Change codegen of LLVM intrinsics to be name-based, and add llvm linkage support for bf16(xN) and i1xN

[sayantn]

This PR changes how LLVM intrinsics are codegen

Explanation of the changes

Current procedure

This is the same for all functions, LLVM intrinsics are not treated specially

• We get the LLVM Type of a function simply using the argument types. For example, the following function

  #[link_name = "llvm.sqrt.f32"]
  fn sqrtf32(a: f32) -> f32;

  will have LLVM type simply f32 (f32) due to the Rust signature

Pros

• Simpler to implement, no extra complexity involved due to LLVM intrinsics

Cons

• LLVM intrinsics have a well-defined signature, completely defined by their name (and if it is overloaded, the type parameters). So, this process of converting Rust signatures to LLVM signatures may not work, for example the following code generates LLVM IR without any problem

  #[link_name = "llvm.sqrt.f32"]
  fn sqrtf32(a: i32) -> f32;

  but the generated LLVM IR is invalid, because it has wrong signature for the intrinsic (Godbolt, adding -Zverify-llvm-ir to it will fail compilation). I would expect this code to not compile at all instead of generating invalid IR.
• LLVM intrinsics that have types in their signature that can't be accessed from Rust (notable examples are the AMX intrinsics that have the x86amx type, and (almost) all intrinsics that have vectors of i1 types) can't be linked to at all. This is a (major?) roadblock in the AMX and AVX512 support in stdarch.
• If code uses an non-existing LLVM intrinsic, even -Zverify-llvm-ir won't complain. Eventually it will error out due to the non-existing function (courtesy of the linker). I don't think this is a behavior we want.

What this PR does

• When linking to non-overloaded intrinsics, we use the function LLVMIntrinsicGetType to directly get the function type of the intrinsic from LLVM.
• We then use this LLVM definition to verify the Rust signature, and emit a proper error if it doesn't match, instead of silently emitting invalid IR.

Note

This PR only focuses on non-overloaded intrinsics, overloaded can be done in a future PR

Regardless, the undermentioned functionalities work for all intrinsics

• If we can't find the intrinsic, we check if it has been AutoUpgraded by LLVM. If not, that means it is an invalid intrinsic, and we error out.
• (Yet to be implemented) Don't allow intrinsics from other archs to be declared, e.g. error out if an AArch64 intrinsic is declared when we are compiling for x86

Pros

• It is now not possible (or at least, it would require significantly more leaps and bounds) to introduce invalid IR using non-overloaded LLVM intrinsics.
• As we are now doing the matching of Rust signatures to LLVM intrinsics ourselves, we can now add bypasses to enable linking to such non-Rust types (e.g. matching 8192-bit vectors to x86amx and injecting llvm.x86.cast.vector.to.tile and llvm.x86.cast.tile.to.vectors in callsite)

Note

I don't intend for these bypasses to be permanent (at least the bf16 and i1 ones, the x86amx bypass seems inevitable). A better approach will be introducing a bf16 type in Rust, and allowing repr(simd) with bools to get Rust-native i1xNs. These are meant to be short-time, as I mentioned, "bypass"es. They shouldn't cause any major breakage even if removed, as link_llvm_intrinsics is perma-unstable.

This PR adds bypasses for bf16 (via i16), bf16xN (via i16xN), i1xN (via iM, where M is the smallest power of 2 s.t. M >= N, unless N <= 4, where we use M = 8). This will unblock AVX512-VP2INTERSECT and a lot of bf16 intrinsics in stdarch. This PR also automatically destructures structs if the types don't exactly match (this is required for us to start emitting hard errors on mismmatches).

Cons

• This only works for non-overloaded intrinsics (at least for now). Improving this to work with overloaded intrinsics too will involve significantly more work.

Possible ways to extend this to overloaded intrinsics (future)

Parse the mangled intrinsic name to get the type parameters

LLVM has a stable mangling of intrinsic names with type parameters (in LLVMIntrinsicCopyOverloadedName2), so we can parse the name to get the type parameters, and then just do the same thing.

Pros

• For most intrinsics, this will work perfectly, and is a easy way to do this.

Cons

• The LLVM mangling is not perfectly reversible. When we have TargetExt types or identified structs, their name is a part of the mangling, making it impossible to reverse. Even more complexities arise when there are unnamed identified structs, as LLVM adds more mangling to the names.

Use the IITDescriptor table and the Rust function signature

We can use the base name to get the IITDescriptors of the corresponding intrinsic, and then manually implement the matching logic based on the Rust signature.

Pros

• Doesn't have the above mentioned limitation of the parsing approach, has correct behavior even when there are identified structs and TargetExt types. Also, fun fact, Rust exports all struct types as literal structs (unless it is emitting LLVM IR, then it always uses named identified structs, with mangled names)

Cons

• Doesn't actually use the type parameters in the name, only uses the base name and the Rust signature to get the llvm signature (although we can check that it is the correct name). It means there would be no way to (for example) link against llvm.sqrt.bf16 until we have bf16 types in Rust. Because if we are using u16s (or any other type) as bf16s, then the matcher will deduce that the signature is u16 (u16) not bf16 (bf16) (which would lead to an error because u16 is not a valid type parameter for llvm.sqrt), even though the intended type parameter is specified in the name.
• Much more complex, and hard to maintain as LLVM gets new IITDescriptorKinds

These 2 approaches might give different results for same function. Let's take

#[link_name = "llvm.is.constant.bf16"]
fn foo(a: u16) -> bool

The name-based approach will decide that the type parameter is bf16, and the LLVM signature is i1 (bf16) and will inject some bitcasts at callsite.
The IITDescriptor-based approach will decide that the LLVM signature is i1 (u16), and will see that the name given doesn't match the expected name (llvm.is.constant.u16), and will error out.

