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Michael Goulet 828a3a41b3
Rollup merge of #137417 - taiki-e:riscv-atomic, r=Amanieu 
rustc_target: Add more RISC-V atomic-related features

This is a continuation of https://github.com/rust-lang/rust/pull/130877 and adds a few target features, including `zacas`, which was experimental in LLVM 19 and marked non-experimental in LLVM 20.

This adds the following target features to unstable riscv_target_feature:

- `za64rs` (Za64rs Extension 1.0): Reservation Set Size of at Most 64 Bytes
  ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.0-rc2/llvm/lib/Target/RISCV/RISCVFeatures.td#L227-L228), [available since LLVM 18](8649328060))
- `za128rs` (Za128rs Extension 1.0): Reservation Set Size of at Most 128 Bytes
  ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.0-rc2/llvm/lib/Target/RISCV/RISCVFeatures.td#L230-L231), [available since LLVM 18](8649328060))
  - IIUC, `za*rs` can be referenced when implementing helpers to reduce contention in synchronization primitives, like [`crossbeam_utils::CachePadded`](https://docs.rs/crossbeam-utils/latest/crossbeam_utils/struct.CachePadded.html). (relevant discussion: https://github.com/riscv/riscv-profiles/issues/79)
- `zacas` (Zacas Extension 1.0): Atomic Compare-And-Swap Instructions (`amocas.{w,d,q}{,.aq,.rl,.aqrl}` and `amocas.{b,h}{,.aq,.rl,.aqrl}` when `zabha` is also enabled)
  ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.0-rc2/llvm/lib/Target/RISCV/RISCVFeatures.td#L240-L243), [available as non-experimental since LLVM 20](614aeda93b))
  - This implies `zaamo`.
  - This is used to optimize CAS in existing atomics and/or implement 64-bit/128-bit atomics on riscv32/riscv64 (e.g., https://github.com/taiki-e/portable-atomic/pull/173).
  - Note that [LLVM does not automatically use this instruction for 64-bit/128-bit atomics on riscv32/riscv64 even if this feature is enabled, because doing it changes the ABI](876174ffd7/llvm/docs/RISCVUsage.rst (riscv-zacas-note)). (If the ability to do that is provided by LLVM in the future, it should probably be controlled by another ABI feature similar to `forced-atomics`.)
- `zama16b` (Zama16b Extension 1.0): Atomic 16-byte misaligned loads, stores and AMOs
  ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.0-rc2/llvm/lib/Target/RISCV/RISCVFeatures.td#L255-L256), [available since LLVM 19](b090569685))
  - IIUC, unlike AArch64 FEAT_LSE2 which also makes 16-byte aligned ldp ({i,u}128 load) atomic, this extension only affects instructions that already considered atomic if they were naturally aligned. i.e., fld (f64 load) on riscv32 would not be atomic with or without this extension ([relevant QEMU code](b69801dd6b/target/riscv/insn_trans/trans_rvd.c.inc (L50-L62))).
- `zawrs` (Zawrs Extension 1.0): Wait on Reservation Set (`wrs.nto` and `wrs.sto`)
  ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.0-rc2/llvm/lib/Target/RISCV/RISCVFeatures.td#L258), [available as non-experimental since LLVM 17](d41a73aa94))
  - This is used to optimize synchronization primitives (e.g., Linux uses this for spinlocks (b8ddb0df30)).

Btw, the question of whether `zaamo` is implied by `zabha` or not, which was discussed in https://github.com/rust-lang/rust/pull/130877, has been resolved in LLVM 20, since LLVM now treats `zaamo` as implied by `zabha`/`zacas` (https://github.com/llvm/llvm-project/pull/115694), just like GCC and rustc.

