Using no_std on thumbv6m-none-eabi

[spadarian]

I'm trying to use this awesome library for an embedded project (just inference) but I'm having problems compiling it.

error[E0432]: unresolved import `__alloc::sync`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/no-std-compat-0.4.1/src/generated.rs:173:48
    |
173 |     #[cfg(feature = "alloc")] pub use __alloc::sync::*;
    |                                                ^^^^ could not find `sync` in `__alloc`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `no-std-compat` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...
error[E0463]: can't find crate for `std`
  --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/num-traits-0.2.15/src/lib.rs:21:1
   |
21 | extern crate std;
   | ^^^^^^^^^^^^^^^^^ can't find crate
   |
   = note: the `thumbv6m-none-eabi` target may not support the standard library
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`.

I tried to run cargo build -Zbuild-std as suggested by I still get the error:

error[E0432]: unresolved import `alloc::sync`
 --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/gimli-0.26.2/src/read/dwarf.rs:2:12
  |
2 | use alloc::sync::Arc;
  |            ^^^^ could not find `sync` in `alloc`

It's probably a misconfiguration from my part but I was wondering if you have any suggestions. Thanks!

@antimora (I saw you in some of the no_std related issues 😄)

[coreylowman]

Can you share your Cargo.toml configuration for dfdx?

For no-std you should have both of these:

1. Disable default features default-features=false
2. Enable the no-std feature. features = ["no-std", ...], note that this is different from other no_std packages

If both of those are true this seems like a regression for no-std. We have a CI pipeline that's supposed to check for no-std support but there could be a bug with it

[spadarian]

Yes, both are true.

Apparently, it is an specific problem with the thumbv6m-none-eabi target as it doesn't have some atomic ops. See discussion here.

In my particular case, I just want to do inference (loading a model at compile time), so no sync would be needed (single thread). No idea how difficult or useful would be to have an inference feature that could support this case.

[coreylowman]

Ahhh I see - it seems like something we could fix by enabling the portable_atomic feature of spin?

[spadarian]

Maybe...

Also, not sure what the deal would be with rand_distr which uses the std_math feature.

[coreylowman]

Hmm yeah adding portable_atomics addresses one compilation issue, but then I start getting these very weird errors:

error[E0463]: can't find crate for `core`
  --> C:\Users\clowm\.cargo\registry\src\index.crates.io-6f17d22bba15001f\libm-0.2.1\src\math\tgamma.rs:25:1
   |
25 | extern crate core;
   | ^^^^^^^^^^^^^^^^^^ can't find crate
   |
   = note: the `thumbv6m-none-eabi` target may not be installed
   = help: consider downloading the target with `rustup target add thumbv6m-none-eabi`
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

error[E0463]: can't find crate for `core`
  --> C:\Users\clowm\.cargo\registry\src\index.crates.io-6f17d22bba15001f\rand_core-0.6.3\src\lib.rs:41:5
   |
41 | use core::convert::AsMut;
   |     ^^^^ can't find crate
   |
   = note: the `thumbv6m-none-eabi` target may not be installed
   = help: consider downloading the target with `rustup target add thumbv6m-none-eabi`
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

I definitely have thumbv6m-none-eabi installed via rustup, and since we are doing no-std I don't think building the std library from scratch is what we want...

[coreylowman]

Do you know of no_std crates that compile successfully for this target architecture? We might be able to copy the configuration from there

[spadarian]

I'm using:

• tinyrand
• num-traits with default features disabled and libm

[coreylowman]

Got it thanks! So it looks like with some feature wrangling I was able to reduce the issues I was seeing, and now am just encountering issues with spin. I created an issue on their repo asking for more info here mvdnes/spin-rs#151.

[spadarian]

Thanks mate. I tried what they suggested (portable_atomic_unsafe_assume_single_core) but it didn't work either... I don't remember the error but I will post it when I get to work.

[coreylowman]

This is the final error I'm getting with the fixes for the target:

error[E0432]: unresolved import `__alloc::sync`
   --> C:\Users\clowm\.cargo\registry\src\github.com-1ecc6299db9ec823\no-std-compat-0.4.1\src\generated.rs:173:48
    |
173 |     #[cfg(feature = "alloc")] pub use __alloc::sync::*;
    |                                                ^^^^ could not find `sync` in `__alloc`

Which is with no-std-compat 🙄

This is with an empty project with Cargo.toml:

[package]
name = "no-std-test"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
dfdx = { path = "../dfdx", default-features = false, features = ["no-std"]}

and .cargo/config:

[target.thumbv6m-none-eabi]
rustflags = ["--cfg", "portable_atomic_unsafe_assume_single_core"]

Command to build is:

cargo build --target thumbv6m-none-eabi

[spadarian]

Yes... that was the error.

[coreylowman]

Found a related issue on no-std-compat https://gitlab.com/jD91mZM2/no-std-compat/-/issues/6

[coreylowman]

Okay so I tested out on the main branch of no-std-compat with this dependency:

no-std-compat = { git = "https://gitlab.com/jD91mZM2/no-std-compat.git", branch = "master", default-features = false, features = [ "alloc", "compat_hash" ], optional = true }

This addresses the above error, but it seems like it doesn't include std::sync at all (see errors below). I'm not sure what this means for compiling dfdx on this target. 🤔

error[E0432]: unresolved import `std::sync::Arc`
 --> C:\Users\clowm\Documents\programming\dfdx\src\optim\adam\mod.rs:6:32
  |
6 | use std::{marker::PhantomData, sync::Arc};
  |                                ^^^^^^^^^ no `Arc` in `generated::sync`

error[E0432]: unresolved import `std::sync::Arc`
 --> C:\Users\clowm\Documents\programming\dfdx\src\optim\rmsprop\mod.rs:6:32
  |
6 | use std::{marker::PhantomData, sync::Arc};
  |                                ^^^^^^^^^ no `Arc` in `generated::sync`

error[E0432]: unresolved import `std::sync::Arc`
  --> C:\Users\clowm\Documents\programming\dfdx\src\tensor\cpu\allocate.rs:11:11
   |
11 | use std::{sync::Arc, vec::Vec};
   |           ^^^^^^^^^ no `Arc` in `generated::sync`

[coreylowman]

I think we may be able to get aroudn this with no-std-compat's "compat_sync" feature, which adds std::sync bindings. However it is currently using a very old version of spin (0.7.0)

[alisomay]

Could you make no-std-compat work with thumbv6m-none-eabi in the end?
It is also blocking me to use some crates.

[coreylowman]

No this is a known issue in the no-std-compat crate: https://gitlab.com/jD91mZM2/no-std-compat/-/issues/6