Tracking issue for stable SIMD in Rust

[alexcrichton]

This is a tracking issue for RFC 2325, adding SIMD support to stable Rust. There's a number of components here, including:

• #[target_feature(enable = ...)]
• Names of accepted target features - currently proposed
• #[cfg(target_feature = ...)]
• is_target_feature_detected! macro
• std::arch naming and submodules

The initial implementation of this is being added in #48513 and the next steps would be:

• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

---

Known issues

• is_target_feature_detected! takes different arguments that #[target_feature]

[scottmcm]

My one request for the bikeshed (which the current PR already does and may be obvious, but I'll write it down anyway): Please ensure they're not all in the same module as things like undefined_behaviour and [un]likely, so that those rust-defined things don't get lost in the sea of vendor intrinsics.

[cuviper]

What will be the story for external LLVM? (lacking MCSubtargetInfo::getFeatureTable())

[alexcrichton]

@scottmcm certainly! I'd imagine that if we ever stabilized Rust-related intrinsics they'd not go into the same module (they probably wouldn't even be platform-specific).

@cuviper currently it's an unresolved question, so if it doesn't get fixed it means that using an external LLVM would basically mean that #[cfg(target_feature = ...)] would always expand to false (or the equivalent thereof)

[hanna-kruppe]

I'd imagine that if we ever stabilized Rust-related intrinsics they'd not go into the same module (they probably wouldn't even be platform-specific).

One option raised in the RFC thread (that I personally quite like) was stabilizing std::intrinsics (only the module), keep the stable rust intrinsics in that module (they can already be imported from that location due to a long-standing bug in stability checking) and put these new platform-specific intrinsics in submodules. IIUC this would also satisfy @scottmcm's request.

To be explicit, under that plan the rustdoc page for std::intrinsics would look like this:

---

Modules

• x86_64
• arm
• ...

Functions

• copy
• copy_nonoverlapping
• drop_in_place
• ...

[alexcrichton]

Another naming idea I've just had. Right now the feature detection macro is is_target_feature_enabled!, but since it's so target specific it may be more apt to call it is_x86_target_feature_enabled!. This'll make it a pain to call on x86/x86_64 though which could be a bummer.

[nox]

Why keep all the leading underscores for the intrinsics? Surely even if we keep the same names as what the vendors chose, we can still remove those signs, right?

[BurntSushi]

The point is to expose vendor APIs. The vendor APIs have underscores. Therefore, ours do too.

[nox]

It is debatable that those underscores are actually part of the name. They only have one because C has no modules and namespacing, AFAICT.

[nox]

I would be happy dropping the topic if it was discussed at length already, but I couldn't find any discussion specific to them leading underscores.

[BurntSushi]

@nox rust-lang/stdarch#212 --- My comment above is basically a summary of that. I probably won't say much else on the topic.

[Centril]

@nox, @BurntSushi Continuing the discussion from there... since it hasn't been mentioned before:

Leading _ for identifiers in rust often means "this is not important" - so just taking the names directly from the vendors may wrongly give this impression.

[alexcrichton]

@nox @Centril the recurring theme of stabilizing SIMD in Rust is "it's not our job to make this nice". Any attempt made to make SIMD different than what the vendors define has ended with uncomfortable questions and intrinsics that end up being left out. To that end the driving force for SIMD intrinsics in Rust is to get anything compiling on stable.

Crates like faster are explicitly targeted at making SIMD usage easy, fast, and ergonomic. The standard library's intrinsics are not intended to be widely used nor use for "intro level" problems. Leveraging the SIMD intrinsics is quite unsafe (due to target feature detection/selection) and can come at a high cost if used incorrectly.

Overall, again, the goal is to not enable ergonomic SIMD in Rust right now, but any SIMD in Rust. Following exactly what the vendors say is the easiest way for us to guarantee that all Rust programs will always have access to vendor intrinsics.

[hanna-kruppe]

I agree that the leading underscores are a C artifact, not a vendor choice (the C standard reserves identifiers of this form, so that's what C compilers use for intrinsics). Removing them is neither "trying to make it nicer/more ergonomic" (it's really only a minor aesthetic difference) nor involves any per-intrinsic judgement calls. It's a dead simple mechanical translation for a difference in language rules, almost as much as __m128 _mm_foo(); is mechanically translated to fn _mm_foo() -> __m128;.

[alexcrichton]

@rkruppe do we have a rock solid guarantee that no vendor will ever in the future add the same name without underscores?

[Centril]

@alexcrichton

@rkruppe do we have a rock solid guarantee that no vendor will ever in the future add the same name without underscores?

Can't speak for CPU vendors, but the probability seems very very low. Why would they add an intrinsic where the difference is only an underscore..? Further, as Rust's influence grows, they might not do this simply because of Rust.

[hanna-kruppe]

A name like mm_foo (no leading underscore at all) is not reserved in the C language, so it can't be used for compiler-supplied extensions without breaking legal C programs. There are a few theoretical possibilities for a vendor to nevertheless create intrinsics without leading underscores:

• they could expose it only in C++ (with namespacing) -- or, for that matter, another language that isn't C
• they could break legal C programs (very unlikely, and I'll eat my hat if GCC or Clang developers accept this)
• A future version of C adds some way of doing namespacing, and people start using it for intrinsics

All extremely unlikely. The first one seems like the only one that doesn't sound like science fiction to me, and if that happens we'd have other problems anyway (such intrinsics may use function overloading and other features Rust doesn't have).

