squiid/rust
1
0
Fork 0
Code Issues Pull requests Projects Activity

Rollup merge of #50932 - nnethercote:seen-Predicates, r=eddyb

Browse source 
Optimize seen Predicate filtering.

This speeds up a few rustc-perf benchmark runs, most notably ones
involving 'coercions', the best by 2%.
This commit is contained in:
kennytm 2018-05-23 00:26:15 +08:00 committed by GitHub
parent bc76e8b029 2ff632484c
commit 5b67b13d1e
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
1 changed files with 18 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

21
src/librustc/traits/select.rs
View file

@ -3356,13 +3356,28 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
predicate: predicate.value
}))
}).collect();
// We are performing deduplication here to avoid exponential blowups
// (#38528) from happening, but the real cause of the duplication is
// unknown. What we know is that the deduplication avoids exponential
// amount of predicates being propogated when processing deeply nested
// amount of predicates being propagated when processing deeply nested
// types.
let mut seen = FxHashSet();
predicates.retain(|i| seen.insert(i.clone()));
//
// This code is hot enough that it's worth avoiding the allocation
// required for the FxHashSet when possible. Special-casing lengths 0,
// 1 and 2 covers roughly 75--80% of the cases.
if predicates.len() <= 1 {
// No possibility of duplicates.
} else if predicates.len() == 2 {
// Only two elements. Drop the second if they are equal.
if predicates[0] == predicates[1] {
predicates.truncate(1);
}
} else {
// Three or more elements. Use a general deduplication process.
let mut seen = FxHashSet();
predicates.retain(|i| seen.insert(i.clone()));
}
self.infcx().plug_leaks(skol_map, snapshot, predicates)
}
}