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Significant optimization regression due to failing remove unused branch #7637

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@xuruiyang2002

Description

@xuruiyang2002

Given the following code:

(module
  (import "External" "external_function" (func $external_function))
  (func $_start call $main)
  (func $main
  (local $0 i32)
    (local $1 i32)
    block  ;; label = @1
      block (result i32)  ;; label = @2
        i32.const 1
        local.set $0
        i32.const 0
      end
      br_if 0 (;@1;)
      i32.const 1
      local.set $1
    end
    local.get $1
    local.set $0
    i32.const 1
    local.set $1
    block  ;; label = @1
      local.get $0
      br_if 0 (;@1;)
      i32.const 0
      i32.load8_u
      local.tee $0
      br_if 0 (;@1;)
      call $external_function
      i32.const 0
      local.set $1
    end
    i32.const 0
    local.get $1
    local.tee $0
    i32.store)
  (memory $0 258 258)
  (export "_start" (func $_start)))

wasm-opt (4f9374c) cannot remove unused branch by -O3, while -O2 can. In details, the -O2 produces (simplified):

 (func $_start
  (i32.store
   (i32.const 0)
   (i32.const 1)
  )
 )

while -O3 produces (simplified):

 (func $_start
  (local $0 i32)
  (block $block1
   (br_if $block1
    (local.tee $0
     (i32.const 1)
    )
   )
   (br_if $block1
    (i32.load8_u
     (i32.const 0)
    )
   )
   (call $external_function)
   (local.set $0
    (i32.const 0)
   )
  )
  (i32.store
   (i32.const 0)
   (local.get $0)
  )
 )

which is a significant optimization regression.

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