Wasm-opt not deduplicating repeated struct.gets on unchanged local

[biggs0125]

Consider the larger wasm snippet below. There are multiple instances of the same group of instructions:

local.get $var0
struct.get $#Top $field0

$var0 doesn't change between these struct.gets. I would expect wasm-opt to be able to identify this, do the lookup once and store it in a local prior to entering the block. This would save both code size and performance. We have some functions with hundreds of these patterns so the code size savings from this could be significant.

...
block $label1
  block $label0 (result (ref $#Top))
    local.get $var0
    br_on_non_null $label0
    local.get $var0
    global.get $C6307 SymbolConstant(#[]=)
    local.get $var4
    call $_typeArgumentsToList
    local.get $var3
    call $_positionalParametersToList
    local.get $var10
    call $_namedParametersToMap
    call $_Invocation.method
    call $NoSuchMethodError._throwWithInvocation
    unreachable
  end $label0
  local.tee $var0
  struct.get $#Top $field0
  i32.const 15
  i32.eq
  br_if $label1
  local.get $var0
  struct.get $#Top $field0
  i32.const 16
  i32.eq
  if
    local.get $var0
    ref.as_non_null
    local.get $var4
    local.get $var3
    global.get $C5979 WasmArray<Object>[0]
    call $__ConstMap&_HashFieldBase&MapMixin&_HashBase&_OperatorEqualsAndHashCode&_LinkedHashMapMixin&_MapCreateIndexMixin&_UnmodifiableMapMixin.[]= invocation type checker
    drop
    return
  end
  local.get $var0
  struct.get $#Top $field0
  i32.const 17
  i32.eq
  br_if $label1
  local.get $var0
  struct.get $#Top $field0
  i32.const 18
  i32.eq
  if
    local.get $var0
    ref.as_non_null
    local.get $var4
    local.get $var3
    global.get $C5979 WasmArray<Object>[0]
    i32.const 0
    call_indirect (param (ref $#Top) (ref $Array<_Type>) (ref $Array<Object?>) (ref $Array<Object?>)) (result (ref null $#Top)) $&
    drop
    return
  end
  local.get $var0
  struct.get $#Top $field0
  i32.const 51
  i32.eq
  if
    local.get $var0
    ref.as_non_null
    local.get $var4
    local.get $var3
    global.get $C5979 WasmArray<Object>[0]
    call $CrossappCacheStorage.[]= invocation type checker
    drop
    return
  end
...

[kripken]

Atm we deduplicate such operations inside basic blocks, but not between them, which is the case here. Doing so might help size, but VMs do GVN so runtime speed should not be affected at least. I agree this is worth doing, just explaining why it has not been a high priority.

There was an effort to do something general here that includes dead store elimination as well, #3858, but it ended up too complex. Perhaps we can estimate the benefit here somehow and consider a more targeted fix, like just making the LocalCSE pass work across basic blocks.