Optimize away poison guards when std is built with panic=abort #100603

tmandry · 2022-08-15T23:30:01Z

Note: To take advantage of this PR, you will have to use -Zbuild-std or build your own toolchain. rustup toolchains always link to a libstd that was compiled with panic=unwind, since it's compatible with panic=abort code.

When std is compiled with panic=abort we can remove a lot of the poison machinery from the locks. This changes the Flag and Guard types to be ZSTs. It also adds an uninhabited member to PoisonError so the compiler knows it can optimize away the Result::Err paths, and make LockResult<T> layout-equivalent to T.

Is this a breaking change?

PoisonError::new now panics if invoked from a libstd built with panic="abort" (or any non-unwind strategy). It is unclear to me whether to consider this a breaking change.

In order to encounter this behavior, both of the following must be true:

Using a libstd with `panic="abort"`

This is pretty uncommon. We don't build libstd with that in rustup, except in (Tier 2-3) platforms that do not support unwinding, most notably wasm.

Most people who do this are using cargo's -Z build-std feature, which is unstable.

panic="abort" is not a supported option in Rust's build system. It is possible to configure it using CARGO_TARGET_xxx_RUSTFLAGS, but I believe this only works on non-host platforms.

Creating `PoisonError` manually

This is also unlikely. The only common use case I can think of is in tests, and you can't run tests with panic="abort" without the unstable -Z panic_abort_tests flag.

It's possible that someone is implementing their own locks using std's PoisonError and defining "thread failure" to mean something other than "panic". If this is the case then we would break their code if it was used with a panic="abort" libstd. The locking crates I know of don't replicate std's poison API, but I haven't done much research into this yet.

I've touched on a fair number of considerations here. Which ones do people consider relevant?

rustbot · 2022-08-15T23:30:04Z

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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rust-highfive · 2022-08-15T23:30:05Z

r? @thomcc

(rust-highfive has picked a reviewer for you, use r? to override)

tmandry · 2022-08-15T23:32:25Z

@rustbot label +T-libs-api -T-libs
r? rust-lang/t-libs-api

thomcc · 2022-08-15T23:35:54Z

I'm in favor of this, but am unsure if it constitutes a breaking change. Either way, I think this probably needs an FCP. I'm going to reassign to someone on t-libs-api for that.

r? @m-ou-se

tmandry · 2022-08-31T00:39:38Z

@rustbot label +I-libs-api-nominated

m-ou-se · 2022-09-06T15:19:04Z

Why not leave PoisonError the way it was, with an always working new function? It seems the performance benefits come from the changes to Flag and Guard, even if PoisonError remains unchanged.

tmandry · 2022-09-06T22:36:49Z

The reason for the change to PoisonError is to make code paths that check the result of locking a Mutex trivially optimize away. I'd like the lock code we generate to be the same as if you hand-rolled an implementation without poisoning.

In theory this can be done with enough inlining and optimizations turned on, without a change to PoisonError. I can investigate that. It probably requires playing with #[inline] attributes a bit, which can impact performance in other ways, but hopefully the inliner behaves reasonably predictably.

the8472 · 2022-12-02T21:50:11Z

library/std/src/sync/poison.rs

@@ -267,6 +313,7 @@ where
 {
    match result {
        Ok(t) => Ok(f(t)),
+        #[cfg(panic = "unwind")]


map_result isn't public API so instead of removing the variant you could add an unreachable_unchecked in this

Dylan-DPC · 2023-10-07T11:02:24Z

@tmandry any updates on this? thanks

dtolnay · 2024-01-26T03:45:03Z

@rust-lang/libs-api:
@rfcbot fcp merge

I am in favor of this, including the change to make PoisonError::new abort in the narrow circumstances described above.

rfcbot · 2024-01-26T03:45:05Z

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

Amanieu · 2024-02-03T06:47:29Z

I was actually looking into something similar, but where std::thread::panicking was changed to always return false when built with panic=abort. This would allow crates like scopeguard to optimize away scope guards that only trigger when unwinding (defer_on_unwind).

