Add option for preserving function order?

[matthew-mojira]

We are working on a project in Wizard that handles the earliest functions in a Wasm module specially. Unfortunately wasm-opt seems to reorder all the functions. Some of our functions really need to be first. We noticed that there is a --preserve-type-order option, which helps our use case, but we need something similar for functions. Is this something that could be added to binaryen? @titzer

[kripken]

Functions should not be literally reordered (as while that reduces LEB size, it can increase compressed size in practice, so the reorder-functions pass is not run by default). What you are seeing could be something else like duplicate function elimination which merges functions, or global DCE which removes unused functions (causing function indexes to change). You can try to just disable those passes:

wasm-opt -O3 --skip-pass=duplicate-function-elimination --skip-pass=remove-unused-module-elements [..]

(Though that does disable a significant amount of optimization.)

If that's not it, you can find which pass is causing the change using BINARYEN_PASS_DEBUG=3 in the env, which emits a wasm file after each pass.