homepage

lookdict_index() (and the rest of the files in dictobject.c) are using unnecessarily complicated perturb mechanism. And, in fact, it's slower than the simpler case.
Instead of this:
for (size_t perturb = hash;;) {
 perturb >>= PERTURB_SHIFT;
 i = mask & ((i << 2) + i + perturb + 1);
....
it should do this:
for (size_t perturb = hash;;) {
 i = mask & ((i << 1) + perturb + 1);
 perturb >>= PERTURB_SHIFT;
....
This would not only save an instruction (a minor issue), but it would also reduce collisions.
I've attached a file which calculates frequencies of collisions for demonstration purposes. It shows that the calculation, as it stands right now, does not create a 1-1 mapping even on the 1st iteration through the loop. Moving PERTURB_SHIFT to the line before the calculation does reduce the density of the collision space. But using the calculation, which I proposed, eliminates collisions on the 1st iteration completely and reduces it on most subsequent iterations.