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A comment on The humble for loop in Rust. Parent comment

Re:The humble for loop in Rust by hedgehog on Thursday December 12, 2024 @04:29PM (#42403548)
It seems `map` should have less restrictive semantics (specifically ordering) than `for`, does that allow more optimization? I don't know much about Rust internals.
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Re:The humble for loop in Rust by ironhaven on Thursday December 12, 2024 @04:46PM (#42403746)
Reading the godbolt it looks like for the push loop llvm is unable to remove the `grow_one` capacity check after every push. Becaue of this the Vec could possibly reallocate after every push meaning it can't auto vectorize.
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Re:The humble for loop in Rust by mastax on Thursday December 12, 2024 @05:21PM (#42404026)
It's a little bit surprising to me that LLVM can't eliminate the grow_one check. It looks like there's a test ensure it's not needed in the easier case of vec.push(vec.pop()) [0]. With the iterator the optimization is handled in the standard library using specialization and TrustedLen[1].

[0]: https://github.com/rust-lang/rust/blob/master/tests/codegen/...

[1]: https://github.com/rust-lang/rust/blob/d4025ee454169fbd22f57...

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