Use volatile access instead of #[used] for on_tls_callback
#121596
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…llaumeGomez Rollup of 7 pull requests Successful merges: - rust-lang#119748 (Increase visibility of `join_path` and `split_paths`) - rust-lang#120820 (Enable CMPXCHG16B, SSE3, SAHF/LAHF and 128-bit Atomics (in nightly) in Windows x64) - rust-lang#121000 (pattern_analysis: rework how we hide empty private fields) - rust-lang#121376 (Skip unnecessary comparison with half-open range patterns) - rust-lang#121596 (Use volatile access instead of `#[used]` for `on_tls_callback`) - rust-lang#121669 (Count stashed errors again) - rust-lang#121783 (Emitter cleanups) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#121596 - ChrisDenton:tls, r=joboet Use volatile access instead of `#[used]` for `on_tls_callback` The first commit adds a volatile load of `p_thread_callback` when registering a dtor so that the compiler knows if the callback is used or not. I don't believe the added volatile instruction is otherwise significant in the context. In my testing using the volatile load allowed the compiler to correctly reason about whether `on_tls_callback` is used or not, allowing it to be omitted entirely in some cases. Admittedly it usually is used due to `Thread` but that can be avoided (e.g. in DLLs or with custom entry points that avoid the offending APIs). Ideally this would be something the compiler could help a bit more with so we didn't have to use tricks like `#[used]` or volatile. But alas. I also used this as an opportunity to clean up the `unused` lints which I don't think serve a purpose any more. The second commit removes the volatile load of `_tls_used` with `#cfg[target_thread_local]` because `#[thread_local]` already implies it. And if it ever didn't then `#[thread_local]` would be broken when there aren't any dtors.
This was referenced Apr 9, 2024
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The first commit adds a volatile load of
p_thread_callbackwhen registering a dtor so that the compiler knows if the callback is used or not. I don't believe the added volatile instruction is otherwise significant in the context. In my testing using the volatile load allowed the compiler to correctly reason about whetheron_tls_callbackis used or not, allowing it to be omitted entirely in some cases. Admittedly it usually is used due toThreadbut that can be avoided (e.g. in DLLs or with custom entry points that avoid the offending APIs). Ideally this would be something the compiler could help a bit more with so we didn't have to use tricks like#[used]or volatile. But alas.I also used this as an opportunity to clean up the
unusedlints which I don't think serve a purpose any more.The second commit removes the volatile load of
_tls_usedwith#cfg[target_thread_local]because#[thread_local]already implies it. And if it ever didn't then#[thread_local]would be broken when there aren't any dtors.