For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

@JSerFeng Sorry for delay, I think you are looking for this - #15497

It is ready and tests, just forgot to merge 😞 We need just rebase and we can merge, also it solve more variants

Not the same #15497, this PR can bailout statements if an explicit dead control flow is detected in inner statement.

eg. current behavior is as follows

if (true) {
  return;
} else {
  inner statements
}

outer statements

Currently we only bailout inner statements inside If statement, but I want to skip the outer as well as we know exactly the rest statements is unreachable

@JSerFeng Oh, very big sorry, I just noticed that I gave you the wrong link 😞 I think you are looking for #14357? Right?

I did rebase and also wrote TODO for tomorrow, will finish tomorrow too 😄

I think you are looking for #14357? Right?

Oh I see, it's similar

Do I need to do anything to help ?

No, will finish soon 👍

@JSerFeng @dmichon-msft I added basic dead control flow check (#14357), but there are some limitations currently:

• try/catch with if:

function test() {
  try {
    if (true) {
      throw new Error("should throw");
    }
	
    require("fail");
  } catch(e) {
    require('./used');
  }
}

Now it only works then you have return instead throw inside if (without if return and throw works fine), I think we need to improve nested check here, should not be hard, I don't implement it because I don't see this code often - rare case.

• switch (case):

function test() {
  switch ("production") {
    return require("fail");
  }
  case "production":
    return require("./used");
  default:
    return require("fail");
  }
}

Rare case, if you want you can improve. if/return/throw inside case work fine.

• We don't have return and throw at top level scope

This limitation because of we need to handle export and module.export after return and throw otherwise our build failed when you have import { func } from "./file.js"; and output throw an error "Can't found func export". Any feedback on this welcome.

Examaple:

function func() {}

throw new Error("test"):

export { func };

• Did I miss something?

So you can rebase and improve above cases

Yes, I think the one, try/catch with if can be improved.

We can't change terminate state if inTry because we can't pass that terminate state outside of TryStatement, so I only check if current statement is exactly in TryStatement other than other statement

@JSerFeng Sorry for delay here, I made some improvements (without loop) and fix edge cases with catch/finally, feedback welcome and we can merge