GitLab now enforces expiry dates on tokens that originally had no set expiration date. Those tokens were given an expiration date of one year later. Please review your personal access tokens, project access tokens, and group access tokens to ensure you are aware of upcoming expirations. Administrators of GitLab can find more information on how to identify and mitigate interruption in our documentation.
Transforms x+y to x - (-y) only when there is a clear benefit.
Profiling our code generator revealed that freezing of additions takes a considerable amount of time.
This is because it heavily relies on sympy's is_negative which is really slow.
(We had just fixed a similar performance issue in our own code which also relied on the assumptions system. It is just slow.)
However, it is not only slow, it is also overeager to use it in the freezer.
Before this MR, the freezer would behave strangely in presence of negative symbols:
Moreover, it would freeze x - 2*y (which in sympy's representation is Add(Symbol('x'), Mul(Integer(-1), Integer(2), Symbol('y')))) to PsSub(x, PsMul(2, y)).
I do not see a benefit of this form over PsAdd(x, PsMul(-2, y)).
Therefore, this MR changes the freezer such that is_negative is not used.
Additions are only frozen to PsSub if the second argument is a multiplication with -1.
In this case, the resulting code benefits from emitting PsSub: x - y instead of x + -1 * y.
As mentioned before, the performance benefit is dramatic.
Freezing one particular expression tree used to take 0.4s and that is now reduced to mere milliseconds.
Removing the check for -1, too, improves the performance even more but it is hard to measure such short times.
(While 0.4s might seem insignificant to begin with, it is not in our scenario, as we must handle a multitude of such expressions and it really adds up. Overall, the generation time dropped from 12.6s to 7.5s. Most of the remainder is due to CSE.)
