Stuart Cook 6ebe590e41
Rollup merge of #135847 - edwloef:slice_ptr_rotate_opt, r=scottmcm
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.
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The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.

One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:

// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb

After specifying those variations, you can write different expected, or explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:, like so:

// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT:                        emitted-only-for-bbb
// bbb-NOT:  emitted-only-for-aaa
// bbb-SAME:                       emitted-only-for-bbb