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Use std based dedup in projection

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Unstable sort was added recently, and the code that is being modified is 3 years old. As quicksort doesn't allocate it will likely perform as well as, or better than linear search.
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Tatsuyuki Ishi 2018-01-22 19:58:13 +09:00 • committed by GitHub
parent bc072ed0ca
commit 0bbc4221dc
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1 changed files with 6 additions and 15 deletions
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21
src/librustc/traits/project.rs
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@ -824,21 +824,12 @@ fn project_type<'cx, 'gcx, 'tcx>(
// Drop duplicates.
//
// Note: `candidates.vec` seems to be on the critical path of the
// compiler. Replacing it with an hash set was also tried, which would
// render the following dedup unnecessary. It led to cleaner code but
// prolonged compiling time of `librustc` from 5m30s to 6m in one test, or
// ~9% performance lost.
if candidates.vec.len() > 1 {
let mut i = 0;
while i < candidates.vec.len() {
let has_dup = (0..i).any(|j| candidates.vec[i] == candidates.vec[j]);
if has_dup {
candidates.vec.swap_remove(i);
} else {
i += 1;
}
}
}
// compiler. Replacing it with an HashSet was also tried, which would
// render the following dedup unnecessary. The original comment indicated
// that it was 9% slower, but that data is now obsolete and a new
// benchmark should be performed.
candidates.vec.sort_unstable();
candidates.vec.dedup();
// Prefer where-clauses. As in select, if there are multiple
// candidates, we prefer where-clause candidates over impls. This