Comment by misnome | Hacker News Reader

Comment by misnome | Hacker News Reader

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> ISPC: https://ispc.github.io/ an open source compiler for a SPMD language which compiles it to efficient SIMD code

I've been learning ispc lately and it does seem like a wonderful solution, you avoid having to build separate implementations for every instruction set and/or worrying about per-compiler massaging to get it to recognise the vectorisation opportunities. The arguments for having a domain-specific language variant and why it was written (https://pharr.org/matt/blog/2018/04/30/ispc-all.html is a good read) seem like persuasive arguments.

However, outside of the projects in the above list - it doesn't seem to have very wide usage. There are still commits coming in/responding to some issues so it doesn't seem dead, but there are many issues untouched or just untriaged. There isn't much discussion about using it, or people asking for advice. The mailing list has about a message a month.

Is it merely just an extremely highly specialised domain? Is it just that CUDA/OpenCL is a more efficient solution for most cases where one would otherwise consider it? Are there too many ASM/intrinsic experts out there to bother learning?

ISPC is really awesome, but you're right it is much less known than CUDA/OpenCL. Part of that might just be lack of marketing effort and focus (you don't hear much about it compared to e.g. CUDA) and the team working on it is far smaller than that on CUDA. There has been some wider adoption, like Unreal Engine 4 using it now: https://devmesh.intel.com/projects/intel-ispc-in-unreal-engi... which is super cool, so hopefully we'll see more of that.

As far as support from other languages I did write this wrapper for using ISPC from Rust https://github.com/Twinklebear/ispc-rs (but that's just me again), and there has been work on a WebASM+SIMD backend which is really exciting. Intel does also have an ISPC based texture compressor (https://github.com/GameTechDev/ISPCTextureCompressor) which I think does have some popularity.

However, the domain is pretty specialized, and I think the fraction of people who really care about CPU performance and are willing to port or write part of their code in another language is smaller still. It's also possible that a lot of those who would do so have their own hand written intrinsics wrappers already. Migrating to ISPC would reduce a lot of maintenance effort on such projects, but when they already have momentum in the other direction it can be harder to switch. I think that on the CPU ISPC is easier and better than OpenCL for performance and tight integration with the "host" language, since you can directly share pointers and even call back and forth between the "host" and "kernel".

ISPC’s creator Matt Pharr works at NVIDIA, they have a series of blog post explaining the history of ISPC.

Link: https://pharr.org/matt/blog/2018/04/18/ispc-origins.html

At work, I had a project involving a DSL for Monte Carlo simulations. The DSL was an internal DSL in Scala, our interpreter was in Scala, and we transpiled to ISPC (for servers/VMs that didn't expose a GPU) and OpenCL.

I generally liked ISPC, but I really didn't like that it tried to look as close as possible to C but departed from C in unnecessary ways. With Monte Carlo simulations, we deal with a lot of probabilities represented as doubles in the range [0.0, 1.0]. The biggest pain is that operations between a double and any integral type cast the double to the integral type, whereas in C, the integral type gets implicitly cast to a double. I understand the implicit casting rules were changed to give the fastest speed rather than minimize worst-case rounding error. I could understand getting rid of implicit casts, or maybe I could understand changing rules to improve accuracy and know that the user could easily use a profiler to discover any performance problems this caused. However, in our case, uint32_t * double = (uint32_t) 0, which then would get implicitly cast back to a double if being assigned to a variable. My interne was beating his head against the wall for the better part of an afternoon before I gave him a bit of debugging help. All of his probabilities were coming out 0% and 100% for his component.

I actually emailed the authors with a bug report when I found the implicit casting rules differed so radically from C and were in the direction away from accuracy. (Note there's no rounding error when converting uint32_t to a 64-bit IEEE-754 double.) They were very nice, and pointed us to where this behavior was documented.

If you're going out of your way to make your language look like C and interoperate seamlessly with C, you should have really strong justifications for the places where you radically depart from C's semantics.

> However, outside of the projects in the above list - it doesn't seem to have very wide usage.

ISPC is pretty popular in the HPC world.

Is it? I haven't heard about it actually being popular anywhere. It definitely works well, but I haven't seen it talked about much except in the case of embree, Intel's ray tracing library. It doesn't seem like there is any funding for it, though it actually works so well already it doesn't seem to need big leaps in progress to be valuable.

> Is it? I haven't heard about it actually being popular anywhere.

I know 3 simulators running on supercomputers in the neurosciences domains that use it + some graph processing over supercomputers tools.

It is true that is is not extremely well known, but it is used.

That's great, but you said it was popular in the HPC world. I would love that to be true, but I don't know of a way to see the big picture.

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