[mypyc] feat: cache len for iterating over immutable types #19656
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BobTheBuidler
commented
Aug 14, 2025
| L0: | ||
| r0 = 'abc' | ||
| source = r0 | ||
| r1 = CPyStr_Size_size_t(source) |
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this CPyStr_Size twice in 4 lines thing looks pretty ugly, but its better than what we had before. This will be optimized a bit better in my follow-up PR
JukkaL
approved these changes
Aug 14, 2025
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This looks reasonable. I suspect that this will rarely yield any significant performance improvement, but the implementation is simple enough and it could help in the future more once we optimize other things (almost all the change volume is in irbuild tests, which are easier to maintain since they can be updated automatically).
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I did see a ~0.4% improvement when I benchmarked self-check, but that was on the larger PR with the other len-related change as well. In any case the IR looks prettier now! |
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Currently, if a user uses an immutable type as the sequence input for a for loop, the length is checked once at each iteration which, while necessary for some container types such as list and dictionaries, is not necessary for iterating over immutable types tuple, str, and bytes.
This PR modifies the codebase such that the length is only checked at the first iteration, and reused from there.