::cell::Cell;
+
+pub struct Mutex {
+	// This platform has no threads, so we can use a Cell here.
+	locked: Cell<bool>,
+}
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {} // no threads on this platform
+
+impl Mutex {
+	#[inline]
+	pub const fn new() -> Mutex {
+		Mutex {
+			locked: Cell::new(false),
+		}
+	}
+
+	/// Locks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already locked by the current thread and the
+	/// `unsafe_lock` feature is enabled.
+	#[inline]
+	pub unsafe fn lock(&self) {
+		if self.locked.replace(true) {
+			if cfg!(feature = "unsafe_lock") {
+				// safety: it's UB to lock this when it's already locked
+				unsafe { core::hint::unreachable_unchecked() };
+			} else {
+				panic!("deadlock on a platform with no threads");
+			}
+		}
+	}
+
+	/// If the mutex is unlocked, it is locked, and this function returns
+	/// `true'. Otherwise, `false` is returned.
+	#[inline]
+	pub unsafe fn try_lock(&self) -> bool {
+		self.locked.replace(true) == false
+	}
+
+	/// Unlocks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already unlocked or if it has been locked on
+	/// a different thread.
+	#[inline]
+	pub unsafe fn unlock(&self) {
+		self.locked.set(false);
+	}
+
+	#[inline]
+	pub unsafe fn is_locked(&self) -> bool {
+		self.locked.get()
+	}
+}
+
+pub struct RwLock {
+	// This platform has no threads, so we can use a Cell here.
+	mode: Cell<isize>,
+}
+
+unsafe impl Send for RwLock {}
+unsafe impl Sync for RwLock {} // no threads on this platform
+
+impl RwLock {
+	#[inline]
+	pub const fn new() -> RwLock {
+		RwLock { mode: Cell::new(0) }
+	}
+
+	#[inline]
+	pub unsafe fn read(&self) {
+		let m = self.mode.get();
+		if m >= 0 {
+			self.mode.set(m + 1);
+		} else {
+			unsafe { core::hint::unreachable_unchecked() };
+		}
+	}
+
+	#[inline]
+	pub unsafe fn try_read(&self) -> bool {
+		let m = self.mode.get();
+		if m >= 0 {
+			self.mode.set(m + 1);
+			true
+		} else {
+			false
+		}
+	}
+
+	#[inline]
+	pub unsafe fn write(&self) {
+		if self.mode.replace(isize::MAX) != 0 {
+			unsafe { core::hint::unreachable_unchecked() };
+		}
+	}
+
+	#[inline]
+	pub unsafe fn try_write(&self) -> bool {
+		if self.mode.get() == 0 {
+			self.mode.set(-1);
+			true
+		} else {
+			false
+		}
+	}
+
+	#[inline]
+	pub unsafe fn read_unlock(&self) {
+		self.mode.set(self.mode.get() - 1);
+	}
+
+	#[inline]
+	pub unsafe fn write_unlock(&self) {
+		assert_eq!(self.mode.replace(0), -1);
+	}
+}
diff --git a/src/pthread.rs b/src/pthread.rs
new file mode 100755
index 0000000..b549e1d
--- /dev/null
+++ b/src/pthread.rs
@@ -0,0 +1,118 @@
+use core::cell::UnsafeCell;
+use core::ops::Deref;
+
+use alloc::boxed::Box;
+
+use libc::{
+	pthread_mutex_lock, pthread_mutex_t, pthread_mutex_trylock, pthread_mutex_unlock, EBUSY,
+	PTHREAD_MUTEX_INITIALIZER,
+};
+
+use crate::lazy_box::{LazyBox, LazyInit};
+
+struct RawMutex(UnsafeCell<pthread_mutex_t>);
+
+impl LazyInit for RawMutex {
+	fn init() -> Box<Self> {
+		// We need to box this no matter what, because copying a pthread mutex
+		// results in undocumented behavior.
+		let mutex = Box::new(Self(UnsafeCell::new(PTHREAD_MUTEX_INITIALIZER)));
+
+		if !cfg!(feature = "unsafe_lock") {
+			// A pthread mutex initialized with PTHREAD_MUTEX_INITIALIZER will have
+			// a type of PTHREAD_MUTEX_DEFAULT, which has undefined behavior if you
+			// try to re-lock it from the same thread when you already hold a lock
+			// (https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_init.html).
