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[aarch64] add target feature outline-atomics
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Enable outline-atomics by default as enabled in clang by the following commit
https://reviews.llvm.org/rGc5e7e649d537067dec7111f3de1430d0fc8a4d11

Performance improves by several orders of magnitude when using the LSE instructions
instead of the ARMv8.0 compatible load/store exclusive instructions.

Tested on Graviton2 aarch64-linux with
x.py build && x.py install && x.py test
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Sebastian Pop committed Sep 30, 2021
1 parent 3e8f32e commit 0f9f241
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5 changes: 5 additions & 0 deletions compiler/rustc_codegen_llvm/src/llvm_util.rs
Original file line number Diff line number Diff line change
Expand Up @@ -416,6 +416,11 @@ pub fn llvm_global_features(sess: &Session) -> Vec<String> {
// -Ctarget-features
features.extend(sess.opts.cg.target_feature.split(',').flat_map(&filter));

// FIXME: Move outline-atomics to target definition when earliest supported LLVM is 12.
if get_version() >= (12, 0, 0) && sess.target.llvm_target.contains("aarch64-unknown-linux") {
features.push("+outline-atomics".to_string());
}

features
}

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16 changes: 16 additions & 0 deletions src/test/assembly/asm/aarch64-outline-atomics.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// min-llvm-version: 12.0
// assembly-output: emit-asm
// compile-flags: -O
// compile-flags: --target aarch64-unknown-linux-gnu
// needs-llvm-components: aarch64
// only-aarch64

#![crate_type = "rlib"]

use std::sync::atomic::{AtomicI32, Ordering::*};

pub fn compare_exchange(a: &AtomicI32) {
// On AArch64 LLVM should outline atomic operations.
// CHECK: __aarch64_cas4_relax
let _ = a.compare_exchange(0, 10, Relaxed, Relaxed);
}

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