Optimization issue where allocation is not eliminated

Dart uses iterators pervasively, especially over lists (e.g. for (final value in list) { ... }). The iterable (list) provides a get:iterator method. The iterator then has a iterator.moveNext() and iterator.current. If we know which list type it is we'd like to avoid the iterator overhead.

We have been doing inlining on the dart2wasm side to avoid that. Now I'm experimenting with adjusting our inliner and have noticed an issue in binaryen:

• if dart2wasm does more inlining the iterator allocation disppears after binaryen optimizations
• if dart2wasm does less inlining, binaryen does the inlining for us, but the iterator allocation stays

Example: The dart code looks like this:

final a = A();
main() {
  a.doIt();
  a.doIt();
}

class A {
  final a1 = <int>[];
  final a2 = <double>[];
  final a3 = <String>[];

  void doIt() {
    for (final x in a1) {
      print(x);
    }
    for (final x in a2) {
      print(x);
    }
    for (final x in a3) {
      print(x);
    }
  }
}

Here we want to avoid the allocation of the iterator (a _GrowableListIterator wasm struct).

See allocation_elimination_issue.tar.gz. Unpacking with tar xvzf allocation_elimination_issue.tar.gz will give

# Dart2wasm does the inlining                                           
test.before.wasm
test.before.wasm.wat
# The test.before.wasm optimized via -Os -Os -Os
test.before.opt.wasm
test.before.opt.wasm.wat

# Dart2wasm does less inlining, binaryen does the rest (it inlines `call $"_GrowableListIterator.current= implicit setter"`)         
test.after.wasm
test.after.wasm.wat
# The test.after.wasm optimized via -Os -Os -Os
test.after.opt.wasm
test.after.opt.wasm.wat

Here we can see that (func $A.doit ...) is much larger if we leave the inlining to binaryen, as it doesn't eliminiate the _GrowableListIterator struct allocations.

(Optimization via wasm-opt --enable-gc --enable-reference-types --enable-multivalue --enable-exception-handling --enable-nontrapping-float-to-int --enable-sign-ext --enable-bulk-memory --enable-threads --enable-simd '--no-inline=*<noInline>*' --traps-never-happen -g -Os -Os -Os test.before.wasm -o test.before.opt.wasm and same for test.after.wasm -> test.after.opt.wasm)