Question

In Z3 there are 2 modes: automatic reference counting and manual.

I understand how manual ref counting works. Thanks to example.

But how does Z3 know when to delete AST node in automatic ref-counting case?
Since Z3_ast is a struct from C language => it is impossible to track all assignments and usages of Z3_ast outside Z3 after it was created.

Or Z3 track references inside Z3 only? That is no updates to ref counters are made if you do for example: ast1 = ast2.

Answer 1

The automatic mode uses a very simple policy. Whenever an AST is returned to the user, Z3 stores it on a stack S and increments its reference counter.
When the Z3_push function is executed, Z3 saves the size of the stack S. When the matching Z3_pop is executed, the size of the stack S is restored, and the reference counter of the ASTs popped from the stack is decremented.
This mode is very easy to use, but it has a main problem: memory consumption. For example, if Z3_push and Z3_pop are not used, then all ASTs created by the user will never be deleted.