However it would be a (slight) breaking change since panicking could be called from the panic hook and would incorrectly return false there.

rfcbot · 2024-02-03T06:47:35Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2024-02-13T07:09:26Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

dtolnay

Thank you!

dtolnay · 2024-02-13T13:10:51Z

@bors r+

dtolnay · 2024-02-13T13:15:12Z

@bors ping

dtolnay · 2024-02-13T13:15:43Z

@bors ping

bors · 2024-02-13T13:15:46Z

😪 I'm awake I'm awake

dtolnay · 2024-02-13T13:15:58Z

@bors r+

bors · 2024-02-13T13:16:00Z

📌 Commit 6b9289c has been approved by dtolnay

It is now in the queue for this repository.

bors · 2024-02-14T10:07:16Z

⌛ Testing commit 6b9289c with merge 81b757c...

bors · 2024-02-14T12:02:55Z

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 81b757c to master...

rust-timer · 2024-02-14T13:16:32Z

Finished benchmarking commit (81b757c): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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Bootstrap: 632.818s -> 633.405s (0.09%)
Artifact size: 306.16 MiB -> 306.10 MiB (-0.02%)

Upgrade toolchain to 2024-02-17. Relevant PR: - rust-lang/rust#120500 - rust-lang/rust#100603 Fixes #87 Fixes #3034 Fixes #3037

Pkgsrc changes: * Adapt checksums and patches, some have beene intregrated upstream. Upstream chnages: Version 1.78.0 (2024-05-02) =========================== Language -------- - [Stabilize `#[cfg(target_abi = ...)]`] (rust-lang/rust#119590) - [Stabilize the `#[diagnostic]` namespace and `#[diagnostic::on_unimplemented]` attribute] (rust-lang/rust#119888) - [Make async-fn-in-trait implementable with concrete signatures] (rust-lang/rust#120103) - [Make matching on NaN a hard error, and remove the rest of `illegal_floating_point_literal_pattern`] (rust-lang/rust#116284) - [static mut: allow mutable reference to arbitrary types, not just slices and arrays] (rust-lang/rust#117614) - [Extend `invalid_reference_casting` to include references casting to bigger memory layout] (rust-lang/rust#118983) - [Add `non_contiguous_range_endpoints` lint for singleton gaps after exclusive ranges] (rust-lang/rust#118879) - [Add `wasm_c_abi` lint for use of older wasm-bindgen versions] (rust-lang/rust#117918) This lint currently only works when using Cargo. - [Update `indirect_structural_match` and `pointer_structural_match` lints to match RFC] (rust-lang/rust#120423) - [Make non-`PartialEq`-typed consts as patterns a hard error] (rust-lang/rust#120805) - [Split `refining_impl_trait` lint into `_reachable`, `_internal` variants] (rust-lang/rust#121720) - [Remove unnecessary type inference when using associated types inside of higher ranked `where`-bounds] (rust-lang/rust#119849) - [Weaken eager detection of cyclic types during type inference] (rust-lang/rust#119989) - [`trait Trait: Auto {}`: allow upcasting from `dyn Trait` to `dyn Auto`] (rust-lang/rust#119338) Compiler -------- - [Made `INVALID_DOC_ATTRIBUTES` lint deny by default] (rust-lang/rust#111505) - [Increase accuracy of redundant `use` checking] (rust-lang/rust#117772) - [Suggest moving definition if non-found macro_rules! is defined later] (rust-lang/rust#121130) - [Lower transmutes from int to pointer type as gep on null] (rust-lang/rust#121282) Target changes: - [Windows tier 1 targets now require at least Windows 10] (rust-lang/rust#115141) - [Enable CMPXCHG16B, SSE3, SAHF/LAHF and 128-bit Atomics in tier 1 Windows] (rust-lang/rust#120820) - [Add `wasm32-wasip1` tier 2 (without host tools) target] (rust-lang/rust#120468) - [Add `wasm32-wasip2` tier 3 target] (rust-lang/rust#119616) - [Rename `wasm32-wasi-preview1-threads` to `wasm32-wasip1-threads`] (rust-lang/rust#122170) - [Add `arm64ec-pc-windows-msvc` tier 3 target] (rust-lang/rust#119199) - [Add `armv8r-none-eabihf` tier 3 target for the Cortex-R52] (rust-lang/rust#110482) - [Add `loongarch64-unknown-linux-musl` tier 3 target] (rust-lang/rust#121832) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Bump Unicode to version 15.1.0, regenerate tables] (rust-lang/rust#120777) - [Make align_offset, align_to well-behaved in all cases] (rust-lang/rust#121201) - [PartialEq, PartialOrd: document expectations for transitive chains] (rust-lang/rust#115386) - [Optimize away poison guards when std is built with panic=abort] (rust-lang/rust#100603) - [Replace pthread `RwLock` with custom implementation] (rust-lang/rust#110211) - [Implement unwind safety for Condvar on all platforms] (rust-lang/rust#121768) - [Add ASCII fast-path for `char::is_grapheme_extended`] (rust-lang/rust#121138) Stabilized APIs --------------- - [`impl Read for &Stdin`] (https://doc.rust-lang.org/stable/std/io/struct.Stdin.html#impl-Read-for-%26Stdin) - [Accept non `'static` lifetimes for several `std::error::Error` related implementations] (rust-lang/rust#113833) - [Make `impl<Fd: AsFd>` impl take `?Sized`] (rust-lang/rust#114655) - [`impl From<TryReserveError> for io::Error`] (https://doc.rust-lang.org/stable/std/io/struct.Error.html#impl-From%3CTryReserveError%3E-for-Error) These APIs are now stable in const contexts: - [`Barrier::new()`] (https://doc.rust-lang.org/stable/std/sync/struct.Barrier.html#method.new) Cargo ----- - [Stabilize lockfile v4](rust-lang/cargo#12852) - [Respect `rust-version` when generating lockfile] (rust-lang/cargo#12861) - [Control `--charset` via auto-detecting config value] (rust-lang/cargo#13337) - [Support `target.<triple>.rustdocflags` officially] (rust-lang/cargo#13197) - [Stabilize global cache data tracking] (rust-lang/cargo#13492) Misc ---- - [rustdoc: add `--test-builder-wrapper` arg to support wrappers such as RUSTC_WRAPPER when building doctests] (rust-lang/rust#114651) Compatibility Notes ------------------- - [Many unsafe precondition checks now run for user code with debug assertions enabled] (rust-lang/rust#120594) This change helps users catch undefined behavior in their code, though the details of how much is checked are generally not stable. - [riscv only supports split_debuginfo=off for now] (rust-lang/rust#120518) - [Consistently check bounds on hidden types of `impl Trait`] (rust-lang/rust#121679) - [Change equality of higher ranked types to not rely on subtyping] (rust-lang/rust#118247) - [When called, additionally check bounds on normalized function return type] (rust-lang/rust#118882) - [Expand coverage for `arithmetic_overflow` lint] (rust-lang/rust#119432) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Update to LLVM 18](rust-lang/rust#120055) - [Build `rustc` with 1CGU on `x86_64-pc-windows-msvc`] (rust-lang/rust#112267) - [Build `rustc` with 1CGU on `x86_64-apple-darwin`] (rust-lang/rust#112268) - [Introduce `run-make` V2 infrastructure, a `run_make_support` library and port over 2 tests as example] (rust-lang/rust#113026) - [Windows: Implement condvar, mutex and rwlock using futex] (rust-lang/rust#121956)

