4 ms·

It seems `map` should have less restrictive semantics (specifically ordering) than `for`, does that allow more optimization? I don't know much about Rust intern

• ironhaven 2y ago
  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.
  • mastax 2y ago
    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/vec_pop_push_noop.rs#L8 https://github.com/rust-lang/rust/blob/master/tests/codegen/... [1]: https://github.com/rust-lang/rust/blob/d4025ee454169fbd22f5773f54348310ab6a47bb/library/alloc/src/vec/mod.rs#L3543 https://github.com/rust-lang/rust/blob/d4025ee454169fbd22f57...