Use UnsafeCell for fast constant thread locals
#122583
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joboet
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…enton Rollup of 7 pull requests Successful merges: - rust-lang#122323 (configure.py: add flag for loongarch64 musl-root) - rust-lang#122372 (prevent notifying the same changes more than once) - rust-lang#122390 (Bump windows-bindgen to 0.55.0) - rust-lang#122401 (Check library crates for all tier 1 targets in PR CI) - rust-lang#122489 (Bump `cargo update` PR more often) - rust-lang#122583 (Use `UnsafeCell` for fast constant thread locals) - rust-lang#122590 (bootstrap: Don't name things copy that are not copies) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#122583 - Zoxc:tls-non-mut, r=joboet Use `UnsafeCell` for fast constant thread locals This uses `UnsafeCell` instead of `static mut` for fast constant thread locals. This changes the type of the TLS shims to return `&UnsafeCell<T>` instead of `*mut T` which means they are always non-null so LLVM can optimize away the check for `Some` in `LocalKey::with` if `T` has no destructor. LLVM is currently unable to do this optimization as we lose the fact that `__getit` always returns `Some` as it gets optimized to just returning the value of the TLS shim.
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This uses
UnsafeCellinstead ofstatic mutfor fast constant thread locals. This changes the type of the TLS shims to return&UnsafeCell<T>instead of*mut Twhich means they are always non-null so LLVM can optimize away the check forSomeinLocalKey::withifThas no destructor.LLVM is currently unable to do this optimization as we lose the fact that
__getitalways returnsSomeas it gets optimized to just returning the value of the TLS shim.