Move README.wasm -> wasm-qt/README.md

Most of the folders now have README.md in Markdown format and it is
possible to create html docs out of them which is available at:

https://docs.libreoffice.org/

The content of README file was previously converted to Markdown
format in commit d19299f579e820c34b38ee56b26125dc30f96ada, so only
the rename of the README file was needed.

Change-Id: Ib030676a70194fdb24aea066ec985c2f95ee4b3e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/123449
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Tested-by: Jenkins

2 files changed, 302 insertions, 1 deletions

diff --git a/wasm-qt/README b/wasm-qt/README
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-See /README.wasm
diff --git a/wasm-qt/README.md b/wasm-qt/README.md
new file mode 100644
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+# Support for Emscripten Cross Build
+
+This module provides support for emscripten cross build
+
+## Status
+
+    $ make
+    $ emrun --serve_after_close instdir/program/ui-previewer.html
+
+The `ui-previewer` "binary" will "crash" with memory alignment problems.
+
+You can run the WASM mandelbrot Qt example, if you copy its HTML
+and the qtloader.js from the Qt's example folder after build with:
+
+    $ emrun --serve_after_close workdir/LinkTarget/Executable/mandelbrot.html
+
+REMINDER: Always start new tabs in the browser, reload might fail / cache!
+
+
+## Setup for the LO WASM build (with Qt)
+
+We're using Qt 5.15 with the officially supported emscripten v1.39.8.
+But there are several potential problems with threads and exceptions, so this will likely
+change later to a newer emscripten.
+
+Qt WASM is not yet used with LO, just if you're wondering!
+
+- 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 1.39.8
+    ./emsdk activate --embedded 1.39.8
+
+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>
+
+I originally build the Qt 5.15 branch, but probably better to build a tag like v5.15.2.
+
+So:
+
+    git clone https://github.com/qt/qt5.git
+    cd qt5
+    git checkout v5.15.2
+    ./init-repository
+    ./configure -xplatform wasm-emscripten -feature-thread -compile-examples -prefix $PWD/qtbase
+    make -j<CORES> module-qtbase module-qtdeclarative
+
+Building with examples will break with some of them, but at that point Qt already works.
+
+At some point Qt configure failed for me with:
+
+"Checking for target architecture... Project ERROR: target architecture detection binary not found."
+
+What seems to have fixed this was to run "emsdk activate 1.39.8" again.
+
+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
+
+Enabling multi-thread support in Firefox is a bit of work with older versions:
+
+- <https://bugzilla.mozilla.org/show_bug.cgi?id=1477743#c7>
+- <https://wiki.qt.io/Qt_for_WebAssembly#Multithreading_Support>
+- <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer>
+
+
+### 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 (work in progress), that gets your
+defaults right (and currently disables a ton of 3rd party stuff which
+is not essential).
+
+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`
+
+### 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>
+
+
+## Workaround for eventual clang WASM compiler bug
+
+````
+sc/source/core/data/attarray.cxx:378:44: error: call to member function 'erase' is ambiguous
+                        aNewCondFormatData.erase(nIndex);
+                        ~~~~~~~~~~~~~~~~~~~^~~~~
+include/o3tl/sorted_vector.hxx:86:15: note: candidate function
+    size_type erase( const Value& x )
+              ^
+include/o3tl/sorted_vector.hxx:97:10: note: candidate function
+    void erase( size_t index )
+````
+
+This is currently patched by using `x.erase` (`x.begin() + nIndex`).
+
+There shouldn't be an ambiguity, because of "[WebAssembly] Change size_t to `unsigned long`."
+<https://reviews.llvm.org/rGdf07a35912d78781ed6a62a7c032bfef5085a4f5#change-IrS9f6jH6PFq>,
+from "Jul 23 2018" which pre-dates the emscripten tag 1.39.8 from 02/14/2020 by ~1.5y.
+
+
+## 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>
+