clang.md

Compiling C/C++ to WebAssembly

Rolling your own compiler

Clang has a WebAssembly target, though it is not easy to use currently. First, a custom build must be made.

To build clang:

git clone http://llvm.org/git/llvm.git
cd llvm/tools
git clone http://llvm.org/git/clang.git
cd ../projects
git clone http://llvm.org/git/compiler-rt.git
mkdir ../build
cd ../build
cmake -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly -DLLVM_TARGETS_TO_BUILD= ..
cmake --build .
cd ../..

This will take anything from 1 to 5 hours.

To build binaryen:

git clone https://github.com/WebAssembly/binaryen.git
cd binaryen
mkdir build
cd build
cmake ..
cmake --build .
cd ../..

Using this compiler

Now everything is set to finally compile Hello World!

The compilation process has four steps:

• compiling (clang)
• linking to LLVM IR (llc)
• compiling to WebAssembly S-expressions (s2wasm)
• compiling to WebAssembly binary (wasm-as)

Note: the last step can also be accomplished with wabt (previously called sexpr-wasm-prototype).

Cheat sheet:

clang -emit-llvm --target=wasm32-unknown-unknown-elf -nostdlib -S hello.c
llc -o hello.s hello.ll
s2wasm -o hello.wast hello.s
wasm-as -o hello.wasm hello.wast

There you go, you have your very first WebAssembly binary.