test.js

// d8 --allow-natives-syntax --trace-opt --trace-deopt test.js

var o = { x: 0, y: 0 };
print(%HasFastProperties(o));  // => true
Object.setPrototypeOf(o, { random: 'stuff', stuff: function() { } });
print(%HasFastProperties(o));  // => true

function foo (o) {
  return o.x;
}

function bar (o) {
  return o.random;
}

function baz (o) {
  return o.stuff();
}

function opt(f, o) {
    f(o); f(o); f(o);
    %OptimizeFunctionOnNextCall(f);
    f(o);
}

opt(foo, o);
opt(bar, o);
opt(baz, o);

// No deoptimization below
print('{{{')
Object.setPrototypeOf(o, { emptiness: 'is form', form: 'is emptiness', stuff: function ( ) { } });
print('}}}')
print(%HasFastProperties(o));  // => true

// Eager deopts in each call below
foo(o);
bar(o);
baz(o);