Improve selectify

Open

#4,166 opened on Sep 18, 2021 2 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Stale
Tech stack: cpp, wasm
Domain: compilers, testing-qa

Research direction

Start with the selectify logic near src/passes/RemoveUnusedBrs.cpp#L1077 and the existing cases in lit/remove-unused-brs.wast. Add the supplied early-return regression case and verify that the test expects the if without a false arm to be canonicalized so selectify can produce the shown select form.

Written by the indexing model from the issue text.

Description

I think I already mentioned a similar problem, but it was a long time ago.

If we have an "if" branch without a false arm, you selectify stops working:

function selectify_early_return(x: i32): i32 {
  if (x) return x;
  return 1;
}

Just add this test into lit/remove-unused-brs.wast as:

  ;; CHECK:      (func $selectify-early-return (param $x i32) (result i32)
  ;; CHECK-NEXT:  (if
  ;; CHECK-NEXT:   (local.get $x)
  ;; CHECK-NEXT:   (return
  ;; CHECK-NEXT:    (local.get $x)
  ;; CHECK-NEXT:   )
  ;; CHECK-NEXT:  )
  ;; CHECK-NEXT:  (i32.const 1)
  ;; CHECK-NEXT: )
  (func $selectify-early-return (param $x i32) (result i32)
    (if
      (local.get $x)
      (return (local.get $x))
    )
    (i32.const 1)
  )

But obversely we expect something like this insted:

(func $selectify-early-return (param $x i32) (result i32)
  (select
    (local.get $0)
    (i32.const 1)
    (local.get $0)
  )
)

The main problem here: https://github.com/WebAssembly/binaryen/blob/main/src/passes/RemoveUnusedBrs.cpp#L1077
selectify just skip optimisation if ifFalse is not exists. So it is necessary to canonicalze such cases before applying selectify. Any idea how best to do this?

Dominant language: WebAssembly
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