Hello,
I attach patch that needs one problem to be resolved. Gains for
following test code:

test_fun() {
    arr[20000]=""
    repeat 10000; do
        arr[10000]="x"
    done
}

are huge: 37969 ms with no optimization, 244 ms with optimization. The
screensaver runs for 495 sec without optimization, 51 sec with opt
(still slower than hash which runs for 23 sec).

I changed:

        if (v->end <= pre_assignment_length)
            post_assignment_length += pre_assignment_length - v->end +
            1;

to:

        if (v->end <= pre_assignment_length)
            post_assignment_length += pre_assignment_length - v->end;

And this allows to test post* == pre*. I think the "+ 1" isn't needed,
v->end can be treated as "number of elements affected already".
Subtracting the number of elements from pre_assignment_length gives
correct value: elements that will be copied from old.

The thing that has to be resolved: special setters. The patch contains
one special handler tested:

if ( pre_assignment_length != post_assignment_length ||
v->pm->gsu.a->setfn == set_region_active ) {

But this doesn't work as the symbol `set_region_active` isn't available.
How to solve this, and what are other special handlers? One test is
failing (A06) – append to typeset -U array, because the special setter
isn't called.

-- 
  Sebastian Gniazdowski
  psprint@fastmail.com