+			// This is the case even if PTHREAD_MUTEX_DEFAULT == PTHREAD_MUTEX_NORMAL
+			// (https://github.com/rust-lang/rust/issues/33770#issuecomment-220847521) -- in that
+			// case, `pthread_mutexattr_settype(PTHREAD_MUTEX_DEFAULT)` will of course be the same
+			// as setting it to `PTHREAD_MUTEX_NORMAL`, but not setting any mode will result in
+			// a Mutex where re-locking is UB.
+			//
+			// In practice, glibc takes advantage of this undefined behavior to
+			// implement hardware lock elision, which uses hardware transactional
+			// memory to avoid acquiring the lock. While a transaction is in
+			// progress, the lock appears to be unlocked. This isn't a problem for
+			// other threads since the transactional memory will abort if a conflict
+			// is detected, however no abort is generated when re-locking from the
+			// same thread.
+			//
+			// Since locking the same mutex twice will result in two aliasing &mut
+			// references, we instead create the mutex with type
+			// PTHREAD_MUTEX_NORMAL which is guaranteed to deadlock if we try to
+			// re-lock it from the same thread, thus avoiding undefined behavior.
+			unsafe {
+				let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
+				libc::pthread_mutexattr_init(attr.as_mut_ptr());
+				let attr = PthreadMutexAttr(&mut attr);
+				libc::pthread_mutexattr_settype(attr.0.as_mut_ptr(), libc::PTHREAD_MUTEX_NORMAL);
+				libc::pthread_mutex_init(mutex.0.get(), attr.0.as_ptr());
+			}
+		}
+
+		mutex
+	}
+}
+
+impl Deref for RawMutex {
+	type Target = UnsafeCell<pthread_mutex_t>;
+
+	fn deref(&self) -> &Self::Target {
+		&self.0
+	}
+}
+
+pub struct Mutex(LazyBox<RawMutex>);
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {}
+
+impl Mutex {
+	#[inline]
+	pub const fn new() -> Self {
+		Self(LazyBox::new())
+	}
+
+	/// Locks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already locked by the current thread and the
+	/// `unsafe_lock` feature is enabled.
+	#[inline]
+	pub unsafe fn lock(&self) {
+		// safety: the pointer is valid
+		pthread_mutex_lock(self.0.get());
+	}
+
+	/// If the mutex is unlocked, it is locked, and this function returns
+	/// `true'. Otherwise, `false` is returned.
+	#[inline]
+	pub unsafe fn try_lock(&self) -> bool {
+		// safety: the pointer is valid
+		unsafe { pthread_mutex_trylock(self.0.get()) == 0 }
+	}
+
+	/// Unlocks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already unlocked or if it has been locked on
+	/// a different thread.
+	#[inline]
+	pub unsafe fn unlock(&self) {
+		// safety: the pointer is valid
+		pthread_mutex_unlock(self.0.get());
+	}
+
+	pub unsafe fn is_locked(&self) -> bool {
+		if self.try_lock() {
+			unsafe {
+				self.unlock();
+			}
+			false
+		} else {
+			true
+		}
+	}
+}
diff --git a/src/spin.rs b/src/spin.rs
new file mode 100755
index 0000000..c81a1e7
--- /dev/null
+++ b/src/spin.rs
@@ -0,0 +1,68 @@
+use core::sync::atomic::{AtomicBool, Ordering};
+
+pub struct Mutex(AtomicBool);
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {}
+
+impl Mutex {
+	#[inline]
+	pub const fn new() -> Mutex {
+		Mutex(AtomicBool::new(false))
+	}
+
+	/// Locks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already locked by the current thread and the
+	/// `unsafe_lock` feature is enabled.
+	#[inline]
+	pub unsafe fn lock(&self) {
+		// Can fail to lock even if the spinlock is not locked. May be more
+		// efficient than `try_lock` hen called in a loop.
+		loop {
+			if self.try_lock_weak() {
+				break;
+			}
+
+			while self.is_locked() {
+				core::hint::spin_loop();
+			}
+		}
+	}
+
+	/// If the mutex is unlocked, it is locked, and this function returns
+	/// `true'. Otherwise, `false` is returned.
+	#[inline]
+	pub unsafe fn try_lock(&self) -> bool {
+		// The reason for using a strong compare_exchange is explained here:
+		// https://github.com/Amanieu/parking_lot/pull/207#issuecomment-575869107
+		self.lock
+			.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
+			.is_ok()
+	}
+
+	/// Unlocks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already unlocked or if it has been locked on
+	/// a different thread.