[alexreg]

It is debatable that those underscores are actually part of the name. They only have one because C has no modules and namespacing, AFAICT.

This. The whole point is that the underscore-leading names were chosen so as to specifically not clash with user-defined functions. Which means they should never be using non-underscore names. It's against well-established C conventions. Hence, we should just rename them to follow Rust conventions, with no real chance there will be any name clash in the future, providing the vendors stay sane and respect C conventions.

[alexcrichton]

@Centril "probability seems very very low" is what I would say as well, but we're talking about stability of functions in the standard library, so "low probability" won't cut it unfortunately.

@rkruppe I definitely agree, yeah, but "extremely unlikely" to me says "follow the vendor spec to the letter and we can figure out ergonomics later".

[alexcrichton]

Another point worth mentioning for staying exactly to the upstream spec is that I believe it actually boosts learnability. You'll have instant familiarity with any SIMD/intrinsic code written in C, of which there's already quite a lot!

If we stray from the beaten path then we'll have to have a section of the documentation which is very clear about defining the mappings between intrinsic names and what we actually expose in Rust.

[pythoneer]

I don't think renaming (no leading underscore or any other alteration) is useful. This is simply not the goal and only introduces pain points. I cannot think of a reason other than "i like that more" to justify that. It only introduces the possibility to naming clashes and "very very unlikely" is not convincing because we can prevent this 100% by not doing it altogether.

I think its the best choice to follow the vendor naming schema as close as possible and i think we should even break compatibility if we ever introduce an error in the "public API" without doing some renaming like _mm_intr_a to _mm_intr_a2 and start diverging the exact naming schema introduced by the vendor.

[nox]

@alexcrichton But as @rkruppe said, removing the leading underscore isn't about ergonomics, it's about not porting C defects to Rust blindly.

[nox]

Sorry for the double post, but I also want to add that arguing that a vendor may release an unprefixed intrinsic with the same name as a prefixed one is to me as hypothetical as arguing that bool may not be a single byte on some platform we would like to support.

[pythoneer]

@nox but why stop by the _? We could also fully rename the function with ps and pd into f32 and f64 which would be something "more Rust". Its somewhat arbitrary to just remove the leading underscore. And we could argue back and forth what is ergonomics and what isn't but i don't think there is a very good line to distinguish that to a point every body agrees.

[nox]

@pythoneer Because the name is what the vendor decided, with a leading underscore because of nondescript limitations of C.

[pythoneer]

@nox and the explicit goal of stdsimd is to expose this (however defect) vendor defined interface.

[alexreg]

@nox and the explicit goal of stdsimd is to expose this (however defect) vendor defined interface.

Interface, sure, but not necessarily the naming conventions!

[chorman0773]

Can we get the AVX-512 intrinsics stabilized please?

I specifically need _mm_rolv_epi64 to avoid confusing codegen from llvm which caused me 4 hours of headaches trying to figure out, while folding down a shift+shift+or into a proper rotate.

[jplatte]

Not sure where else to ask: How would I find out about the state of (non-simd) WASM intrinsics? I'm specifically wondering whether the three memory_atomic_ fn's (two of them being required for parking_lot_core's thread parking) could be stabilized soon-ish.

[Ciel-MC]

Some builds have been failing on nightly because of this, and just to check if I'm understanding everything correctly, this has been stabilized on nightly, is that correct? Which is why the feature attribute fails.

[Amanieu]

The stdsimd feature has been split into sub-features: #27731 (comment)

Here is the full set of new tracking issues for what stdsimd was previous tracking:

• Tracking issue for core::arch::{x86, x86_64}::has_cpuid #60123

• Tracking issue for WebAssembly atomics #77839

• Tracking Issue for std::arch::wasm64 #90599

• Tracking Issue for AVX512 intrinsics #111137

• Tracking Issue for Intel Restricted Transactional Memory intrinsics #111138

• Tracking Issue for PowerPC arch intrinsics #111145

• Tracking Issue for _MM_SHUFFLE #111147

• Tracking Issue for stdarch_mips_feature_detection #111188

• Tracking Issue for stdarch_arm_feature_detection #111190

• Tracking Issue for stdarch_powerpc_feature_detection #111191

• Tracking Issue for stdarch_riscv_feature_detection #111192

• Tracking Issue for WebAssembly relaxed SIMD intrinsics #111196

• Tracking Issue for MIPS arch intrinsics #111198

• Tracking Issue for NVPTX arch intrinsics #111199

• Tracking Issue for NEON intrinsics on 32-bit ARM #111800

• Tracking Issue for RISC-V Ratified Extensions Intrinsics #114544

• Tracking Issue for ARM CRC32 intrinsics #117215

• Tracking Issue for AArch64 TME intrinsics #117216

• Tracking Issue for AArch64 prefetch intrinsic #117217

• Tracking Issue for ARM hint intrinsics #117218

• Tracking Issue for ARM barrier intrinsics #117219

• Tracking Issue for NEON FCMA intrinsics #117222

• Tracking Issue for NEON I8MM intrinsics #117223

• Tracking Issue for NEON dot product intrinsics #117224

• Tracking Issue for AArch64 SHA3 intrinsics #117225

• Tracking Issue for AArch64 SM3 and SM4 intrinsics #117226

• Tracking Issue for NEON FRINTTS intrinsics #117227

• Tracking Issue for 32-bit ARM DSP intrinsics #117237

I'm going to close this issue since it isn't tracking anything anymore.