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Optimize poison guards to ZSTs when panic=abort

6b9289c

tmandry force-pushed the zst-guards branch from 5951bf5 to 6b9289c Compare August 15, 2022 23:36

rustbot added the I-libs-api-nominated The issue / PR has been nominated for discussion during a libs-api team meeting. label Aug 31, 2022

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Upgrade tooclhain to 2024-02-17 model-checking/kani#3032

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Upgrade rust toolchain to 2024-02-17 model-checking/kani#3040

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Upgrade rust toolchain to 2024-02-17 (#3040)

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Upgrade toolchain to 2024-02-17. Relevant PR: - rust-lang/rust#120500 - rust-lang/rust#100603 Fixes #87 Fixes #3034 Fixes #3037

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Optimize away poison guards when std is built with panic=abort #100603

Optimize away poison guards when std is built with panic=abort #100603

tmandry commented Aug 15, 2022 •

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edited by Amanieu

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Optimize away poison guards when std is built with panic=abort #100603

Optimize away poison guards when std is built with panic=abort #100603

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tmandry commented Aug 15, 2022 • edited Loading

Is this a breaking change?

Using a libstd with panic="abort"

Creating PoisonError manually

rustbot commented Aug 15, 2022

rust-highfive commented Aug 15, 2022

tmandry commented Aug 15, 2022

thomcc commented Aug 15, 2022

tmandry commented Aug 31, 2022

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tmandry commented Sep 6, 2022

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Overall result: no relevant changes - no action needed

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tmandry commented Aug 15, 2022 •

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Using a libstd with `panic="abort"`

Creating `PoisonError` manually

rfcbot commented Jan 26, 2024 •

edited by Amanieu

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