WASM: add readme with build howto

Change-Id: I2ce1787abfaa2cf6990d71e88de2d027b08f1489
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/111129
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>

1 files changed, 282 insertions, 0 deletions

diff --git a/README.wasm b/README.wasm
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+= 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 it's 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).
+
+QT5DIR=/dir/of/git_qt5/qtbase
+
+--host=wasm64-local-emscripten
+--disable-coinmp
+--disable-cups
+--disable-dbus
+--disable-dconf
+--disable-dynamic-loading
+--disable-extension-integration
+--disable-extensions
+--disable-extension-update
+--disable-firebird-sdbc
+--disable-gio
+--disable-gstreamer-1-0
+--disable-ldap
+--disable-lpsolve
+--disable-mariadb-sdbc
+--disable-nss
+--disable-odk
+--disable-online-update
+--disable-opencl
+--disable-pdfimport
+--disable-postgresql-sdbc
+--disable-python
+--disable-randr
+--disable-report-builder
+--disable-scripting
+--disable-sdremote-bluetooth
+--enable-ccache
+--without-helppack-integration
+--without-java
+--without-junit
+--without-system-dicts
+--with-fonts
+--with-theme=no
+
+Many will also be encoded in configure.ac at some point, some already is.
+
+
+== 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 != buildir 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 an other 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 follwing 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. Alternativel< you can provide
+-s USE_PTHREADS=1, but both don't seem to work relyable, 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
+woulden'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
+