Calling the llvmint::cttz_iX is broken

[gnzlbg]

I am trying to call the llvmint::cttz_i8(1i8, false) intrinsic from the llvmint crate and am getting the following error:

is_zero_undef argument of bit counting intrinsics must be a constant int
 %17 = call i8 @llvm.cttz.i8(i8 %14, i1 zeroext %16)

The problem seems to be that rustc is not propagating false to LLVM properly.

This was also reported to the llvmint crate: huonw/llvmint#5

[nagisa]

Did you find trailing_zeros method on integers to be too stable? There’s always the std::intrinsics module which contains proper implementation of the intrinsic. I doubt this will ever be fixed, primarily because calling llvm intrinsics will never be stable and there’s no reason to fix it.

[gnzlbg]

Is the handling of zero by that intrinsic defined or undefined? The documentation does not say, and one might want to do both (hence why llvmint::cttz_iX takes two parameters).

[nagisa]

I think the best correct solution you can hope for here is new intrinsics in std::intrinsics which could be added provided there’s a strong usecase for zero-is-undef (performance is unlikely to be one).

re zero: answered on the other issue: #34381.

[gnzlbg]

I think the best correct solution you can hope for here is new intrinsics in std::intrinsics which could be added provided there’s a strong usecase for zero-is-undef (performance is unlikely to be one).

So this turned into an XY Problem.

What I actually wanted was to use the BMI 1.0 tzcnt intrinsic without writting any assembly.

Ideally, std::intrinsics::cttz would use this if the architecture supports BMI 1.0. LLVM will use that instruction automatically if available when calling through the llvm.cttz.iX intrinsic (and will skip the zero check if it can prove that the value cannot be zero), but since there is no architecture flag for BMI 1.0 I don't know how to tell LLVM via rustc that the architecture supports it (and I don't know if the LLVM version used by rustc implements it, but it is new enough that I would guess it does).

So to work around the lack of a feature flag I ended up adding my own feature flags to my crate and trying to call llvm intrinsics for the particular architectures directly and running into this.

[eefriedman]

-C target-feature=+bmi

[gnzlbg]

-C target-feature=+bmi

@eefriedman Thanks! Are those documented somewhere (i've tried googling for the supported target-features without any luck)? Is that for BMI 1.0 or BMI 2.0?

EDIT: found it! The command llc -mattr=help can be used to list all target features!

[gnzlbg]

Is there a way to detect the target-feature's enabled from within a rust program? Something like #[cfg(target_feature = "bmi")] that is true if that target feature is enabled?

[Mark-Simulacrum]

I suspect that this is no longer an issue judging by the comments, with the proper solution being target-feature=+bmi. The original issue is likely impossible to reproduce today since the llvmint crate has not received updates in 2 years. As such, I'm going to close.