v8/test/mjsunit/regress/regress-convert-enum.js

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// Flags: --expose-gc

// Create a transition tree A (no descriptors) -> B (descriptor for a) -> C
// (descriptor for a and c), that all share the descriptor array [a,c]. C is the
// owner of the descriptor array.
var o = {};
o.a = 1;
o.c = 2;

// Add a transition B -> D where D has its own descriptor array [a,b] where b is
// a constant function.
var o1 = {};
o1.a = 1;

// Install an enumeration cache in the descriptor array [a,c] at map B.
for (var x in o1) { }
o1.b = function() { return 1; };

// Return ownership of the descriptor array [a,c] to B and trim it to [a].
o = null;
gc();

// Convert the transition B -> D into a transition to B -> E so that E uses the
// instance descriptors [a,b] with b being a field.
var o2 = {};
o2.a = 1;
o2.b = 10;

// Create an object with map B and iterate over it.
var o3 = {};
o3.a = 1;

for (var y in o3) { }