Support for WebAssembly SIMD

[omnisip]

Hi there,

Are you intending on adding support for WebAssembly SIMD?

[gquintin]

Hi @omnisip,

We do not master web technologies very well. I have searched a bit the internet and found "emscripten". As I understand it C/C++ code can be compiled into LLVM IR and then emscripten takes the latter to produce WebAssembly. So NSIMD should work out of the box to produce WebAssembly.

But I feel I am missing something. Can you elaborate?

Thanks.

[gquintin]

Discussing it with a colleague, I (think I) found and understood that you want in fact a new SIMD architecture for NSIMD namely WebAssembly which wraps WebAssembly intrinsics. Is it correct?

[omnisip]

Yes basically

…

On Fri, Dec 18, 2020, 08:15 gquintin ***@***.***> wrote: Discussing it with a colleague, I (think I) found and understood that you want in fact a new SIMD architecture for NSIMD namely WebAssembly which wraps WebAssembly intrinsics <https://github.com/llvm/llvm-project/blob/master/clang/lib/Headers/wasm_simd128.h>. Is it correct? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#81 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACKQJWLDJ5XXYVW7ZAJPFDSVNW23ANCNFSM4VAKDTJA> .

[gquintin]

@omnisip, is there a "simple" way to test an implementation? Is it necessary to have a browser to execute the resulting code? On my testing servers I do not have any GUI installed and would like to continue this way. Can you give us any insight on this?

[omnisip]

Sure. There are at least a few ways to get going. First, if any of the functions you use were part of resort standardization efforts (bulk majority presumably) -- it's been backported to nodejs 14 by adding the flags --experimental-wasm-simd. See here: https://emscripten.org/docs/porting/simd.html

If you need the newer stuff you can compile v8 from source and run d8 which is like a nodejs light.

For your purposes, in case you weren't planning on using it already, I'd use emscripten.

I'm CC'ing @tlively here who works on that as part of standardization efforts.

[gquintin]

@omnisip, thank you for your answer, also I assume you plan to support AVX and so on in the future, so I should take that into account in NSIMD and prepare for 256-bits WebAssembly and so on. Is it correct?

[omnisip]

The answer is that we expect to add 256 bit, but probably not until after the current standard is finalized.

I would first start with 128 before messing with 256.

[gquintin]

Hi @omnisip, I have successfully built and install emscripten, binaryen and d8 (from v8) and tried a C++ Hello World:

$ cat test.cpp
#include <iostream>

int main() {
  std::cout << "Hello World!\n";
  return 0;
}
$ em++ test.cpp
$ d8 a.out.js
Hello World!

I will try some SIMD now.

[gquintin]

@omnisip, I use tools/dev/gm.py to compile d8 but I did not find any installation prefix parameter. Do you know how to do this?

[tlively]

You might find it easier to install nightly V8 from jsvu than to build it yourself.

[gquintin]

@tlively thanks for the tip. SIMD seems ok.

#include <iostream>
#include <wasm_simd128.h>

int main() {
  float a[128], b[128], c[128];
  for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++) {
    a[i] = (float)i;
    b[i] = (float)(sizeof(a) / sizeof(a[0]) - i);
    c[i] = 0.0;
  }

  for (int i = 0; i < sizeof(a) / sizeof(a[0]); i += 4) {
    v128_t va = wasm_v128_load((void *)(a + i));
    v128_t vb = wasm_v128_load((void *)(b + i));
    v128_t vc = wasm_f32x4_add(va, vb);
    wasm_v128_store((void *)(c + i), vc);
  }

  for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++) {
    std::cout << a[i] << " + " << b[i] << " = " << c[i] << "\n";
  }
  return 0;
}

It compiles fine and executes fine:

$ em++ -msimd128 test.cpp
$ d8 --experimental-wasm-simd a.out.js
0 + 128 = 128
1 + 127 = 128
2 + 126 = 128
[...]
126 + 2 = 128
127 + 1 = 128

Wrapping the intrinsics into NSIMD should not take long but I have to find a contiguous pack of 3/4 hours.

[tlively]

Note that the intrinsics are not quite stable yet because the proposal is still being finalized. I don't anticipate many changes to existing intrinsics, but there will be new ones added. Hopefully they will be stabilized in the next few weeks.