webassembly: Support WASM_OBJECT_FILES=1

Why

Enabling WASM_OBJECT_FILES=1 makes the compiler produce object files in the wasm format instead of llvm bitcode. This makes linking significantly faster, especially when not optimizing.

Currently this requires building upstream llvm/clang from source, instead of using the "fastcomp" fork shipped with emsdk. This mode will eventually be the default for emscripten.

Known/encountered issues:

• wasm-opt crash:

   shared:ERROR: '/Users/msorvig/dev/binaryen/bin/wasm-opt ./gui_opengl.wasm.pre -o ./gui_opengl.wasm -O3' failed (-10) 

  (work around by building with -O0)

• Computed goto is not supported: the QML compiler does not build.

  qv4instr_moth.cpp:175:6: error: WebAssembly
        hasn't implemented computed gotos 

• Warnings (mostly harmless?):

  wasm-ld: warning: function signature mismatch: _ZNK20QPlatformIntegration12possibleKeysEPK9QKeyEvent
  >>> defined as () -> void in /Users/msorvig/code/qt-webassembly-stock-llvm/qt-webassembly-build-wasm/qtbase/plugins/platforms/libqwasm.a(moc_qwasmintegration.o)
  >>> defined as (i32, i32, i32) -> void in /Users/msorvig/code/qt-webassembly-stock-llvm/qt-webassembly-build-wasm/qtbase/lib/libQt5Gui.a(qplatformintegration.o)

Getting started:

(Note: this is "how-I-did-it", not necessarily "how-it-should-be-done".)

(Note the 2nd: See also v8.dev/blog for official instructions on how to install using emsdk)

1. Clone the following repositories:

• emscripten : https://github.com/kripken/emscripten.git
• binaryen: https://github.com/WebAssembly/binaryen.git
• llvm, https://github.com/llvm/llvm-project.git

2. Check out the version you want to use

• emscripten and binaryen have matching emsdk version tags, for example "1.38.23".
• llvm has its own version numbers, and must be synced up manually. Currently:
  • emscripten <= 1.38.23 : llvm 8.0 (8.0.0-rc2 branch)
  • emscripten > 1.38.23 : llvm 9.0 (master branch)
    (em++/emcc will complain if you have the incorrect llvm version)

3. Build

• emscripten is all python, no build needed.
• binaryen: "cmake -GNinja && ninja"
• llvm:

  cmake -GNinja
  ../path/to/llvmcheckout
  -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi;lld"
  -DCMAKE_BUILD_TYPE=Release
  -DLLVM_TARGETS_TO_BUILD=WebAssembly
  -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly
  

  (one of the WebAssembly targets may be redundant)

3. Configure Environment

I use the following:

export EMSDK="/Users/msorvig/dev/emsdks/emscripten-1.38.23"
export PATH="$EMSDK:$PATH"
export LLVM="/Users/msorvig/dev/emsdks/llvm-8.0.0-build/bin"
export BINARYEN="/Users/msorvig/dev/emsdks/binaryen-1.38.23"
export EM_CONFIG="/Users/msorvig/dev/emsdks/.emscripten-vanillallvm-1.38.23"
export EM_CACHE="/Users/msorvig/dev/emsdks/.emscripten-vanillallvm-cache-1.38.23"

Where .emscripten-vanillallvm-1.38.23 is a copy of the .emscripten config file that emsdk generates.