floor(), floorf(), ceil() and ceilf() Don't actually work on no-std (linker error)

[AldaronLau]

The linker prints out: undefined symbol: fmodf and undefined symbol: fmod. I noticed that if I don't use any of these functions the linker error goes away. I've been trying to figure out why this is happening all day, and finally tracked it down to these functions. I believe the code that generates the code looking for this external symbol is caused by the force_eval!() macro invocation. To me it looks like maybe LLVM is trying to optimize based on that and attempting to use the actual libm.

It would also be nice if there was some documentation on what force_eval()! is for because I can't figure it out.

I'm running rustc 1.59.0-nightly (0b42deacc 2021-12-09) cross compiling from Linux (x86) to aarch64.

[Lokathor]

force_eval!() is a questionable attempt to try and make the float operation happen at runtime rather than at compile time, so that the floating point flags are correct according to the spec.

What makes it questionable is that LLVM optimizes float ops as if there's no FP environmental flags anyway, so... it shouldn't actually affect anything to just remove all uses of the macro.

[Amanieu]

Aren't fmodf and fmod provided by this crate anyways? Why would it result in a linker error?

Can you provide example code to reproduce this issue?

[AldaronLau]

@Amanieu yes, but they aren't #[no_mangle] so it can't find it. I tried, I am using it on no-std, and not super familiar with building standandalone apps on no-std so making example code has proven to be difficult. If you want to check the project in question, it's at https://github.com/ardaku/arc/tree/57b77f656088bc04f1fd8e3adb65db6ed3fb4566 - currently we're forcing it to link by defining empty functions in src/symbols.rs, but if they are removed then it fails to link.

[Amanieu]

fmod is re-exported by compiler-builtins with #[no_mangle], but only on a few specific targets. Perhaps we should expose a compiler-builtins-math feature that works like compiler-builtins-mem?

[YXL76]

I also had this problem when compiling to target x86_64-unknown-none, cannot find fmax and fmin

[kevinaboos]

We have also experienced this issue on custom x86_64-based targets for quite a long time. My workaround has just been to write #[no_mangle] wrappers around the missing functions, but it'd be nice to have all of them exported correctly so linking works as expected. 👍

[HTGAzureX1212]

Just a heads up - I ran into this message myself quite some time before and went ahead to export all functions in this crate with #[no_mangle] and extern "C" for convenience, in a single crate: https://crates.io/crates/externc-libm