r? `@Amanieu`

`@rustbot` label +O-riscv +A-target-feature
2025-02-24 19:21:47 -05:00
.github Auto merge of #137023 - Kobzol:bump-sccache, r=marcoieni 2025-02-20 05:59:04 +00:00
compiler Rollup merge of #137417 - taiki-e:riscv-atomic, r=Amanieu 2025-02-24 19:21:47 -05:00
library Rollup merge of #137321 - aviraxp:patch-1, r=cuviper 2025-02-24 19:21:46 -05:00
LICENSES Synchronize Unicode license text from unicode.org 2024-11-20 00:54:12 -08:00
src Remove dyn_compatible_for_dispatch 2025-02-24 18:48:40 +00:00
tests Rollup merge of #137417 - taiki-e:riscv-atomic, r=Amanieu 2025-02-24 19:21:47 -05:00
.clang-format Add .clang-format 2024-06-26 05:56:00 +08:00
.editorconfig Don't apply editorconfig to llvm 2025-02-09 16:21:14 -05:00
.git-blame-ignore-revs Git blame ignore recent formatting commit 2025-02-09 12:00:23 -08:00
.gitattributes Revert "Stop git from merging generated files" 2024-12-12 07:20:11 +01:00
.gitignore ignore vendor directory in git status 2025-02-12 21:16:36 -05:00
.gitmodules Update to LLVM 20 2025-02-14 11:02:28 +01:00
.ignore Add .ignore file to make config.toml searchable in vscode 2024-06-24 10:15:16 +02:00
.mailmap Rollup merge of #134965 - Noratrieb:slightly-late-for-boxing-day-sadly-but-still-almost, r=BoxyUwU 2025-01-03 22:12:42 +01:00
Cargo.lock Auto merge of #137285 - yotamofek:pr/rustdoc/pulldown-escaping, r=GuillaumeGomez 2025-02-24 07:11:04 +00:00
Cargo.toml Move error_index_generator to the rustbook workspace 2025-02-17 10:58:15 -08:00
CODE_OF_CONDUCT.md
config.example.toml Update references to cc_detect.rs 2025-02-20 10:41:29 +00:00
configure Ensure ./configure works when configure.py path contains spaces 2024-02-16 18:57:22 +00:00
CONTRIBUTING.md docs: update contributing docs for submodule/subtree changes 2025-01-20 21:53:59 +08:00
COPYRIGHT dist: Re-work how we describe the licence of Rust in our distributions 2024-12-09 10:18:55 +00:00
INSTALL.md add clarity for custom path installation 2024-10-06 07:37:00 -05:00
LICENSE-APACHE Remove appendix from LICENCE-APACHE 2019-12-30 14:25:53 +00:00
license-metadata.json Add italic for newly added sans serif fonts 2025-01-29 11:01:14 +01:00
LICENSE-MIT dist: Re-work how we describe the licence of Rust in our distributions 2024-12-09 10:18:55 +00:00
README.md Grammar fixes 2024-12-09 17:17:27 -05:00
RELEASES.md Re-add missing empty lines in the releases notes 2025-02-16 12:15:55 +01:00
REUSE.toml Fix tidy errors 2025-01-29 11:01:14 +01:00
rust-bors.toml Increase timeout for new bors bot 2024-03-13 08:31:07 +01:00
rustfmt.toml Update rustfmt.toml to match the rust repo 2025-02-13 14:54:50 +00:00
triagebot.toml jubilee cleared out the review queue 2025-02-21 21:12:39 -08:00
x fix(x): fix a regex used to find python executable 2024-11-06 16:40:02 +03:00
x.ps1 use & instead of start-process in x.ps1 2023-12-09 09:46:16 -05:00
x.py Reformat Python code with ruff 2024-12-04 23:03:44 +01:00

README.md

This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.

Why Rust?

  • Performance: Fast and memory-efficient, suitable for critical services, embedded devices, and easily integrated with other languages.

  • Reliability: Our rich type system and ownership model ensure memory and thread safety, reducing bugs at compile-time.

  • Productivity: Comprehensive documentation, a compiler committed to providing great diagnostics, and advanced tooling including package manager and build tool (Cargo), auto-formatter (rustfmt), linter (Clippy) and editor support (rust-analyzer).

Quick Start

Read "Installation" from The Book.

Installing from Source

If you really want to install from source (though this is not recommended), see INSTALL.md.

Getting Help

See https://www.rust-lang.org/community for a list of chat platforms and forums.

Contributing

See CONTRIBUTING.md.

License

Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.

Trademark

The Rust Foundation owns and protects the Rust and Cargo trademarks and logos (the "Rust Trademarks").

If you want to use these names or brands, please read the media guide.

Third-party logos may be subject to third-party copyrights and trademarks. See Licenses for details.