Halide supports WebAssembly (Wasm) code generation from Halide using the LLVM backend.

As WebAssembly itself is still under active development, Halide's support has some limitations. Some of the most important:

Sign-extension operations are enabled by default (but can be avoided via Target::WasmMvpOnly).
Non-trapping float-to-int conversions are enabled by default (but can be avoided via Target::WasmMvpOnly).
Fixed-width SIMD (128 bit) can be enabled via Target::WasmSimd128.
Threads have very limited support via Target::WasmThreads; see below for more details.
Halide's JIT for Wasm is extremely limited and really useful only for internal testing purposes.

Additional Tooling Requirements:

In additional to the usual install of LLVM and clang, you'll need lld.
Locally-installed version of Emscripten, 1.39.19+

Note that for all of the above, earlier versions might work, but have not been tested.

AOT Limitations

Halide outputs a Wasm object (.o) or static library (.a) file, much like any other architecture; to use it, of course, you must link it to suitable calling code. Additionally, you must link to something that provides an implementation of libc; as a practical matter, this means using the Emscripten tool to do your linking, as it provides the most complete such implementation we're aware of at this time.

Halide ahead-of-time tests assume/require that you have Emscripten installed and available on your system, with the EMSDK environment variable set properly.

JIT Limitations

It's important to reiterate that the WebAssembly JIT mode is not (and will never be) appropriate for anything other than limited self tests, for a number of reasons:

It actually uses an interpreter (from the WABT toolkit [https://github.com/WebAssembly/wabt]) to execute wasm bytecode; not surprisingly, this can be very slow.
Wasm effectively runs in a private, 32-bit memory address space; while the host has access to that entire space, the reverse is not true, and thus any define_extern calls require copying all halide_buffer_t data across the Wasm<->host boundary in both directions. This has severe implications for existing benchmarks, which don't currently attempt to account for this extra overhead. (This could possibly be improved by modeling the Wasm JIT's buffer support as a device model that would allow lazy copy-on-demand.)
Host functions used via define_extern or HalideExtern cannot accept or return values that are pointer types or 64-bit integer types; this includes things like const char * and user_context. Fixing this is tractable, but is currently omitted as the fix is nontrivial and the tests that are affected are mostly non-critical. (Note that halide_buffer_t* is explicitly supported as a special case, however.)
Threading isn't supported at all (yet); all parallel() schedules will be run serially.
The .async() directive isn't supported at all, not even in serial-emulation mode.
You can't use Param<void *> (or any other arbitrary pointer type) with the Wasm jit.
You can't use Func.debug_to_file(), Func.set_custom_do_par_for(), Func.set_custom_do_task(), or Func.set_custom_allocator().
The implementation of malloc() used by the JIT is incredibly simpleminded and unsuitable for anything other than the most basic of tests.
GPU usage (or any buffer usage that isn't 100% host-memory) isn't supported at all yet. (This should be doable, just omitted for now.)

Note that while some of these limitations may be improved in the future, some are effectively intrinsic to the nature of this problem. Realistically, this JIT implementation is intended solely for running Halide self-tests (and even then, a number of them are fundamentally impractical to support in a hosted-Wasm environment and are disabled).

In sum: don't plan on using Halide JIT mode with Wasm unless you are working on the Halide library itself.

Using V8 as the interpreter

There is experimental support for using V8 as the interpreter in JIT mode, rather than WABT. This is enabled by the CMake command line options -DWITH_V8=ON -DWITH_WABT=OFF (only one of them can be used at a time). You must build V8 locally V8, then specify the path to the library and headers as CMake options. This is currently only tested on x86-64-Linux and requires v8 version 9.8.177 as a minimum.

The canonical instructions to build V8 are at v8.dev, and there are examples for embedding v8. The process for Halide is summarized below.

Install depot_tools
Fetch v8 source code (and install required dependencies):
$ gclient

$ mkdir ~/v8 && cd ~/v8

$ fetch v8

$ cd ~/v8/v8

$ git checkout origin/9.8.177
Create a build configuration: tools/dev/v8gen.py x64.release.sample
Turn off pointer compression: ‘echo 'v8_enable_pointer_compression = false’ >> out.gn/x64.release.sample/args.gn
Disable the GDB-JIT interface (conflicts with LLVM):echo 'v8_enable_gdbjit = false' >> out.gn/x64.release.sample/args.gn
Build the static library:autoninja -C out.gn/x64.release.sample v8_monolith`

With V8 built, we can pass the CMake options:

V8_INCLUDE_DIR, path to V8 includes, e.g. $HOME/v8/v8/include
V8_LIBRARY, path to V8 static library, e.g. $HOME/v8/v8/out.gn/x64.release.sample/obj/libv8_monolith.a

