Cleanup comment

[LibreOffice.git] / static / README.wasm.md

# Support for Emscripten Cross Build

This module provides support for emscripten cross build

## Status

The build generates a Writer-only LO build. You should be able to run either

    $ emrun --serve_after_close instdir/program/qt_soffice.html
    $ emrun --serve_after_close workdir/LinkTarget/Executable/qt_vcldemo.html
    $ emrun --serve_after_close workdir/LinkTarget/Executable/qt_wasm-qt5-mandelbrot.html

REMINDER: Always start new tabs in the browser, reload might fail / cache!
INFO: latest browser won't work anymore with 0.0.0.0 and need 127.0.0.1.

## Setup for the LO WASM build (with Qt)

We're using Qt 5.15.2 with Emscripten 2.0.31. There are a bunch of Qt patches
to fix the most grave bugs. Also newer Emscripten versions have various bugs
with the FS image support.

- See below under Docker build for another build option

### Setup emscripten

<https://emscripten.org/docs/getting_started/index.html>

    git clone https://github.com/emscripten-core/emsdk.git
    ./emsdk install 2.0.31
    ./emsdk activate --embedded 2.0.31

Example `bashrc` scriptlet:

    EMSDK_ENV=$HOME/Development/libreoffice/git_emsdk/emsdk_env.sh
    [ -f "$EMSDK_ENV" ] && \. "$EMSDK_ENV" 1>/dev/null 2>&1

### Setup Qt

<https://doc.qt.io/qt-5/wasm.html>

Most of the information from <https://doc.qt.io/qt-6/wasm.html> is still valid for Qt5;
generally the Qt6 WASM documentation is much better, because it incorporated many
information from the Qt Wiki.

FWIW: Qt 5.15 LTS is not maintained publicly and Qt WASM has quite a few bugs. Most
WASM fixes from Qt 6 are needed for Qt 5.15 too. They can mainly be cherry-picked from:
- git log origin/dev src/plugins/platforms/wasm/
- git log --grep wasm origin/dev

We will probably offer our own Qt repository clone at some point.

But even the public Qt 5.15 branch is still broken, so better start with the v5.15.2 tag.

    git clone https://github.com/qt/qt5.git
    cd qt5
    git checkout v5.15.2
    ./init-repository --module-subset=qtbase
    ./configure -xplatform wasm-emscripten -feature-thread -prefix $PWD/install-5.15.2
    make -j<CORES> module-qtbase

Optionally you can add the configure flag "-compile-examples". But then you also have to
patch at least mkspecs/wasm-emscripten/qmake.conf with EXIT_RUNTIME=0, otherwise they will
fail to run. In addition, building with examples will break with some of them, but at that
point Qt already works and also most examples.
Building with examples will break with some of them, but at that point Qt already works.
Or just skip them. Other interesting flags might be "-nomake tests -no-pch -ccache".

Linking takes quite a long time, because emscripten-finalize rewrites the whole WASM files
with some options. This way the LO WASM needs at least 64GB RAM. For faster link times add
"-s WASM_BIGINT=1", change to ASSERTIONS=1 nd use -g3 to prevent rewriting the WASM file
and generating source maps (see emscripten.py, finalize_wasm, and avoid modify_wasm = True).
This is just needed for Qt examples, as LO already uses the correct flags!

The install is not really needed, as LO currently just uses qtbase on its own. You can do

    make -j<CORES> install
or
    make -j8 -C qtbase/src install_subtargets

Current Qt fails to start the demo webserver: <https://bugreports.qt.io/browse/QTCREATORBUG-24072>

Use `emrun --serve_after_close` to run Qt WASM demos.

### Setup LO

`autogen.sh` is patched to use emconfigure. That basically sets various
environment vars, especially `EMMAKEN_JUST_CONFIGURE`, which will create the
correct output file names, checked by `configure` (`a.out`).

There's a distro config for WASM, but it just provides --host=wasm32-local-emscripten, which
should be enough setup. The build itself is a cross build and the cross-toolset just depends
on a minimal toolset (gcc, libc-dev, flex, bison); all else is build from source, because the
final result is not depending on the build system at all.

Recommended configure setup is thusly:

* grab defaults
    `--with-distro=LibreOfficeWASM32`

* local config
    `QT5DIR=/dir/of/git_qt5/qtbase`

* if you want to use ccache on both sides of the build
    `--with-build-platform-configure-options=--enable-ccache`
    `--enable-ccache`

FWIW: it's also possible to build an almost static Linux LibreOffice by just using
--disable-dynloading --enable-customtarget-components. System externals are still
linked dynamically, but everything else is static.

