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TVM WebAssembly and Javascript Backend

This folder contains TVM WebAssembly and Javascript backend through Emscripten.

Installation

While the LLVM main branch support webassembly as a target. We still need a good runtime with libc and other system library support. Emscripten toolchain offers that nicely. The general idea is to build TVM against the fastcomp LLVM backend in the Emscripten project and allow us to generate asmjs-unknown-emscripten as a backend target.

Setup Emscripten

Checkout Emscripten Portable SDK Downloads to download emsdk-portable and unzip it on a local folder. Follow the installation guide from emscripten document.

./emsdk update
./emsdk install latest
./emsdk activate latest

Because we need to compile against the LLVM backend of emscripten, we will need the source and llvm library. Which can be installed via following command.

./emsdk install clang-incoming-64bit
./emsdk activate clang-incoming-64bit

Setup Environment Variable

In normal setting, we can setup the necessary environment variable with the following command.

source /path-to-emsdk-portable/emsdk_env.sh

However, this will put emscripten's clang and llvm path ahead of the current system path. What you can do is to set the path manually, by putting emscripten's path after the PATH like the following ones. You can get the detailed path by type ./emsdk activate

export PATH=${PATH}:/emsdk-related-path-here

Build TVM with Fastcomp LLVM

To build TVM with Emscripten's Fastcomp LLVM, we can modify the LLVM_CONFIG in config.mk to point to fastcomp's llvm-config and build TVM normally.

LLVM_CONFIG = /path/to/emsdk-portable/clang/fastcomp/build_incoming_64/bin/llvm-config

Build TVM Web Runtime

The above command gives us the TVM compiling environment. Now we need to build runtime, to do so, make sure we set the environment correctly as in previous section and type

make web

This will create lib/libtvm_web_runtime.bc and lib/libtvm_web_runtime.js.

Use TVM to Generate Javascript Library

The general idea is to use TVM as normally and set target to be llvm -target=asmjs-unknown-emscripten -system-lib.

The following code snippet from tests/web/prepare_test_libs.py demonstrate the compilation process.

import tvm
from tvm.contrib import emscripten
import os
def prepare_test_libs(base_path):
    target = "llvm -target=asmjs-unknown-emscripten -system-lib"
    if not tvm.module.enabled(target):
        raise RuntimeError("Target %s is not enbaled" % target)
    n = tvm.var("n")
    A = tvm.placeholder((n,), name='A')
    B = tvm.compute(A.shape, lambda *i: A(*i) + 1.0, name='B')
    s = tvm.create_schedule(B.op)
    fadd1 = tvm.build(s, [A, B], target, name="add_one")
    obj_path = os.path.join(base_path, "test_add_one.bc")
    fadd1.save(obj_path)
    emscripten.create_js(os.path.join(base_path, "test_module.js"), obj_path)

if __name__ == "__main__":
    curr_path = os.path.dirname(os.path.abspath(os.path.expanduser(__file__)))
    prepare_test_libs(os.path.join(curr_path, "../../lib"))

In this workflow, we use TVM to generate a .bc file and statically link that with the lib/libtvm_web_runtime.bc(emscripten.create_js will help you do that). The result js library is a library that contains both TVM runtime and the compiled function.

Run the Generated Library

The following code snippet from tests/web/test_module_load.js demonstrate how to run the compiled library.

// Load Emscripten Module, need to change path to root/lib
const path = require("path");
process.chdir(path.join(__dirname, "../../lib"));
var Module = require("../../lib/test_module.js");
// Bootstrap TVMruntime with emscripten module.
const tvm_runtime = require("../../web/tvm_runtime.js");
const tvm = tvm_runtime.create(Module);

// Load system library, the compiled functions is registered in sysLib.
var sysLib = tvm.systemLib();

function randomArray(length, max) {
  return Array.apply(null, Array(length)).map(function() {
    return Math.random() * max;
  });
}

function testAddOne() {
  // grab pre-loaded function
  var faddOne = sysLib.getFunction("add_one");
  tvm.assert(tvm.isPackedFunc(faddOne));
  var n = 124;
  var A = tvm.empty(n).copyFrom(randomArray(n, 1));
  var B = tvm.empty(n);
  // call the function.
  faddOne(A, B);
  // verify
  for (var i = 0; i < B.length; ++i) {
    tvm.assert(B[i] == A[i] + 1);
  }
  faddOne.release();
}

testAddOne();
sysLib.release();

Current example supports static linking, which is the preferred way to get more efficiency in javascript backend.

Proxy based RPC

We can now use javascript end to start an RPC server and connect to it from python side, making the testing flow easier.

The following is an example to reproduce this. This requires everything to be in the git source and setup PYTHONPATH(instead of use setup.py install)

  • run "python -m tvm.exec.rpc_proxy --example-rpc=1" to start proxy.
  • Open broswer, goto the server webpage click Connect to proxy.
    • Alternatively run "node web/example_rpc_node.js"
  • run "python tests/web/websock_rpc_test.py" to run the rpc client.

The general idea is to use Emscripten's dynamic linking to dynamically load modules.