[armedbear-devel] Performance improvement special binding read/write

[Erik Huelsmann]

Last weekend, I ran Maxima tests since some time again. I don't
remember the older performance figures, although I remember the total
test suite - possibly performance tuned - to take 3300seconds before.
Last weekend, it was 3900seconds, giving me a feeling of performance
regression. So I decided to take another look.

I found two things, one of which is that we still have slow special
binding value lookup. Peter Graves suggested some time ago that we
switch to the same scheme used by SBCL and CCL because he found that
XCL benefitted ~10% on his tests when he made the same switch.

The scheme used by SBCL/CCL differs from the one used by ABCL in the
way the "active" binding is being accessed. ABCL manages all bindings
as a linked list, with the most recent one as the active one. This
means O(n) access time on the number of special bindings.

In SBCL/CCL, the active binding is stored in an array, with a linked
list to record which values to restore when unwinding the binding.
Upon restore, this solution takes more cycles than our solution: we
simply change the head of the linked list, whereas the SBCL/CCL
solution needs to set all the array elements.


Running the application which we know tests special bindings
performance the best [Maxima], I see a strong performance improvement:
3900 seconds before and 2300 seconds after the change. Performance
improved by ~40%!

I'll commit this change after cleaning up.


Bye,

Erik.