diff options
Diffstat (limited to 'library/std/src/sys/wasm')
| -rw-r--r-- | library/std/src/sys/wasm/alloc.rs | 166 | ||||
| -rw-r--r-- | library/std/src/sys/wasm/atomics/futex.rs | 34 | ||||
| -rw-r--r-- | library/std/src/sys/wasm/atomics/thread.rs | 55 | ||||
| -rw-r--r-- | library/std/src/sys/wasm/env.rs | 9 | ||||
| -rw-r--r-- | library/std/src/sys/wasm/mod.rs | 77 |
5 files changed, 341 insertions, 0 deletions
diff --git a/library/std/src/sys/wasm/alloc.rs b/library/std/src/sys/wasm/alloc.rs new file mode 100644 index 000000000..6dceb1689 --- /dev/null +++ b/library/std/src/sys/wasm/alloc.rs @@ -0,0 +1,166 @@ +//! This is an implementation of a global allocator on wasm targets when +//! emscripten is not in use. In that situation there's no actual runtime for us +//! to lean on for allocation, so instead we provide our own! +//! +//! The wasm instruction set has two instructions for getting the current +//! amount of memory and growing the amount of memory. These instructions are the +//! foundation on which we're able to build an allocator, so we do so! Note that +//! the instructions are also pretty "global" and this is the "global" allocator +//! after all! +//! +//! The current allocator here is the `dlmalloc` crate which we've got included +//! in the rust-lang/rust repository as a submodule. The crate is a port of +//! dlmalloc.c from C to Rust and is basically just so we can have "pure Rust" +//! for now which is currently technically required (can't link with C yet). +//! +//! The crate itself provides a global allocator which on wasm has no +//! synchronization as there are no threads! + +use crate::alloc::{GlobalAlloc, Layout, System}; + +static mut DLMALLOC: dlmalloc::Dlmalloc = dlmalloc::Dlmalloc::new(); + +#[stable(feature = "alloc_system_type", since = "1.28.0")] +unsafe impl GlobalAlloc for System { + #[inline] + unsafe fn alloc(&self, layout: Layout) -> *mut u8 { + // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access. + // Calling malloc() is safe because preconditions on this function match the trait method preconditions. + let _lock = lock::lock(); + unsafe { DLMALLOC.malloc(layout.size(), layout.align()) } + } + + #[inline] + unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 { + // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access. + // Calling calloc() is safe because preconditions on this function match the trait method preconditions. + let _lock = lock::lock(); + unsafe { DLMALLOC.calloc(layout.size(), layout.align()) } + } + + #[inline] + unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) { + // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access. + // Calling free() is safe because preconditions on this function match the trait method preconditions. + let _lock = lock::lock(); + unsafe { DLMALLOC.free(ptr, layout.size(), layout.align()) } + } + + #[inline] + unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 { + // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access. + // Calling realloc() is safe because preconditions on this function match the trait method preconditions. + let _lock = lock::lock(); + unsafe { DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size) } + } +} + +#[cfg(target_feature = "atomics")] +mod lock { + use crate::sync::atomic::{AtomicI32, Ordering::SeqCst}; + + static LOCKED: AtomicI32 = AtomicI32::new(0); + + pub struct DropLock; + + pub fn lock() -> DropLock { + loop { + if LOCKED.swap(1, SeqCst) == 0 { + return DropLock; + } + // Ok so here's where things get a little depressing. At this point + // in time we need to synchronously acquire a lock, but we're + // contending with some other thread. Typically we'd execute some + // form of `i32.atomic.wait` like so: + // + // unsafe { + // let r = core::arch::wasm32::i32_atomic_wait( + // LOCKED.as_mut_ptr(), + // 1, // expected value + // -1, // timeout + // ); + // debug_assert!