llvm/lib/Target/WebAssembly/README.txt

//===-- README.txt - Notes for WebAssembly code gen -----------------------===//

This WebAssembly backend is presently in a very early stage of development.
The code should build and not break anything else, but don't expect a lot more
at this point.

For more information on WebAssembly itself, see the design documents:
  * https://github.com/WebAssembly/design/blob/master/README.md

The following documents contain some information on the planned semantics and
binary encoding of WebAssembly itself:
  * https://github.com/WebAssembly/design/blob/master/AstSemantics.md
  * https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md

Interesting work that remains to be done:
* Write a pass to restructurize irreducible control flow. This needs to be done
  before register allocation to be efficient, because it may duplicate basic
  blocks and WebAssembly performs register allocation at a whole-function
  level. Note that LLVM's GPU code has such a pass, but it linearizes control
  flow (e.g. both sides of branches execute and are masked) which is undesirable
  for WebAssembly.

//===---------------------------------------------------------------------===//

set_local instructions have a return value. We should (a) model this,
and (b) write optimizations which take advantage of it. Keep in mind that
many set_local instructions are implicit!

//===---------------------------------------------------------------------===//

Load and store instructions can have a constant offset. We should (a) model
this, and (b) do address-mode folding with it.

//===---------------------------------------------------------------------===//

Br, br_if, and tableswitch instructions can support having a value on the
expression stack across the jump (sometimes). We should (a) model this, and
(b) extend the stackifier to utilize it.

//===---------------------------------------------------------------------===//