Enable outline-atomics by default on more AArch64 platforms by tgross35 · Pull Request #144938 · rust-lang/rust (original) (raw)

The baseline Armv8.0 ISA doesn't have atomics instructions, but in
practice most hardware is at least Armv8.1-A (2014), which includes
single-instruction atomics as part of the LSE feature. As a performance
optimization for these cases, GCC and LLVM have the -moutline-atomics flag
to turn atomic operations into calls to symbols like __aarch64_cas1_acq.
These can do runtime feature detection and use the LSE instructions if
available, falling back to more portable load-exclusive/store-exclusive
loops.

Since the recent 3b50253 ("compiler-builtins: plumb LSE support
for aarch64 on linux") our builtins support this LSE optimization, and
since 6936bb9 ("Dynamically enable LSE for aarch64 rust provided
intrinsics"), std will set the flag as part of its startup code. The first
commit in this PR configures this to work on all platforms built with
outline-atomics, not just Linux.

Thus, enable outline-atomics by default on Android, OpenBSD, Windows,
and Fuchsia platforms that don't have LSE in the baseline. The feature is
already enabled on Linux. Platform-specific details are included in each
commit message.

The current implementation can still be accessed by setting
-Ctarget-feature=-outline-atomics. Setting -Ctarget-feature=+lse or
a relevant CPU will use the single-instruction atomics without the call
overhead. https://rust.godbolt.org/z/dsdrzszoe

Link: https://learn.arm.com/learning-paths/servers-and-cloud-computing/lse/intro/
Original Clang outline-atomics benchmarks: https://reviews.llvm.org/D91157#2435844

try-job: aarch64-msvc-*
try-job: arm-android
try-job: dist-android
try-job: dist-aarch64-llvm-mingw
try-job: dist-aarch64-msvc
try-job: dist-various-*
try-job: test-various