std: Use native #[thread_local] TLS on wasm

This commit moves `thread_local!` on WebAssembly targets to using the
`#[thread_local]` attribute in LLVM. This was recently implemented
upstream and is [in the process of being documented][dox]. This change
only takes affect if modules are compiled with `+atomics` which is
currently unstable and a pretty esoteric method of compiling wasm
artifacts.

This "new power" of the wasm toolchain means that the old
`wasm-bindgen-threads` feature of the standard library can be removed
since it should now be possible to create a fully functioning threaded
wasm module without intrusively dealing with libstd symbols or
intrinsics. Yay!

[dox]: WebAssembly/tool-conventions#116

Author: alexcrichton

src/librustc_codegen_ssa/back/linker.rs

@@ -904,6 +904,9 @@ impl<'a> WasmLd<'a> {
         //   linker will synthesize this function, and so we need to make sure
         //   that our usage of `--export` below won't accidentally cause this
         //   function to get deleted.
+        //
+        // * `--export=*tls*` - when `#[thread_local]` symbols are used these
+        //   symbols are how the TLS segments are initialized and configured.
         let atomics = sess.opts.cg.target_feature.contains("+atomics") ||
             sess.target.target.options.features.contains("+atomics");
         if atomics {
@@ -912,6 +915,10 @@ impl<'a> WasmLd<'a> {
             cmd.arg("--import-memory");
             cmd.arg("--passive-segments");
             cmd.arg("--export=__wasm_init_memory");
+            cmd.arg("--export=__wasm_init_tls");
+            cmd.arg("--export=__tls_size");
+            cmd.arg("--export=__tls_align");
+            cmd.arg("--export=__tls_base");
         }
         WasmLd { cmd, sess, info }
     }

src/librustc_target/spec/wasm32_base.rs

@@ -132,6 +132,14 @@ pub fn options() -> TargetOptions {
         // non-relative calls and such later on).
         relocation_model: "static".to_string(),
 
+        // When the atomics feature is activated then these two keys matter,
+        // otherwise they're basically ignored by the standard library. In this
+        // mode, however, the `#[thread_local]` attribute works (i.e.
+        // `has_elf_tls`) and we need to get it to work by specifying
+        // `local-exec` as that's all that's implemented in LLVM today for wasm.
+        has_elf_tls: true,
+        tls_model: "local-exec".to_string(),
+
         .. Default::default()
     }
 }

src/libstd/Cargo.toml

@@ -75,11 +75,6 @@ panic_immediate_abort = ["core/panic_immediate_abort"]
 # requires rebuilding the standard library to use it.
 wasm_syscall = []
 
-# An off-by-default features to enable libstd to assume that wasm-bindgen is in
-# the environment for hooking up some thread-related information like the
-# current thread id and accessing/getting the current thread's TCB
-wasm-bindgen-threads = []
-
 # Enable std_detect default features for stdarch/crates/std_detect:
 # https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/Cargo.toml
 std_detect_file_io = []

src/libstd/sys/wasi/mod.rs

@@ -47,6 +47,8 @@ pub mod stdio;
 pub mod thread;
 #[path = "../wasm/thread_local.rs"]
 pub mod thread_local;
+#[path = "../wasm/fast_thread_local.rs"]
+pub mod fast_thread_local;
 pub mod time;
 pub mod ext;
 

src/libstd/sys/wasm/fast_thread_local.rs

@@ -0,0 +1,9 @@
+#![unstable(feature = "thread_local_internals", issue = "0")]
+
+pub unsafe fn register_dtor(_t: *mut u8, _dtor: unsafe extern fn(*mut u8)) {
+    // FIXME: right now there is no concept of "thread exit", but this is likely
+    // going to show up at some point in the form of an exported symbol that the
+    // wasm runtime is oging to be expected to call. For now we basically just
+    // ignore the arguments, but if such a function starts to exist it will
+    // likely look like the OSX implementation in `unix/fast_thread_local.rs`
+}

src/libstd/sys/wasm/mod.rs

@@ -37,6 +37,8 @@ pub mod stack_overflow;
 pub mod thread;
 pub mod time;
 pub mod stdio;
+pub mod thread_local;
+pub mod fast_thread_local;
 
 pub use crate::sys_common::os_str_bytes as os_str;
 