Other things that this PR does

• disables all ABI checks only for the unadjusted ABI to facilitate the implementation of AMX (otherwise passing 8192-bit vectors to the intrinsic won't be allowed). This is "safe" because this ABI is only used to link to LLVM intrinsics, and passing vectors of any lengths to LLVM intrinsics is fine, because they don't exist in machine level. (moved to another PR)
• Removes unnecessary bitcasts in cg_llvm/builder::check_call (now renamed as cast_arguments due to its new counterpart cast_return). This was old code from when Rust used to pass non-erased lifetimes to LLVM.

Reviews are welcome, as this is my first time actually contributing to rustc

After CI is green, we would need a try build and a rustc-perf run.

@rustbot label T-compiler A-codegen A-LLVM
r? codegen

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[dianqk]

I think you can use LLVMGetIntrinsicDeclaration, LLVMGetIntrinsicDeclaration or some functions in Intrinsic.h in declare_raw_fn, as a reference: https://github.com/llvm/llvm-project/blob/d35ad58859c97521edab7b2eddfa9fe6838b9a5e/llvm/lib/AsmParser/LLParser.cpp#L330-L335.

[sayantn]

That can be used to improve performance, I am not really focusing on performance in this PR. I want to currently emphasize the correctness of the codegen.

[sayantn]

Oh wait, I probably misunderstood your comment, you meant using the llvm declaration by itself. Yeah, that would be better, thanks for the info. I will update the impl when I get the chance

[dianqk]

Oh wait, I probably misunderstood your comment, you meant using the llvm declaration by itself. Yeah, that would be better, thanks for the info. I will update the impl when I get the chance

I think you can just focus on non-overloaded functions for this PR. Overloaded functions and type checking that checking Rust function signatures using LLVM defined can be subsequent PRs.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[nikic]

@sayantn Taking the address of an intrinsic is invalid LLVM IR.

[rustbot]

This PR modifies run-make tests.

cc @jieyouxu

[sayantn]

There was an invalid intrinsic in run-make/simd-ffi/simd.rs 🙂. The problem with invalid intrinsics (as opposed to invalid signatures of valid intrinsics) is that they silently compile, even -Zverify-llvm-ir doesn't complain. They just generate a call to an unknown function in asm, which will eventually fail linking, but most tests don't link at all.

[sayantn]

After CI is green, I think we should do a try build and an rustc-perf run, because this modifies quite a bit in the codegen of rust and llvm intrinsics, which might have some performance footprint (I am especially concerned about rust-intrinsics, I got rid of the signature table on grounds that we don't need to do it anymore, but we might still need it as a cache)

[sayantn]

Can anyone help here? IDK what's going on, in my system (x86_64-unknown-linux-gnu), I can actually see the note (I did ./x test --stage 1 tests/ui/codegen --bless, no changes)

[workingjubilee]

Assuming they can stand separately, can we split up the intrinsic changes from the extern "unadjusted" changes? It really feels like we are going to want to be able to bisect these individually, unfortunately.

[sayantn]

@workingjubilee If you mean I should separate the implementation changes of Rust intrinsics into another PR, yeah no problem, but I feel like they go with this PR. I can try separating them into clean commits, but separating them into different PRs will imo not be good. But I don't think I should separate the loosening of ABI check rules for extern "unadjusted" functions, it is required for the AMX bypass to make sense.

PS: I don't really like that we use the same term for rustc builtins and LLVM builtins, but oh well

[sayantn]

Sorry for the repeated force pushes, I refactored my code quite a bit, and restructured the commits

[sayantn]

As @workingjubilee suggested I will split this PR into 2, this will only add bypasses for i1xN and bf16(xN), I will open another PR for x86amx once this merges.

[sayantn]

@nikic I think this PR is now ready for review, I have moved some parts to another PR that I will send after this is merged

@rustbot ready

[sayantn]

a little more context for why we destructure structs - LLVM typically returns literal non-packed structs. So to match them, Rust code often uses repr(packed) structs. LLVM autoupgrade understands that there is no functional difference between {T, U} and <{T, U}>, and auto-injects some destructurings. But as here we are trying to match the types exactly, it would error as packed structs are not "equal" to non-packed ones. Moreover there is no Rust-native way to creating non-packed LLVM structs with the exact fields we want (in general), because Rust might add extra fields to ensure alignment. Which leaves us with the only option being packed structs, which requires this workaround.

It would have been nice if LLVM offered some way to "pack"/"un-pack" structs, i.e. convert between <{T, U}> and {T, U}, but as it doesn't, we have no way other than simply destructure and restructure them (which is, kinda good actually, as it lets us make these autocasts "deep"

[sayantn]

Do we need to add attributes even if the signature is getting AutoUpgraded (because at this point we can safely assume that the function is getting upgraded, we would have detected it earlier if it was invalid? I assume the sext/zext will matter if LLVM is autoupgrading i32 to i1x8 or sth, but cc @nikic to confirm. If they don't matter even for upgradable intrinsics, we can reduce this to just a name check