Change Backtrace::enabled atomic from SeqCst to Relaxed

[dtolnay]

This atomic is not synchronizing anything outside of its own value, so we don't need the Acquire/Release guarantee that all memory operations prior to the store are visible after the subsequent load, nor the SeqCst guarantee of all threads seeing all of the sequentially consistent operations in the same order.

Using Relaxed reduces the overhead of Backtrace::capture() in the case that backtraces are not enabled.

Benchmark

#![feature(backtrace)]

use std::backtrace::Backtrace;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use std::time::Instant;

fn main() {
    let begin = Instant::now();
    let mut threads = Vec::new();
    for _ in 0..64 {
        threads.push(thread::spawn(|| {
            for _ in 0..10_000_000 {
                let _ = Backtrace::capture();
                static LOL: AtomicUsize = AtomicUsize::new(0);
                LOL.store(1, Ordering::Release);
            }
        }));
    }
    for thread in threads {
        let _ = thread.join();
    }
    println!("{:?}", begin.elapsed());
}

Before: 6.73 seconds
After: 5.18 seconds

[rust-highfive]

r? @m-ou-se

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[m-ou-se]

@bors r+

[bors]

📌 Commit 984b10d has been approved by m-ou-se

[bors]

⌛ Testing commit 984b10d with merge b0dcf2c76b3159084138c63b18bd31373c9e679b...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

The job x86_64-msvc-1 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- process::tests::test_interior_nul_in_arg_is_error stdout ----
thread 'process::tests::test_interior_nul_in_arg_is_error' panicked at 'assertion failed: `(left == right)`
  left: `NotFound`,
 right: `InvalidInput`', library\std\src\process\tests.rs:309:19
---- process::tests::test_interior_nul_in_args_is_error stdout ----
thread 'process::tests::test_interior_nul_in_args_is_error' panicked at 'assertion failed: `(left == right)`
  left: `NotFound`,
  left: `NotFound`,
 right: `InvalidInput`', library\std\src\process\tests.rs:317:19
---- process::tests::test_interior_nul_in_current_dir_is_error stdout ----
thread 'process::tests::test_interior_nul_in_current_dir_is_error' panicked at 'assertion failed: `(left == right)`
  left: `NotFound`,
  left: `NotFound`,
 right: `InvalidInput`', library\std\src\process\tests.rs:325:19
---- sys::windows::process::tests::windows_exe_resolver stdout ----
---- sys::windows::process::tests::windows_exe_resolver stdout ----
thread 'sys::windows::process::tests::windows_exe_resolver' panicked at 'assertion failed: resolve_exe(OsStr::new(\"rustc\"), child_paths).is_ok()', library\std\src\sys\windows\process\tests.rs:174:5

failures:
    process::tests::test_interior_nul_in_arg_is_error
    process::tests::test_interior_nul_in_args_is_error
    process::tests::test_interior_nul_in_args_is_error
    process::tests::test_interior_nul_in_current_dir_is_error
    sys::windows::process::tests::windows_exe_resolver

test result: FAILED. 879 passed; 4 failed; 3 ignored; 0 measured; 0 filtered out; finished in 13.83s

error: test failed, to rerun pass '-p std --lib'


command did not execute successfully: "\\\\?\\D:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage0\\bin\\cargo.exe" "test" "--target" "x86_64-pc-windows-msvc" "-Zbinary-dep-depinfo" "-j" "8" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace profiler compiler-builtins-c" "--manifest-path" "D:\\a\\rust\\rust\\library/test/Cargo.toml" "-p" "std" "--"


Build completed unsuccessfully in 0:40:00
Build completed unsuccessfully in 0:40:00
make: *** [Makefile:72: ci-subset-1] Error 1

[AngelicosPhosphoros]

How about storing boolean as is in atomic?
I managed to remove 1 branch and make code shorter by this:
godbolt

[ehuss]

The CI error was due to some infrastructure problem. I would retry, but do you want to respond to the comment at #92139 (comment) first?

[dtolnay]

That comment seems orthogonal to the change in this PR.

@bors retry