Support stable Rust builds for 32-bit ARM targets by gating ARMv7 NEON code

[Metater]

This is a human:
Thank you so much for making this crate! I am using it in my Unity proximity voice chat library. This pairs super well with Opus and RNNoise to create a full voice chat system.

The following is written by AI:

I ran into build failures for 32-bit ARM targets on stable Rust:

• armv7-unknown-linux-gnueabihf
• armv7-linux-androideabi

The same project builds successfully for Windows, macOS, iOS, Linux x86_64/aarch64, and Android arm64/x86/x86_64. Only the 32-bit ARM jobs fail.

Environment

• Crate: aec3 = "0.3.0"
• Rust: stable
• Build command examples:

cross build --release --target armv7-unknown-linux-gnueabihf --lib
cargo build --release --target armv7-linux-androideabi --lib

Failure

The build fails with E0658 around 32-bit ARM NEON intrinsics and #[target_feature(enable = "neon")].

Example errors:

error[E0658]: use of unstable library feature `stdarch_arm_neon_intrinsics`
   --> aec3-0.3.0/src/audio_processing/agc2/rnn_vad/vector_math.rs:180:46
    |
180 |         accumulator = vaddq_f32(accumulator, vmulq_f32(x_i, y_i));
    |                                              ^^^^^^^^^
    |
    = note: see issue #111800 <https://github.com/rust-lang/rust/issues/111800> for more information

error[E0658]: the target feature `neon` is currently unstable
   --> aec3-0.3.0/src/audio_processing/aec3/adaptive_fir_filter.rs:345:18
    |
345 | #[target_feature(enable = "neon")]
    |                  ^^^^^^^^^^^^^^^
    |
    = note: see issue #150246 <https://github.com/rust-lang/rust/issues/150246> for more information

Other reported sites include:

src/audio_processing/aec3/adaptive_fir_filter.rs
src/audio_processing/aec3/adaptive_fir_filter_erl.rs
src/audio_processing/aec3/matched_filter.rs
src/audio_processing/aec3/vector_math.rs
src/audio_processing/agc2/rnn_vad/vector_math.rs

Likely cause

It looks like the crate has scalar fallbacks, but the ARMv7 NEON implementations are still compiled for target_arch = "arm". On stable Rust, 32-bit ARM NEON intrinsics and #[target_feature(enable = "neon")] are still gated.

Relevant Rust tracking issues:

• Tracking Issue for NEON intrinsics on 32-bit ARM rust-lang/rust#111800
• Tracking Issue for arm_target_feature rust-lang/rust#150246

Requested support

Would you be open to supporting stable Rust builds for these 32-bit ARM targets?

Possible approaches:

1. Gate NEON implementations to target_arch = "aarch64" only, since AArch64 NEON is stable/common.
2. Add a crate feature such as disable-neon / portable-simd-off / scalar-armv7 that forces scalar code on ARMv7.
3. Use conditional compilation so ARMv7 stable builds compile scalar paths while nightly users can opt into ARMv7 NEON.
4. Add CI coverage for armv7-unknown-linux-gnueabihf and/or armv7-linux-androideabi.

For my Unity plugin packaging, I temporarily removed Linux ARMv7 and Android armeabi-v7a from CI, but I would like to restore them if aec3 can compile these targets on stable Rust.

Thanks!

[RubyBit]

Thanks for the nice words :)

Pushed 0.3.1 which hopefully fixes your issue and compiles on those platforms.

[Metater]

Awesome, thank you! I will try that out soon.

Would you be willing to help me out with configuring AEC3 and AGC2?

With my test setup, it seems like AEC3 decreases the audio quality a bit (even when echo cancellation is not needed, using headphones and looping the microphone back through the speakers to test my playback algorithm). The echo cancellation does work really well in cases when it is needed however. For instance, blasting game music through my laptop speakers and looping the audio playback through my microphone.

And AGC2 seems to be cutting off the beginning and ends of my words and some plosives. It might be my setup. I exposed most of the options in my C# interop and rust wrapper DLL, but I believe I am using the default values for both AEC3 and AGC2.

My Discord is "metater" btw

Thanks,
Connor