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Stuart Cook
6ebe590e41
optimize slice::ptr_rotate for small rotates r? `@scottmcm` This swaps the positions and numberings of algorithms 1 and 2 in `slice::ptr_rotate`, and pulls the entire outer loop into algorithm 3 since it was redundant for the first two. Effectively, `ptr_rotate` now always does the `memcpy`+`memmove`+`memcpy` sequence if the shifts fit into the stack buffer. With this change, an `IndexMap`-style `move_index` function is optimized correctly. Assembly comparisons: - `move_index`, before: https://godbolt.org/z/Kr616KnYM - `move_index`, after: https://godbolt.org/z/1aoov6j8h - the code from `#89714`, before: https://godbolt.org/z/Y4zaPxEG6 - the code from `#89714`, after: https://godbolt.org/z/1dPx83axc related to #89714 some relevant discussion in https://internals.rust-lang.org/t/idea-shift-move-to-efficiently-move-elements-in-a-vec/22184 Behavior tests pass locally. I can't get any consistent microbenchmark results on my machine, but the assembly diffs look promising.