#### Experimental (AKA currently broken) WASM exception + SjLj build

You can build LO with WASM exceptions, which should be "much" faster then the JS
based Emscripten EH handling. For setjmp / longjmp (SjLj) used by the PNG and JPEG
libraries error handling, this needs Emscripten 3.1.3+. That builds, but execution
still fails early with a signature mismatch call to Task::UpdateMinPeriod in LO's
job scheduler code. Unfortunately the build also needs a Qt build with
"-s SUPPORT_LONGJMP=wasm", which is incompatible with the JS EH + SjLj.

The LO configure flag is simply an additional --enable-wasm-exceptions. Qt5 can
be patched in qtbase/mkspecs/wasm-emscripten/qmake.conf with the addition of

    QMAKE_CFLAGS += -s SUPPORT_LONGJMP=wasm
    QMAKE_CXXFLAGS += -s SUPPORT_LONGJMP=wasm

### "Deploying" soffice.wasm

    tar -chf wasm.tar --xform 's/.*program/lo-wasm/' instdir/program/soffice.* \
        instdir/program/qt*

Your HTTP server needs to provide additional headers:
* add_header Cross-Origin-Opener-Policy same-origin
* add_header Cross-Origin-Embedder-Policy require-corp

The default html to use should be qt_soffice.html

### Debugging setup

Since a few months you can use DWARF information embedded by LLVM into the WASM
to debug WASM in Chrome. You need to enable an experimental feature and install
an additional extension. The whole setup is described in:

https://developer.chrome.com/blog/wasm-debugging-2020/

This way you don't need source maps (much faster linking!) and can resolve local
WASM variables to C++ names!

Per default, the WASM debug build splits the DWARF information into an additional
WASM file, postfixed '.debug.wasm'.

### Using Docker to cross-build with emscripten

If you prefer a controlled environment (sadly emsdk install/activate
is _not_ stable over time, as e.g. nodejs versions evolve), that is
easy to replicate across different machines - consider the docker
images we're providing.

Config/setup file see
<https://git.libreoffice.org/lode/+/ccb36979563635b51215477455953252c99ec013>

Run

    docker-compose build

in the lode/docker dir to get the container prepared. Run

    PARALLELISM=4 BUILD_OPTIONS= BUILD_TARGET=build docker-compose run --rm \
        -e PARALLELISM -e BUILD_TARGET -e BUILD_OPTIONS builder

to perform an actual `srcdir != builddir` build; the container mounts
checked-out git repo and output dir via `docker-compose.yml` (so make
sure the path names there match your setup):

The lode setup expects, inside the lode/docker subdir, the following directories:

- core (`git checkout`)
- workdir (the output dir - gets written into)
- cache (`ccache tree`)
- tarballs (external project tarballs gets written and cached there)


## Ideas for an UNO bridge implementation

My post to Discord #emscripten:

"I'm looking for a way to do an abstract call
from one WASM C++ object to another WASM C++ object, so like FFI / WebIDL,
just within WASM. All my code is C++ and normally I have bridge code, with
assembler to implement the function call /RTTI and exception semantics of the
specified platform. Code is at
<https://cgit.freedesktop.org/libreoffice/core/tree/bridges/source/cpp_uno>.
I've read a bit about `call_indirect` and stuff, but I don't have yet a good
idea, how I could implement this (and  there is an initial feature/wasm branch
for the interested). I probably need some fixed lookup table, like on iOS,
because AFAIK you can't dynamically generate code in WASM. So any pointers or
ideas for an implementation? I can disassemble some minimalistic WASM example
and read clang code for `WASM_EmscriptenInvoke`, but if there were some
standalone code or documentation I'm missing, that would be nice to know."

We basically would go the same way then the other backends. Write the bridge in
C++, which is probably largely boilerplate code, but the function call in WAT
(<https://github.com/WebAssembly/wabt>) based on the LLVM WASM calling
conventions in `WASM_EmscriptenInvoke`. I didn't get a reply to that question for
hours. Maybe I'll open an Emscripten issue, if we really have to implement
this.

WASM dynamic dispatch:

- <https://fitzgeraldnick.com/2018/04/26/how-does-dynamic-dispatch-work-in-wasm.html>

## Tools for problem diagnosis

* `nm -s` should list the symbols in the archive, based on the index generated by ranlib.
  If you get linking errors that archive has no index.


## Emscripten filesystem access with threads

This is closed, but not really fixed IMHO:

- <https://github.com/emscripten-core/emscripten/issues/3922>

## Dynamic libraries `/` modules in emscripten

There is a good summary in:

- <https://bugreports.qt.io/browse/QTBUG-63925>

Summary: you can't use modules and threads.

