[Parallel_libs-commits] [PATCH] D24951: Import/adapt the SLEEF vector math-function library as an LLVM runtime

[Chandler Carruth via Parallel_libs-commits]

chandlerc added a comment.

Focusing on the (very) high level issues:

I think we should probably import this with a name other than "SLEEF". At least for me, that name doesn't convey much. I would choose the name to help guide how the library should be developed and used, hopefully in a way that is reasonable short.

At least two possible and reasonable long-term goals come to my mind:

1. Provide vectorized versions of any math routines whose scalar counterparts are typically provided via `libc` or `libm` such that LLVM can use the system `libc` and/or `libm` without being limited to the vectorized facilities it happens to provide when vectorizing code.

2. Provide vectorized versions of any math routines useful to enable vectorization.

Clearly #2 is a superset of #1 here. The interesting question between these two is whether there is a specific desire to limit the scope to the `libc` and `libm` common set of mathematical functions. I can see good reasons to do that, and I can see good reasons to expand the scope if there are common mathematical routines that come up for users in contexts where vectorization is desired.

If folks want the scope to be narrow along the lines of #1, I would suggest a library name that is reasonably reminiscent of `libm`. `libmv` or `libmvec`? Dunno.

If folks want the scope to be broader, I would probably be inclined to go for a broader name as well. `vector_math` or `vmath`? Other ideas?

Or are there other suggestions on how to describe / scope the library that would lend themselves to other names?

Once the name at the project level is settled, we should pick the installed name. I would strongly suggest something with 'llvm' or at least 'll' in it. I can imagine people wanting to incorporate this code into other vector math libraries which is I think ultimately a good thing. But LLVM should be able to ship as a toolchain a clearly named runtime library that we own and that provides a set of runtime functions the toolchain expects to be able to use fairly directly.

This brings us to the question of how to name the actual routines which you raised in the original patch description and which you raised again with your latest email Hal:

In https://reviews.llvm.org/D24951#557963, @hfinkel wrote:

> A few people have suggested using the Intel vector ABI name-mangling scheme (https://www.cilkplus.org/sites/default/files/open_specifications/Intel-ABI-Vector-Function-2012-v0.9.5.pdf) for these functions.
>
> There are two potential not-mutually-exclusive meanings to this suggestion:
>
> 1. Use our own base names for the functions (e.g. __xcos or __sleef_cos), but identify the vector versions using the vector ABI mangling scheme (e.g. _ZGVxN4v___xcos or _ZGVxN4v___sleef_cos).
> 2. Name our vector functions provide vector-ABI-mangled versions of the libm functions (e.g. _ZGVxN4v_cos).
>
>   Regarding (1), this makes sense to me, the only disadvantage I can see being that the names are less human-readable, what do people prefer? Regarding (2), one potential issue is that, since we (i.e. the provider of a compiler and not the provider of libc, which is true for some of us) don't "own" cos we also don't really "own" _ZGVxN4v_cos either, and providing this when libc might also could lead to conflicts. One option is to use our own base name for the functions, but also provide weak aliases to the standard names with the vector ABI names. Thoughts?


A huge initial question here: do we want to support users of LLVM calling these routines directly from their code? That is, do we want to use this *both* to support LLVM's vectorizers and to support users vectorizing their code?

I'm inclined to say "yes" but I represent a relatively small and specialized set of potential users for these routines. I suspect Hal and others should comment how they expect to proceed here.

If the answer is "yes" then I think we should think carefully about how that API is spelled in user code. That would (IMO) make it important that the routines have quite friendly names and interfaces. I suspect that will end up being the overriding concern with naming these, potentially making things like C++ namespaces part of what we want to think about here. However, I can also see an argument we should design a user-facing API as wrappers for the underlying runtime library.

Past all of that, or if the answer is "no", my concern about naming these routines following the Intel ABI is that it would mean zero consistency between architectures which seems really unfortunate.

My suggested naming of the routines would follow a set of conventions to build each name in a predictable way:

- Library (and LLVM) specific prefix like `__llvm_vector_math`
- Name of the mathematical function
- Vectorization factor and lane type like `v4f32`
- (optional) An architecture and potentially ABI-specific suffix like `x86_64`

This would give us as an example:

  __llvm_vector_math_sin_vNiM_<arch/abi>
  __llvm_vector_math_sin_vNfM_<arch/abi>

I very much want a common prefix (and for that common prefix to be something LLVM-specific) so we don't have collisions and can identify where these routines came from. I would like the component of the name that just identifies the vectorization and lane type to be common across architectures to make maintenance and IR calling these routines much more clear. We have good common naming conventions within the LLVM backend already. If there are more dimensions than vectorization factor and element type, we can add more to this descriptor, I just pulled this example off the top of my head.

The last bit I'm not as confident in... I can see an argument that we should just have the raw name and generate the code for the architecture, but I think for testing and other purposes we will often want multiple variants per architecture and in *source code* may see multiple architectures at the same time. There, having suffixes may make development much more clear. If we go that route, it would be good to have a suffix-free alias that is computed using an IFUNC at runtime or otherwise a reasonable default.

All that said, I really like the idea of using the Intel ABI where appropriate. I would suggest that we have a build mode that includes (weak?) aliases for the `libc` and `libm` routines with the vector ABI applied on x86 so that many things "just work". If there are other contexts in which emitting names under that ABI make sense, that seems good to me as well, I would just always do it with aliases to the more predictable / easy to read names.

Does all of this make sense? Curious if there are drawbacks here I've not thought of...


https://reviews.llvm.org/D24951