bpo-38328: Speed up the creation time of constant list literals. #16498
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brandtbucher changed the title 
brandtbucher commented Sep 30, 2019
| samples = [[], [1,2,3], ['1', '2', '3']] | ||
| for sample in samples: | ||
| check(sample, vsize('Pn') + len(sample)*self.P) | ||
| check(list(sample), vsize('Pn') + len(sample)*self.P) |
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This is a sys.getsizeof test. The 3-element lists are overallocated, so we resize them here.
brandtbucher force-pushed the constant-lists branch from c41475a to d53e48d Compare 
| @serhiy-storchaka Since you proposed something similar a while back, do you mind taking a look at this when you have the chance? |
Python/peephole.c Outdated
| h, i+1, consts, j); | ||
| h, i+1-islist, consts, j); | ||
| if (h >= 0 && islist) { | ||
| h = copy_op_arg(codestr, i, BUILD_LIST_UNPACK, 1, i+1); |
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h is not used. Remove h = .
It can also be replaced with
codestr[i] = PACKOPARG(BUILD_LIST_UNPACK, 1); | Thanks for the feedback @serhiy-storchaka. Tests are passing, ready for another review. |
serhiy-storchaka approved these changes Oct 5, 2019
| Thank @serhiy-storchaka. Is anything else needed here? |
brandtbucher force-pushed the constant-lists branch from d0f4746 to 091e9cd Compare 
| Nothing particular. I just want to fix the issue with list overallocation first. Your PR is good, thank you. |
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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:
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.
https://bugs.python.org/issue38328