The example had a potential race condition that would still pass the test.
If the thread which was supposed to modify it's own thread local was slower than the instruction to
modify in the main thread, then the test would pass even in case of a failure.
This is would be minor if the child thread was waited for since it check using an `assert_eq` for
the same thing, but vice versa.
However, if the `assert_eq` failed this would trigger a panic, which is not at all caught by the
example since the thread is not waited on.
Signed-off-by: benaryorg <binary@benary.org>
std: Enable usage of `thread_local!` through imports
The `thread_local!` macro delegated to an internal macro but it didn't
do so in a macros-and-the-module-system compatible fashion, meaning if a
`#![no_std]` crate imported `std` and tried to use `thread_local!` it
would fail due to missing a lookup of an internal macro.
This commit switches the macro to instead use `$crate` to invoke other
macros, ensuring that it'll work when `thread_local!` is imported alone.
The `thread_local!` macro delegated to an internal macro but it didn't
do so in a macros-and-the-module-system compatible fashion, meaning if a
`#![no_std]` crate imported `std` and tried to use `thread_local!` it
would fail due to missing a lookup of an internal macro.
This commit switches the macro to instead use `$crate` to invoke other
macros, ensuring that it'll work when `thread_local!` is imported alone.
Some code in the TLS implementation in libstd stores `Some(val)` into an
`&mut Option<T>` (effectively) and then pulls out `&T`, but it currently
uses `.unwrap()` which can codegen into a panic even though it can never
panic. With sufficient optimizations enabled (like LTO) the compiler can
see through this but this commit helps it along in normal mode
(`--release` with Cargo by default) to avoid codegen'ing the panic path.
This ends up improving the optimized codegen on wasm by ensuring that a
call to panic pulling in more file size doesn't stick around.
This adds an implementation of thread local storage for the
`wasm32-unknown-unknown` target when the `atomics` feature is
implemented. This, however, comes with a notable caveat of that it
requires a new feature of the standard library, `wasm-bindgen-threads`,
to be enabled.
Thread local storage for wasm (when `atomics` are enabled and there's
actually more than one thread) is powered by the assumption that an
external entity can fill in some information for us. It's not currently
clear who will fill in this information nor whose responsibility it
should be long-term. In the meantime there's a strategy being gamed out
in the `wasm-bindgen` project specifically, and the hope is that we can
continue to test and iterate on the standard library without committing
to a particular strategy yet.
As to the details of `wasm-bindgen`'s strategy, LLVM doesn't currently
have the ability to emit custom `global` values (thread locals in a
`WebAssembly.Module`) so we leverage the `wasm-bindgen` CLI tool to do
it for us. To that end we have a few intrinsics, assuming two global values:
* `__wbindgen_current_id` - gets the current thread id as a 32-bit
integer. It's `wasm-bindgen`'s responsibility to initialize this
per-thread and then inform libstd of the id. Currently `wasm-bindgen`
performs this initialization as part of the `start` function.
* `__wbindgen_tcb_{get,set}` - in addition to a thread id it's assumed
that there's a global available for simply storing a pointer's worth
of information (a thread control block, which currently only contains
thread local storage). This would ideally be a native `global`
injected by LLVM, but we don't have a great way to support that right
now.
To reiterate, this is all intended to be unstable and purely intended
for testing out Rust on the web with threads. The story is very likely
to change in the future and we want to make sure that we're able to do
that!
This commit applies a few code size optimizations for the wasm target to
the standard library, namely around panics. We notably know that in most
configurations it's impossible for us to print anything in
wasm32-unknown-unknown so we can skip larger portions of panicking that
are otherwise simply informative. This allows us to get quite a nice
size reduction.
Finally we can also tweak where the allocation happens for the
`Box<Any>` that we panic with. By only allocating once unwinding starts
we can reduce the size of a panicking wasm module from 44k to 350 bytes.
support pub(restricted) in thread_local! (round 2)
Resurrected #40984 now that the issue blocking it was fixed. Original description:
`pub(restricted)` was stabilized in #40556 so let's go!
Here is a [playground](https://play.rust-lang.org/?gist=f55f32f164a6ed18c219fec8f8293b98&version=nightly&backtrace=1).
I changed the interface of `__thread_local_inner!`, which is supposedly unstable but this is not checked for macros (#34097 cc @petrochenkov @jseyfried), so this may be an issue.
I think LLVM has had support for quite some time now for this, we just never got
around to testing it out and binding it. We've had some trouble landing this in
the past I believe, but it's time to try again!
This commit flags the `#[thread_local]` attribute as being available for Windows
targets and adds an implementation of `register_dtor` in the `thread::local`
module to ensure we can destroy these keys. The same functionality is
implemented in clang via a function called `__tlregdtor` (presumably provided in
some Windows runtime somewhere), but this function unfortunately does not take a
data pointer (just a thunk) which means we can't easily call it. For now
destructors are just run in the same way the Linux fallback is implemented,
which is just keeping track via a single OS-based TLS key.
This test has been deadlocking and causing problems on the bots basically since
its inception. Some memory safety issues were fixed in 987dc84b, but the
deadlocks remained afterwards unfortunately.
After some investigation, I've concluded that this is just a situation where OSX
is not guaranteed to run destructors. The fix in 987dc84b observed that OSX was
rewriting the backing TLS memory to its initial state during destruction while
we weren't looking, and this would have the effect of canceling the destructors
of any other initialized TLS slots.
While very difficult to pin down, this is basically what I assume is happening
here, so there doesn't seem to really be anythig we can do to ensure the test
robustly passes on OSX, so just ignore it for now.
This transitions the standard library's `thread_local!` macro to use the
freshly-added and gated `#[cfg(target_thread_local)]` attribute. This greatly
simplifies the `#[cfg]` logic in play here, but requires that the standard
library expose both the OS and ELF TLS implementation modules as unstable
implementation details.
The implementation details were shuffled around a bit but end up generally
compiling to the same thing.
Closes#26581 (this supersedes the need for the option)
Closes#27057 (this also starts ignoring the option)
Mazdak Farrokhzad