(r == 0 || r == 1); + // } + // + // Unfortunately though in doing so we would cause issues for the + // main thread. The main thread in a web browser *cannot ever + // block*, no exceptions. This means that the main thread can't + // actually execute the `i32.atomic.wait` instruction. + // + // As a result if we want to work within the context of browsers we + // need to figure out some sort of allocation scheme for the main + // thread where when there's contention on the global malloc lock we + // do... something. + // + // Possible ideas include: + // + // 1. Attempt to acquire the global lock. If it fails, fall back to + // memory allocation via `memory.grow`. Later just ... somehow + // ... inject this raw page back into the main allocator as it + // gets sliced up over time. This strategy has the downside of + // forcing allocation of a page to happen whenever the main + // thread contents with other threads, which is unfortunate. + // + // 2. Maintain a form of "two level" allocator scheme where the main + // thread has its own allocator. Somehow this allocator would + // also be balanced with a global allocator, not only to have + // allocations cross between threads but also to ensure that the + // two allocators stay "balanced" in terms of free'd memory and + // such. This, however, seems significantly complicated. + // + // Out of a lack of other ideas, the current strategy implemented + // here is to simply spin. Typical spin loop algorithms have some + // form of "hint" here to the CPU that it's what we're doing to + // ensure that the CPU doesn't get too hot, but wasm doesn't have + // such an instruction. + // + // To be clear, spinning here is not a great solution. + // Another thread with the lock may take quite a long time to wake + // up. For example it could be in `memory.grow` or it could be + // evicted from the CPU for a timeslice like 10ms. For these periods + // of time our thread will "helpfully" sit here and eat CPU time + // until it itself is evicted or the lock holder finishes. This + // means we're just burning and wasting CPU time to no one's + // benefit. + // + // Spinning does have the nice properties, though, of being + // semantically correct, being fair to all threads for memory + // allocation, and being simple enough to implement. + // + // This will surely (hopefully) be replaced in the future with a + // real memory allocator that can handle the restriction of the main + // thread. + // + // + // FIXME: We can also possibly add an optimization here to detect + // when a thread is the main thread or not and block on all + // non-main-thread threads. Currently, however, we have no way + // of knowing which wasm thread is on the browser main thread, but + // if we could figure out we could at least somewhat mitigate the + // cost of this spinning. + } + } + + impl Drop for DropLock { + fn drop(&mut self) { + let r = LOCKED.swap(0, SeqCst); + debug_assert_eq!(r, 1); + + // Note that due to the above logic we don't actually need to wake + // anyone up, but if we did it'd likely look something like this: + // + // unsafe { + // core::arch::wasm32::atomic_notify( + // LOCKED.as_mut_ptr(), + // 1, // only one thread + // ); + // } + } + } +} + +#[cfg(not(target_feature = "atomics"))] +mod lock { + #[inline] + pub fn lock() {} // no atomics, no threads, that's easy! +} diff --git a/library/std/src/sys/wasm/atomics/futex.rs b/library/std/src/sys/wasm/atomics/futex.rs new file mode 100644 index 000000000..f4fbe9f48 --- /dev/null +++ b/library/std/src/sys/wasm/atomics/futex.rs @@ -0,0 +1,34 @@ +use crate::arch::wasm32; +use crate::sync::atomic::AtomicU32; +use crate::time::Duration; + +/// Wait for a futex_wake operation to wake us. +/// +/// Returns directly if the futex doesn't hold the expected value. +/// +/// Returns false on timeout, and true in all other cases. +pub fn futex_wait(futex: &AtomicU32, expected: u32, timeout: Option<Duration>) -> bool { + let timeout = timeout.and_then(|t| t.as_nanos().try_into().ok()).unwrap_or(-1); + unsafe { + wasm32::memory_atomic_wait32( + futex as *const AtomicU32 as *mut i32, + expected as i32, + timeout, + ) < 2 + } +} + +/// Wake up one thread that's blocked on futex_wait on this futex. +/// +/// Returns true if this actually woke up such a thread, +/// or false if no thread was waiting on this futex. +pub fn futex_wake(futex: &AtomicU32) -> bool { + unsafe { wasm32::memory_atomic_notify(futex as *const AtomicU32 as *mut i32, 1) > 0 } +} + +/// Wake up all threads that are waiting on futex_wait on this futex. +pub fn futex_wake_all(futex: &AtomicU32) { + unsafe { + wasm32::memory_atomic_notify(futex as *const AtomicU32 as *mut i32, i32::MAX as u32); + } +} diff --git a/library/std/src/sys/wasm/atomics/thread.rs b/library/std/src/sys/wasm/atomics/thread.rs new file mode 100644 index 000000000..714b70492 --- /dev/null +++ b/library/std/src/sys/wasm/atomics/thread.rs @@ -0,0 +1,55 @@ +use crate::ffi::CStr; +use crate::io; +use crate::num::NonZeroUsize; +use crate::sys::unsupported; +use crate::time::Duration; + +pub struct Thread(!); + +pub const DEFAULT_MIN_STACK_SIZE: usize = 4096; + +impl Thread { + // unsafe: see thread::Builder::spawn_unchecked for safety requirements + pub unsafe fn new(_stack: usize, _p: Box<dyn FnOnce()>) -> io::Result<Thread> { + unsupported() + } + + pub fn yield_now() {} + + pub fn set_name(_name: &CStr) {} + + pub fn sleep(dur: Duration) { + use crate::arch::wasm32; + use crate::cmp; + + // Use an atomic wait to block the current thread artificially with a + // timeout listed. Note that we should never be notified (return value + // of 0) or our comparison should never fail (return value of 1) so we + // should always only resume execution through a timeout (return value + // 2). + let mut nanos = dur.as_nanos(); + while nanos > 0 { + let amt = cmp::min(i64::MAX as u128, nanos); + let mut x = 0; + let val = unsafe { wasm32::memory_atomic_wait32(&mut x, 0, amt as i64) }; + debug_assert_eq!(val, 2); + nanos -= amt; + } + } + + pub fn join(self) {} +} + +pub fn available_parallelism() -> io::Result<NonZeroUsize> { + unsupported() +} + +pub mod guard { + pub type Guard = !; + pub unsafe fn current() -> Option<Guard> { + None + } + pub unsafe fn init() -> Option<Guard> { + None + } +} diff --git a/library/std/src/sys/wasm/env.rs b/library/std/src/sys/wasm/env.rs new file mode 100644 index 000000000..730e356d7 --- /dev/null +++ b/library/std/src/sys/wasm/env.rs @@ -0,0 +1,9 @@ +pub mod os { + pub const FAMILY: &str = ""; + pub const OS: &str = ""; + pub const DLL_PREFIX: &str = ""; + pub const DLL_SUFFIX: &str = ".wasm"; + pub const DLL_EXTENSION: &str = "wasm"; + pub const EXE_SUFFIX: &str = ".wasm"; + pub const EXE_EXTENSION: &str = "wasm"; +} diff --git a/library/std/src/sys/wasm/mod.rs b/library/std/src/sys/wasm/mod.rs new file mode 100644 index 000000000..4159efe2a --- /dev/null +++ b/library/std/src/sys/wasm/mod.rs @@ -0,0 +1,77 @@ +//! System bindings for the wasm/web platform +//! +//! This module contains the facade (aka platform-specific) implementations of +//! OS level functionality for wasm. Note that this wasm is *not* the emscripten +//! wasm, so we have no runtime here. +//! +//! This is all super highly experimental and not actually intended for +//! wide/production use yet, it's still all in the experimental category. This +//! will likely change over time. +//! +//! Currently all functions here are basically stubs that immediately return +//! errors. The hope is that with a portability lint we can turn actually just +//! remove all this and just omit parts of the standard library if we're +//! compiling for wasm. That way it's a compile time error for something that's +//! guaranteed to be a runtime error! + +#![deny(unsafe_op_in_unsafe_fn)] + +pub mod alloc; +#[path = "../unsupported/args.rs"] +pub mod args; +#[path = "../unix/cmath.rs"] +pub mod cmath; +pub mod env; +#[path = "../unsupported/fs.rs"] +pub mod fs; +#[path = "../unsupported/io.rs"] +pub mod io; +#[path = "../unsupported/net.rs"] +pub mod net; +#[path = "../unsupported/os.rs"] +pub mod os; +#[path = "../unix/os_str.rs"] +pub mod os_str; +#[path = "../unix/path.rs"] +pub mod path; +#[path = "../unsupported/pipe.rs"] +pub mod pipe; +#[path = "../unsupported/process.rs"] +pub mod process; +#[path = "../unsupported/stdio.rs"] +pub mod stdio; +#[path = "../unsupported/thread_local_dtor.rs"] +pub mod thread_local_dtor; +#[path = "../unsupported/thread_local_key.rs"] +pub mod thread_local_key; +#[path = "../unsupported/time.rs"] +pub mod time; + +cfg_if::cfg_if! { + if #[cfg(target_feature = "atomics")] { + #[path = "../unix/locks"] + pub mod locks { + #![allow(unsafe_op_in_unsafe_fn)] + mod futex_condvar; + mod futex_mutex; + mod futex_rwlock; + pub(crate) use futex_condvar::{Condvar, MovableCondvar}; + pub(crate) use futex_mutex::{Mutex, MovableMutex}; + pub(crate) use futex_rwlock::{RwLock, MovableRwLock}; + } + #[path = "atomics/futex.rs"] + pub mod futex; + #[path = "atomics/thread.rs"] + pub mod thread; + } else { + #[path = "../unsupported/locks/mod.rs"] + pub mod locks; + #[path = "../unsupported/thread.rs"] + pub mod thread; + } +} + +#[path = "../unsupported/common.rs"] +#[deny(unsafe_op_in_unsafe_fn)] +mod common; +pub use common::*; |