Potential optimization opportunity: Tablify sequences of ifs

[dcodeIO]

So far we have been using sequences of br_ifs to hint to Binaryen that these might be suitable candidates for a br_table if conditions are dense constants potentially. While this works well, there are occasions where it is easier / more idiomatic to emit sequences of normal ifs, either manually or using the Reloopers addBranch, but these don't benefit from RemoveUnusedBrs's tablify. Typical code is:

(local.set $0 (...))
(if
 (i32.or
  (i32.or
   (i32.eq
    (local.get $0)
    (i32.const 1)
   )
   (i32.eq
    (local.get $0)
    (i32.const 2)
   )
  )
  (i32.eq
   (local.get $0)
   (i32.const 3)
  )
 )
 (if
  ...
  (call ...)
 )
 (call ...) ;; or unreachable
)

or

(local.set $0 (...))
(if
 (i32.or
  (i32.or
   (i32.eq
    (local.get $0)
    (i32.const 1)
   )
   (i32.eq
    (local.get $0)
    (i32.const 2)
   )
  )
  (i32.eq
   (local.get $0)
   (i32.const 3)
  )
 )
 (return
  (call ...)
 )
)
(if
 ...
 (return
  (call ...)
 )
)
(call ...) ;; or unreachable

where at some point converting to a br_table seems to become more efficient.

[kripken]

This type of pattern is pretty hard to match in general. How about doing the opposite, creating a br_table in your code, and let the optimizer lower that to ifs if that makes sense? I think that would be much more practical.

[dcodeIO]

I see, hmm. The problem with emitting a br_table is that it can potentially be huge, making the code hard to deal with in our tests, or for anyone looking at untouched output for their own reasons.

But, reading your comment, I'm wondering if there is already something in place (that I missed) to lower such a table right away, upon the Relooper's renderAndDispose? If not, that'd essentially solve the issue as well for me, since one could simply build a table and rely on the Relooper to do the right thing right away, based on optimize levels set on render.

[dcodeIO]

Created a little helper on our end now that builds a br_ifified table, with an API very similar to the Relooper's. Does the job, so the suggestions here aren't particularly important anymore and more a nice to have :)

[tlively]

Closing because the requested feature is no longer necessary.