This is mentioned at the end of:

- <https://github.com/emscripten-core/emscripten/wiki/Linking>

The usage of `MAIN_MODULE` and `SIDE_MODULE` has other problems, a major one IMHO is symbol resolution at runtime only.
So this works really more like plugins in the sense of symbol resolution without dependencies `/` rpath.

There is some clang-level dynamic-linking in progress (WASM dlload). The following link is already a bit old,
but I found it a god summary of problems to expect:

- <https://iandouglasscott.com/2019/07/18/experimenting-with-webassembly-dynamic-linking-with-clang/>


## Mixed information, links, problems, TODO

More info on Qt WASM emscripten pthreads:

- <https://wiki.qt.io/Qt_for_WebAssembly#Multithreading_Support>

WASM needs `-pthread` at compile, not just link time for atomics support. Alternatively you can provide
`-s USE_PTHREADS=1`, but both don't seem to work reliable, so best provide both.
<https://github.com/emscripten-core/emscripten/issues/10370>

The output file must have the prefix .o, otherwise the WASM files will get a
`node.js` shebang (!) and ranlib won't be able to index the library (link errors).

Qt with threads has further memory limit. From Qt configure:
````
Project MESSAGE: Setting PTHREAD_POOL_SIZE to 4
Project MESSAGE: Setting TOTAL_MEMORY to 1GB
````

You can actually allocate 4GB:

- <https://bugzilla.mozilla.org/show_bug.cgi?id=1392234>

LO uses a nested event loop to run dialogs in general, but that won't work, because you can't drive
the browser event loop. like VCL does with the system event loop in the various VCL backends.
Changing this will need some major work (basically dropping Application::Execute).

But with the know problems with exceptions and threads, this might change:

- <https://github.com/emscripten-core/emscripten/pull/11518>
- <https://github.com/emscripten-core/emscripten/issues/11503>
- <https://github.com/emscripten-core/emscripten/issues/11233>
- <https://github.com/emscripten-core/emscripten/issues/12035>

We're also using emconfigure at the moment. Originally I patched emscripten, because it
wouldn't create the correct a.out file for C++ configure tests. Later I found that
the `emconfigure` sets `EMMAKEN_JUST_CONFIGURE` to work around the problem.

ICU bug:

- <https://github.com/emscripten-core/emscripten/issues/10129>

Alternative, probably:

- <https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Intl>

There is a wasm64, but that still uses 32bit pointers!

Old outdated docs:

- <https://wiki.documentfoundation.org/Development/Emscripten>

Reverted patch:

- <https://cgit.freedesktop.org/libreoffice/core/commit/?id=0e21f6619c72f1e17a7b0a52b6317810973d8a3e>

Generally <https://emscripten.org/docs/porting>:

- <https://emscripten.org/docs/porting/guidelines/api_limitations.html#api-limitations>
- <https://emscripten.org/docs/porting/files/file_systems_overview.html#file-system-overview>
- <https://emscripten.org/docs/porting/pthreads.html>
- <https://emscripten.org/docs/porting/emscripten-runtime-environment.html>

This will be interesting:

- <https://emscripten.org/docs/getting_started/FAQ.html#how-do-i-run-an-event-loop>

This didn't help much yet:

- <https://github.com/emscripten-ports>

Emscripten supports standalone WASI binaries:

- <https://github.com/emscripten-core/emscripten/wiki/WebAssembly-Standalone>
- <https://www.qt.io/qt-examples-for-webassembly>
- <http://qtandeverything.blogspot.com/2017/06/qt-for-web-assembly.html>
- <http://qtandeverything.blogspot.com/2020/>
- <https://emscripten.org/docs/api_reference/Filesystem-API.html>
- <https://discuss.python.org/t/add-a-webassembly-wasm-runtime/3957/12>
- <http://git.savannah.gnu.org/cgit/config.git>
- <https://webassembly.org/specs/>
- <https://developer.chrome.com/docs/native-client/>
- <https://emscripten.org/docs/getting_started/downloads.html>
- <https://github.com/openpgpjs/openpgpjs/blob/master/README.md#getting-started>
- <https://developer.mozilla.org/en-US/docs/WebAssembly/Using_the_JavaScript_API>
- <https://github.com/bytecodealliance/wasmtime/blob/main/docs/WASI-intro.md>
- <https://www.ip6.li/de/security/x.509_kochbuch/openssl-fuer-webassembly-compilieren>
- <https://emscripten.org/docs/introducing_emscripten/about_emscripten.html#about-emscripten-porting-code>
- <https://emscripten.org/docs/compiling/Building-Projects.html>