@@ -48,13 +50,10 @@ cfg_if::cfg_if! {
         pub mod mutex;
         #[path = "rwlock_atomics.rs"]
         pub mod rwlock;
-        #[path = "thread_local_atomics.rs"]
-        pub mod thread_local;
     } else {
         pub mod condvar;
         pub mod mutex;
         pub mod rwlock;
-        pub mod thread_local;
     }
 }
 

src/libstd/sys/wasm/thread.rs

@@ -59,48 +59,40 @@ pub mod guard {
     pub unsafe fn init() -> Option<Guard> { None }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(all(target_feature = "atomics", feature = "wasm-bindgen-threads"))] {
-        #[link(wasm_import_module = "__wbindgen_thread_xform__")]
-        extern {
-            fn __wbindgen_current_id() -> u32;
-            fn __wbindgen_tcb_get() -> u32;
-            fn __wbindgen_tcb_set(ptr: u32);
+// This is only used by atomics primitives when the `atomics` feature is
+// enabled. In that mode we currently just use our own thread-local to store our
+// current thread's ID, and then we lazily initialize it to something allocated
+// from a global counter.
+#[cfg(target_feature = "atomics")]
+pub fn my_id() -> u32 {
+    use crate::sync::atomic::{AtomicU32, Ordering::SeqCst};
+
+    static NEXT_ID: AtomicU32 = AtomicU32::new(0);
+
+    #[thread_local]
+    static mut MY_ID: u32 = 0;
+
+    unsafe {
+        // If our thread ID isn't set yet then we need to allocate one. Do so
+        // with with a simple "atomically add to a global counter" strategy.
+        // This strategy doesn't handled what happens when the counter
+        // overflows, however, so just abort everything once the counter
+        // overflows and eventually we could have some sort of recycling scheme
+        // (or maybe this is all totally irrelevant by that point!). In any case
+        // though we're using a CAS loop instead of a `fetch_add` to ensure that
+        // the global counter never overflows.
+        if MY_ID == 0 {
+            let mut cur = NEXT_ID.load(SeqCst);
+            MY_ID = loop {
+                let next = cur.checked_add(1).unwrap_or_else(|| {
+                    crate::arch::wasm32::unreachable()
+                });
+                match NEXT_ID.compare_exchange(cur, next, SeqCst, SeqCst) {
+                    Ok(_) => break next,
+                    Err(i) => cur = i,
+                }
+            };
         }
-        pub fn my_id() -> u32 {
-            unsafe { __wbindgen_current_id() }
-        }
-
-        // These are currently only ever used in `thread_local_atomics.rs`, if
-        // you'd like to use them be sure to update that and make sure everyone
-        // agrees what's what.
-        pub fn tcb_get() -> *mut u8 {
-            use crate::mem;
-            assert_eq!(mem::size_of::<*mut u8>(), mem::size_of::<u32>());
-            unsafe { __wbindgen_tcb_get() as *mut u8 }
-        }
-
-        pub fn tcb_set(ptr: *mut u8) {
-            unsafe { __wbindgen_tcb_set(ptr as u32); }
-        }
-
-        // FIXME: still need something for hooking exiting a thread to free
-        // data...
-
-    } else if #[cfg(target_feature = "atomics")] {
-        pub fn my_id() -> u32 {
-            panic!("thread ids not implemented on wasm with atomics yet")
-        }
-
-        pub fn tcb_get() -> *mut u8 {
-            panic!("thread local data not implemented on wasm with atomics yet")
-        }
-
-        pub fn tcb_set(_ptr: *mut u8) {
-            panic!("thread local data not implemented on wasm with atomics yet")
-        }
-    } else {
-        // stubbed out because no functions actually access these intrinsics
-        // unless atomics are enabled
+        MY_ID
     }
 }

src/libstd/sys/wasm/thread_local.rs

@@ -1,40 +1,26 @@
-use crate::boxed::Box;
-use crate::ptr;
-
 pub type Key = usize;
 
-struct Allocated {
-    value: *mut u8,
-    dtor: Option<unsafe extern fn(*mut u8)>,
-}
-
 #[inline]
-pub unsafe fn create(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
-    Box::into_raw(Box::new(Allocated {
-        value: ptr::null_mut(),
-        dtor,
-    })) as usize
+pub unsafe fn create(_dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
+    panic!("should not be used on the wasm target");
 }
 
 #[inline]
-pub unsafe fn set(key: Key, value: *mut u8) {
-    (*(key as *mut Allocated)).value = value;
+pub unsafe fn set(_key: Key, _value: *mut u8) {
+    panic!("should not be used on the wasm target");
 }
 
 #[inline]
-pub unsafe fn get(key: Key) -> *mut u8 {
-    (*(key as *mut Allocated)).value
+pub unsafe fn get(_key: Key) -> *mut u8 {
+    panic!("should not be used on the wasm target");
 }
 
 #[inline]
-pub unsafe fn destroy(key: Key) {
-    let key = Box::from_raw(key as *mut Allocated);
-    if let Some(f) = key.dtor {
-        f(key.value);
-    }
+pub unsafe fn destroy(_key: Key) {
+    panic!("should not be used on the wasm target");
 }
 
 #[inline]
 pub fn requires_synchronized_create() -> bool {
-    false
+    panic!("should not be used on the wasm target");
 }