[AArch64] stdarch unconditionally uses AtomicI64 in RCPC3 intrinsics, breaking targets with `max-atomic-width = 0`

[Snehal-Reddy]

The recent addition of RCPC3 intrinsics (specifically vldap1_lane_s64, vldap1q_lane_s64, vstl1_lane_s64, and vstl1q_lane_s64) in stdarch causes compilation failures when building core for AArch64 targets that disable 64-bit atomics (e.g., custom targets with "max-atomic-width": 0).

The core library conditionally defines AtomicI64 based on #[cfg(target_has_atomic_load_store = "64")]. However, the generated AArch64 NEON intrinsics module uses AtomicI64 unconditionally within the RCPC3 functions, leading to resolution errors when the target does not support 64-bit atomics.

Reproduction Steps

1. Create a custom target specification file aarch64-no-atomic.json that disables atomics:

 {
     "arch": "aarch64",
     "data-layout":
 "e-m:e-p270:32:32-p271:32:32-p272:64:64-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128-Fn32",
     "llvm-target": "aarch64-unknown-none",
     "max-atomic-width": 0,
     "atomic-cas": false,
     "linker-flavor": "gnu-lld",
     "linker": "rust-lld",
     "panic-strategy": "abort",
     "target-pointer-width": 64
 }

2. Attempt to build core using build-std with a recent Nightly toolchain (e.g., 1.95 or later):

   cargo +nightly build \
       -Z build-std=core \
       --target aarch64-no-atomic.json \
       --release

Compilation fails with errors indicating AtomicI64 cannot be found:

   error[E0433]: failed to resolve: could not find `AtomicI64` in `atomic`
        --> library/core/src/../../stdarch/crates/core_arch/src/aarch64/neon/generated.rs:12863:43
         |
   12863 |     let atomic_src = crate::sync::atomic::AtomicI64::from_ptr(ptr as *mut i64);
         |                                           ^^^^^^^^^ could not find `AtomicI64` in `atomic`

   error[E0425]: cannot find type `AtomicI64` in module `crate::sync::atomic`
        --> library/core/src/../../stdarch/crates/core_arch/src/aarch64/neon/generated.rs:27193:55
         |
   27193 |     let atomic_dst = ptr as *mut crate::sync::atomic::AtomicI64;
         |                                                       ^^^^^^^^^

Suggested Fix

The usage of AtomicI64 in stdarch should be guarded by #[cfg(target_has_atomic = "64")].

[bjorn3]

Are there actually any arm64 cpus that don't support atomics?

[Snehal-Reddy]

Not sure, but ARM does support turning off MMUs/caches. In those cases, it would be beneficial to run with atomics disabled, to prevent undefined behaviour.