is duplicated by:

• SR-7545 Swift should support using realloc for tail allocated classes

Issue Description:

We should support using realloc() for increasing the size of tail allocated objects, like an array buffer.

This could be done by introducing a new built-in: Builtin.reallocWithTailElems_n (n == 1, 2, 3)

It’s similar to Builtin.allocWithTailElems_n, but receives the original object reference as an additional (first) parameter.

On the implementation side we could introduce a new instruction realloc_ref, again with an additional operand, which is the reference to the original object.

Obviously there are some restrictions: The original object must be uniquely reference, i.e. must have a reference count of 1 (and no weak/unowned references). All stored properties, including the tail allocated elements, must be bit-wise movable.

The work to do here is:

• add the new builtins

• add the new SIL instruction

• implement the IRGen part of it

• add a new runtime function swift_reallocObject (similar to swift_allocObject)