Building Emscripten from Source using the SDK¶

What you’ll need¶

First download and install the SDK. The SDK automatically installs and activates most of the Emscripten tools and dependencies.

Then download and install the compiler toolchain for your platform.

Building Emscripten from the main repositories¶

The Emscripten SDK (emsdk) controls which tool and SDK “targets” are installed and activated. To start using the latest upstream git development branch (incoming), run the following commands:

# Clone, pull, and build the latest "sdk" Emscripten environment
./emsdk install sdk-incoming-64bit

# Set the "incoming SDK" as the active version.
./emsdk activate sdk-incoming-64bit

If you want to use the upstream stable branch master, replace -incoming- with -master- in the commands above.

Building Emscripten using your own fork¶

Emscripten Contributors need to be able to build Emscripten from their own fork — in order to use pull requests to merge changes back into Emscripten. This is achieved by first installing source from the main repositories (as described in the previous section), and then replacing the sources with those from the fork.

After installing the main repositories:

1. Get your fork of Fastcomp:

   # From your emsdk "root" (where emsdk is installed). Navigate to the LLVM clone:
   cd clang/fastcomp/src
   
   # Add a git remote link to your LLVM repository fork
   git remote add myremote https://github.com/mygituseraccount/emscripten-fastcomp.git
   
   
   # Fetch the changes in the remote.
   git fetch myremote
   
   # Checkout the code in your LLVM fork to replace the original repository code
   git checkout -b myremote --track myremote/incoming
   

2. Get your fork of Fastcomp-Clang:

   # Navigate to the tools/clang directory ( <emsdk root>/clang/fastcomp/src/tools/clang )
   cd tools/clang
   
   # Add a git remote link to your Clang repository fork
   git remote add myremote https://github.com/mygituseraccount/emscripten-fastcomp-clang.git
   
   
   # Fetch the changes in the remote.
   git fetch myremote
   
   # Checkout the code in your Clang fork to replace the original repository code
   git checkout -b myremote --track myremote/incoming
   

3. Build Fastcomp:

   1. The SDK builds Fastcomp into a branch- and build-specific directory — in this case <emsdk root>/clang/fastcomp/build_incoming_64. We need to build to the same location.

      #Navigate to the /clang/fastcomp/build_incoming_64
      cd ../../../build_incoming_64
      

   2. Configure the build using cmake:

      cmake .. -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86;JSBackend" -DLLVM_INCLUDE_EXAMPLES=OFF -DLLVM_INCLUDE_TESTS=OFF -DCLANG_INCLUDE_EXAMPLES=OFF -DCLANG_INCLUDE_TESTS=OFF
      

      Note

      On Windows you will need Visual Studio 2012 or newer to build.

   3. Determine the number of available cores on your system (Emscripten can run many operations in parallel, so using more cores may have a significant impact on compilation time):

      • On Mac OS X you can get the number of cores using: Apple menu | About this mac | System report. The Hardware overview on the resulting dialog includes a Total number of cores entry.
      • On Linux you can find the number of cores by entering the following command on the terminal: cat /proc/cpuinfo | grep "^cpu cores" | uniq.
      • On Windows the number of cores is listed on the Task Manager | Performance Tab. You can open the Task Manager by entering Ctrl + Shift + Esc from the Desktop.

   4. Call make to build the sources, specifying the number of available cores:

      make -j1
      

      At this point Fastcomp is compiled, and available in the same location as the original.

4. Get your fork of Emscripten:

   # From your emsdk "root" (where emsdk is installed). Navigate to the LLVM clone:
   cd clang/fastcomp/src
   
   # Add a git remote link to your LLVM repository fork, then fetch changes
   git remote add myremote https://github.com/mygituseraccount/emscripten-fastcomp.git
   git fetch myremote
   
   # Add a git remote link to your Clang repository fork.
   git remote add myclangremote https://github.com/mygituseraccount/emscripten-fastcomp-clang.git
   
   # Fetch the changes in the remote.
   git fetch myllvmremote
   
   # Checkout the code in your LLVM fork to replace the original repository code
   git checkout -b myllvmremote --track myremote/incoming
   

You can switch back and forth between remotes (the original sources and your fork, or the incoming and master branches) using the git checkout command. You can switch between the SDK and source using the ./emsdk activate <target> command.