Optimize block tails where a dropped br_if's value is redundant

[xuruiyang2002]

If a block ends with a br_if followed by a value that is the same as the br_if's value (and has no side effects), the value of br_if and br_if itself are redundant and can be removed. For example:

(block $block (result i32)
 ..
 (drop
  (br_if $block
   (value)
   (condition)
  )
 )
 (value)
)

=>

(block $block (result i32)
 ..
 (drop
  (condition)
 )
 (value)
)

Fixes: #7489

[xuruiyang2002]

Thanks! Learned a lot from your comments and good coding taste. I've updated as you suggested.

[kripken]

Looks good aside from two final comments. Also please fuzz this for a while.

[xuruiyang2002]

(Fixed)

Bad news, the fuzzer told me that this PR aborts when dealing with the following wat code:

(module
 (rec
  (type $0 (struct))
  (type $1 (func (param funcref) (result (ref $0))))
  (type $2 (func))
 )
 (func $0 (type $2)
  (nop)
 )
 (func $1 (type $1) (param $0 funcref) (result (ref $0))
  (block $block (result (ref $0))
   (drop
    (br_on_cast $block (ref $0) (ref $0)
     (struct.new_default $0)
    )
   )
   (struct.new_default $0)
  )
 )
 (func $2 (type $2)
  (drop
   (call $1
    (ref.func $0)
   )
  )
 )
)

Above code saved as w.wat

Compile option: bin/wasm-opt w.wat --code-folding --monomorphize --pass-arg=monomorphize-min-benefit@95 --optimize-level=0 --shrink-level=0 -fimfs=99999999 --closed-world --dce --enable-threads --enable-mutable-globals --enable-nontrapping-float-to-int --enable-simd --enable-bulk-memory --enable-sign-ext --enable-exception-handling --enable-tail-call --enable-reference-types --enable-gc --enable-memory64 --enable-extended-const --enable-multimemory --enable-stack-switching --enable-bulk-memory-opt --enable-call-indirect-overlong --closed-world -S -o -

wasm-opt complains at src/wasm-traversal.h:286: :

void wasm::Walker<SubType, VisitorType>::pushTask(TaskFunc, wasm::Expression**) [with SubType = wasm::RemoveUnusedNames; VisitorType = wasm::UnifiedExpressionVisitor<wasm::RemoveUnusedNames>; TaskFunc = void (*)(wasm::RemoveUnusedNames*, wasm::Expression**)]: Assertion `*currp' failed.
Aborted (core dumped)

Obviously, the optimization meets all the condition and performs the update, however, the br->condition is null, and drop cannot ref to a null ptr.

Fixed.

Updated: I've fuzzing for one hour, all is well.

[kripken]

Good, nice, this is exactly what the fuzzer is helpful with!

[xuruiyang2002]

After updating all you suggested, I've also fuzzing for about 15 minutes; and all is well.