Correct the second-biggest design flaw in the bytecode optimizer:
instead of using a de Bruijn-like representation of variable
references in the optimizer pass, use variable objects.
This change is intended to address limitations on programs like the
one in
http://bugs.racket-lang.org/query/?cmd=view&pr=15244
where the optimizer could not perform a straightforward-seeming
transformation due to the constraints of its representation.
Besides handling the bug-report example better, there are other minor
optimization improvements as a side effect of refactoring the code. To
simplify the optimizer's implementation (e.g., eliminate code that I
didn't want to convert) and also preserve success for optimizer tests,
the optimizer ended up getting a little better at flattening and
eliminating `let` forms and `begin`--`let` combinations.
Overall, the optimizer tests in "optimize.rktl" pass, which helps
ensure that no optimizations were lost. I had to modify just a few
tests:
* The test at line 2139 didn't actually check against reordering as
intended, but was instead checking that the bug-report limitation
was intact (and now it's not).
* The tests around 3095 got extra `p` references, because the
optimizer is now able to eliminate an unused `let` around the
second case, but it still doesn't discover the unusedness of `p` in
the first case soon enough to eliminate the `let`. The extra
references prevent eliminating the `let` in both case, since that's
not the point of the tests.
Thanks to Gustavo for taking a close look at the changes.
LocalWords: pkgs rkt
