RemoveUnusedBrs fails to optimize control flow based dead code

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#7,652 opened on Jun 12, 2025 2 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 38/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Stale
Tech stack: wasm
Domain: compilers

Research direction

Start with the RemoveUnusedBrs optimization and reproduce the supplied WebAssembly case using wasm-opt version 2b989ae at O2 and O3. Compare the resulting control flow with the expected O2 output; done means O3 also eliminates the unreachable branch body and following dead calls as shown.

Written by the indexing model from the issue text.

Description

Given the following code:

(module
  (import "env" "dead" (func $dead))
  (func $_start (param $0 i32)
    i32.const 0
    i32.const 0
    call $main)
  (func $main (param $0 i32) (param $1 i32)
    block  ;; label = @1
      i32.const 0
      i32.const 0
      i32.store
      i32.const 0
      i32.load
      local.set $1
      i32.const 1
      local.set $0
      local.get $1
      br_if 0 (;@1;)
      i32.const 0
      i32.const 0
      i32.store
      i32.const 1
      local.set $0
      local.get $0
      br_if 0 (;@1;)
      i32.const 0
      local.set $0
    end
    local.get $0
    if  ;; label = @1
      unreachable
    end
    call $dead
    call $dead
    call $dead
    )
  (memory 1)
  (export "_start" (func $_start)))

wasm-opt version: 2b989ae

O2 can eliminate the dead code because it identifies the statements of if unreachable:

 (func $_start (param $0 i32)
  (i32.store
   (i32.const 0)
   (i32.const 0)
  )
  (if
   (i32.eqz
    (i32.load
     (i32.const 0)
    )
   )
   (then
    (i32.store
     (i32.const 0)
     (i32.const 0)
    )
   )
  )
  (unreachable)
 )

O3 cannot:

 (func $_start (param $0 i32)
  (if
   (block $block (result i32)
    (i32.store
     (i32.const 0)
     (i32.const 0)
    )
    (drop
     (br_if $block
      (i32.const 1)
      (i32.load
       (i32.const 0)
      )
     )
    )
    (i32.store
     (i32.const 0)
     (i32.const 0)
    )
    (i32.const 1)
   )
   (then
    (unreachable)
   )
  )
  (call $dead)
  (call $dead)
  (call $dead)
 )

Dominant language: WebAssembly
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Merged PRs (30d): 77

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