LLVM intrinsic emission is inconsistent

[MabezDev]

Running the following snippet across a few different platforms (x86_64-unknown-linux-gnu, riscv32imc-esp-espidf) has varying results in the intrinsic's used in the llvm-ir output.

let f: f32 = 1.0;
let f = f.mul_add(2.0, 3.0);
println!("f = {}", f);

I would expect the llvm-ir for each target to have calls in llvm intrinsic's, because that's exactly whats in the function body of mul_add, however on the riscv32imc-esp-espidf platform, this doesn't always end up happening.

Adding a call to a new unstable function lerp for example, and the intrinsic is now emitted in the llvm-ir even though its calling the same mul_add functions inside it.

I thought maybe compiler_builtins might have an implementation that is used in the simple example, but that doesn't seem to be the case.

Meta

rustc --version --verbose:

1.55.0, also happens on 1.54.0 too

also on a side note, where can I find the intrinsic's that must be implemented for a std platform? I notice in the case of the float modules, some calls use intrinsic's, some use cmath. I would be great to have a list of the required intrinsic's as we are bringing up Xtensa Rust support.

[nagisa]

What the actual motivation underlying this? What LLVM-IR Rust generates is entirely an implementation detail of the compiler, so I'm not sure we do or will guarantee any specific output in terms of LLVM-IR.

[MabezDev]

The motivation is that I am bootstrapping Xtensa targets, in which we are also running a fork of llvm. When rebasing the rust patchset on top of 1.55, our standard library port stopped compiling, throwing a missing intrinsic error (fma). Specifically it failed in a new function introduced in 1.55, lerp .I noticed that just using mul_add did not emit a intrinsic call, but the inclusion of lerp did.

Overall, I guess it doesn't matter if the compiler does emit it or not. What matters is can the compiler emit it? Is there a list (or an easy way to find) the intrinsics used by the standard library, so we can implement them in our llvm fork?

[nagisa]

https://github.com/rust-lang/rust/blob/master/compiler/rustc_codegen_llvm/src/intrinsic.rs is probably a good way to find out all of the LLVM intrinsic calls that we might emit for some Rust code. grep for llvm. in that file for a list.

[MabezDev]

Thanks for the assistance @nagisa, we've resolved the issue by correctly implementing the basic llvm intrinsics.