WebAssembly lld port

Note: The WebAssembly port is still a work in progress and is be lacking certain features.

The WebAssembly version of lld takes WebAssembly binaries as inputs and produces a WebAssembly binary as its output. For the most part this port tried to mimic the behaviour of traditional ELF linkers and specifically the ELF lld port. Where possible that command line flags and the semantics should be the same.

Object file format

The format the input object files that lld expects is specified as part of the the WebAssembly tool conventions https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md.

This is object format that the llvm will produce when run with the wasm32-unknown-unknown-wasm target. To build llvm with WebAssembly support currently requires enabling the experimental backed using -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly.

Missing features

There are several key features that are not yet implement in the WebAssembly ports:

  • COMDAT support. This means that support for C++ is still very limited.
  • Function stripping. Currently there is no support for --gc-sections so functions and data from a given object will linked as a unit.
  • Section start/end symbols. The synthetic symbols that mark the start and of data regions are not yet created in the output file.