An example to configure Halide with V8 support, build and run an example test:

$ cd /path/to/halide
$ export HL_TARGET=wasm-32-wasmrt-wasm_simd128
$ export HL_JIT_TARGET=${HL_TARGET}
$ cmake -G Ninja \
      -DWITH_WABT=OFF \
      -DWITH_V8=ON \
      -DV8_INCLUDE_DIR=$HOME/v8/v8/include \
      -DV8_LIBRARY=$HOME/v8/v8/out.gn/x64.release.sample/obj/libv8_monolith.a \
      -DHalide_TARGET=${HL_TARGET} \
      /* other cmake settings here as appropriate */
 
$ cmake --build .
$ ctest -L "correctness|generator" -j

To Use Halide For WebAssembly:

Ensure WebAssembly is in LLVM_TARGETS_TO_BUILD; if you use the default ("all") then it's already present, but otherwise, add it explicitly:

-DLLVM_TARGETS_TO_BUILD="X86;ARM;NVPTX;AArch64;PowerPC;Hexagon;WebAssembly

Enabling wasm JIT

If you want to run test_correctness and other interesting parts of the Halide test suite (and you almost certainly will), you'll need to ensure that LLVM is built with wasm-ld:

Ensure that you have lld in LVM_ENABLE_PROJECTS:

cmake -DLLVM_ENABLE_PROJECTS="clang;lld" ...

To run the JIT tests, set HL_JIT_TARGET=wasm-32-wasmrt (possibly adding wasm_simd128) and run CMake/CTest normally. Note that wasm testing is only supported under CMake (not via Make).

Enabling wasm AOT

If you want to test ahead-of-time code generation (and you almost certainly will), you need to install Emscripten locally.

The simplest way to install is probably via the Emscripten emsdk (https://emscripten.org/docs/getting_started/downloads.html).
To run the AOT tests, set HL_TARGET=wasm-32-wasmrt (possibly adding wasm_simd128) and run CMake/CTest normally. Note that wasm testing is only supported under CMake (not via Make).

Running benchmarks

The test_performance benchmarks are misleading (and thus useless) for Wasm, as they include JIT overhead as described elsewhere. Suitable benchmarks for Wasm will be provided at a later date. (See https://github.com/halide/Halide/issues/5119 and https://github.com/halide/Halide/issues/5047 to track progress.)

Using Threads

You can use the wasm_threads feature to enable use of a normal pthread-based thread pool in Halide code, but with some careful caveats:

This requires that you use a wasm runtime environment that provides pthread-compatible wrappers. At this time of this writing, the only environment known to support this well is Emscripten (when using the -pthread flag, and compiling for a Web environment). In this configuration, Emscripten goes to great lengths to make WebWorkers available via the pthreads API. (You can see an example of this usage in apps/HelloWasm.) Note that not all wasm runtimes support WebWorkers; generally, you need a full browser environment to make this work (though some versions of some shell tools may also support this, e.g. nodejs).
There is currently no support for using threads in a WASI environment, due to current limitations in the WASI specification. (We hope that this will improve in the future.)
There is no support for using threads in the Halide JIT environment, and no plans to add them anytime in the near-term future.

Known Limitations And Caveats

Current trunk LLVM (as of July 2020) doesn't reliably generate all of the Wasm SIMD ops that are available; see https://github.com/halide/Halide/issues/5130 for tracking information as these are fixed.
Using the JIT requires that we link the wasm-ld tool into libHalide; with some work this need could possibly be eliminated.
OSX and Linux-x64 have been tested. Windows hasn't; it should be supportable with some work. (Patches welcome.)
None of the apps/ folder has been investigated yet. Many of them should be supportable with some work. (Patches welcome.)
We currently use v8/d8 as a test environment for AOT code; we may want to consider using Node or (better yet) headless Chrome instead (which is probably required to allow for using threads in AOT code).

Known TODO:

There's some invasive hackiness in Codgen_LLVM to support the JIT trampolines; this really should be refactored to be less hacky.
Can we rework JIT to avoid the need to link in wasm-ld? This might be doable, as the wasm object files produced by the LLVM backend are close enough to an executable form that we could likely make it work with some massaging on our side, but it's not clear whether this would be a bad idea or not (i.e., would it be unreasonably fragile).
Buffer-copying overhead in the JIT could possibly be dramatically improved by modeling the copy as a "device" (i.e. copy_to_device() would copy from host -> wasm); this would make the performance benchmarks much more useful.
Can we support threads in the JIT without an unreasonable amount of work? Unknown at this point.