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Auto merge of #114231 - ttsugriy:binary_search_slice, r=cjgillot

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[rustc_data_structures] Use partition_point to find  slice range end.

This PR uses approach introduced in https://github.com/rust-lang/rust/pull/114152 to find
the end of the range. It's much easier to understand and reason about invariants of such
implementation.
Technically it's possible to make it even shorter by returning `&[start..end]` unconditionally
because even if searched item is not present in the slice, `start` and `end` would point at
the same index, so the range would be empty. The reason I decided not to use this shorter
implementation is because it would involve more comparisons in case there are no elements
in the slice with key equal to `key`.

Also, not that it matters much, but this implementation also improves perf according to the
benchmark below:
https://gist.github.com/ttsugriy/63c0ed39ae132b131931fa1f8a3dea55

The results on my M1 macbook air are:
```
Running benches/bin_search_slice_benchmark.rs (target/release/deps/bin_search_slice_benchmark-90fa6d68c3bd1298)
Benchmarking multiply add/binary_search_slice: Collecting 100 samples in estimated 5.0002 s (1
multiply add/binary_search_slice
                        time:   [44.719 ns 44.918 ns 45.158 ns]
                        No change in performance detected.
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe
Benchmarking multiply add/binary_search_slice_new: Collecting 100 samples in estimated 5.0001
multiply add/binary_search_slice_new
                        time:   [36.955 ns 37.060 ns 37.221 ns]
                        No change in performance detected.
Found 7 outliers among 100 measurements (7.00%)
  3 (3.00%) high mild
  4 (4.00%) high severe
```
This commit is contained in:
bors 2024-01-18 18:54:54 +00:00
commit 25f8d01fd8
1 changed files with 4 additions and 21 deletions
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25
compiler/rustc_data_structures/src/binary_search_util/mod.rs
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@ -18,27 +18,10 @@ where
return &[];
};
// Now search forward to find the *last* one.
let mut end = start;
let mut previous = start;
let mut step = 1;
loop {
end = end.saturating_add(step).min(size);
if end == size || key_fn(&data[end]) != *key {
break;
}
previous = end;
step *= 2;
}
step = end - previous;
while step > 1 {
let half = step / 2;
let mid = end - half;
if key_fn(&data[mid]) != *key {
end = mid;
}
step -= half;
}
// Find the first entry with key > `key`. Skip `start` entries since
// key_fn(&data[start]) == *key
let offset = start + 1;
let end = data[offset..].partition_point(|x| key_fn(x) <= *key) + offset;
&data[start..end]
}