All the parts needed to support wasm are included in upstream clang, lld, and compiler-rt, as of the LLVM 8.0 release. However, to use it, you'll need to build WebAssembly-targeted versions of the library parts, and it can be tricky to get all the CMake invocations lined up properly.

To make things easier, we provide prebuilt packages that provide builds of Clang and sysroot libraries.

WASI doesn't yet support setjmp/longjmp or C++ exceptions, as it is waiting for unwinding support in WebAssembly.

By default, the C/C++ toolchain orders linear memory to put the globals first, the stack second, and start the heap after that. This reduces code size, because references to globals can use small offsets. However, it also means that stack overflow will often lead to corrupted globals. The -Wl,--stack-first flag to clang instructs it to put the stack first, followed by the globals and the heap, which may produce slightly larger code, but will more reliably trap on stack overflow.

See the wasm-ld documentation for more information and additional flags. Note flags related to dynamic linking, such -shared and --export-dynamic are not yet stable and are expected to change behavior in the future.