bpo-38328: Speed up the creation time of constant list literals. by brandtbucher · Pull Request #16498 · python/cpython
This PR contains a small change to the peephole optimizer that converts sequences of:
LOAD_CONST(a), LOAD_CONST(b), ..., BUILD_LIST(n)
to
LOAD_CONST((a, b, ...)), BUILD_LIST_UNPACK(1)
The improvement quickly becomes significant for lists larger than a few items:
| Elements | Speedup |
|---|---|
| 5 | ~5% |
| 10 | ~20% |
| 15 | ~25% |
| 20 | ~30% |
| 25 | ~35% |
| 30 | ~45% |
| 35 | ~50% |
This can be tested on any version of Python by comparing the performance of [0, 1, 2, ...] vs [*(0, 1, 2, ...)]. The common cases of empty and single-element lists are not affected by this change.
This is related to bpo-33325, but that was an invasive change for all collection literals that had an unknown affect on performance. I've limited this one to lists and kept it to a few lines in the peephole optimizer.