+	#[inline]
+	pub unsafe fn unlock(&self) {
+		self.locked.set(false);
+	}
+
+	#[inline]
+	pub unsafe fn is_locked(&self) -> bool {
+		self.lock.load(Ordering::Relaxed)
+	}
+
+	#[inline]
+	fn try_lock_weak(&self) -> bool {
+		self.lock
+			.compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+			.is_ok()
+	}
+}
diff --git a/src/windows7.rs b/src/windows7.rs
new file mode 100755
index 0000000..2007d05
--- /dev/null
+++ b/src/windows7.rs
@@ -0,0 +1,119 @@
+use core::cell::UnsafeCell;
+
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct SrwLock {
+	ptr: *mut core::ffi::c_void,
+}
+
+const SRWLOCK_INIT: SrwLock = SrwLock {
+	ptr: core::ptr::null_mut(),
+};
+
+type Boolean = u8;
+
+#[link(name = "kernel32")]
+extern "system" {
+	pub fn AcquireSRWLockShared(srwlock: *mut SrwLock);
+	pub fn AcquireSRWLockExclusive(srwlock: *mut SrwLock);
+	pub fn TryAcquireSRWLockShared(srwlock: *mut SrwLock) -> Boolean;
+	pub fn TryAcquireSRWLockExclusive(srwlock: *mut SrwLock) -> Boolean;
+	pub fn ReleaseSRWLockShared(srwlock: *mut SrwLock);
+	pub fn ReleaseSRWLockExclusive(srwlock: *mut SrwLock);
+}
+
+pub struct Mutex(UnsafeCell<SrwLock>);
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {}
+
+impl Mutex {
+	#[inline]
+	pub const fn new() -> Self {
+		Self(UnsafeCell::new(SRWLOCK_INIT))
+	}
+
+	/// Locks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already locked by the current thread and the
+	/// `unsafe_lock` feature is enabled.
+	#[inline]
+	pub unsafe fn lock(&self) {
+		unsafe {
+			AcquireSRWLockExclusive(self.0.get());
+		}
+	}
+
+	/// If the mutex is unlocked, it is locked, and this function returns
+	/// `true'. Otherwise, `false` is returned.
+	#[inline]
+	pub unsafe fn try_lock(&self) -> bool {
+		unsafe { TryAcquireSRWLockExclusive(self.0.get()) != 0 }
+	}
+
+	/// Unlocks the mutex
+	///
+	/// # Safety
+	///
+	/// UB occurs if the mutex is already unlocked or if it has been locked on
+	/// a different thread.
+	#[inline]
+	pub unsafe fn unlock(&self) {
+		unsafe {
+			ReleaseSRWLockExclusive(self.0.get());
+		}
+	}
+
+	pub unsafe fn is_locked(&self) -> bool {
+		if self.try_lock() {
+			unsafe {
+				self.unlock();
+			}
+			false
+		} else {
+			true
+		}
+	}
+}
+
+pub struct RwLock {
+	inner: UnsafeCell<c::SRWLOCK>,
+}
+
+unsafe impl Send for RwLock {}
+unsafe impl Sync for RwLock {}
+
+impl RwLock {
+	#[inline]
+	pub const fn new() -> RwLock {
+		RwLock {
+			inner: UnsafeCell::new(c::SRWLOCK_INIT),
+		}
+	}
+	#[inline]
+	pub fn read(&self) {
+		unsafe { c::AcquireSRWLockShared(self.inner.get()) }
+	}
+	#[inline]
+	pub fn try_read(&self) -> bool {
+		unsafe { c::TryAcquireSRWLockShared(self.inner.get()) != 0 }
+	}
+	#[inline]
+	pub fn write(&self) {
+		unsafe { c::AcquireSRWLockExclusive(self.inner.get()) }
+	}
+	#[inline]
+	pub fn try_write(&self) -> bool {
+		unsafe { c::TryAcquireSRWLockExclusive(self.inner.get()) != 0 }
+	}
+	#[inline]
+	pub unsafe fn read_unlock(&self) {
+		c::ReleaseSRWLockShared(self.inner.get())
+	}
+	#[inline]
+	pub unsafe fn write_unlock(&self) {
+		c::ReleaseSRWLockExclusive(self.inner.get())
+	}
+}