Avoid excessive reallocations during item-bodies checking #31929
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When foldings Substs, we map over VecPerParamSpace instances using EnumeratedItems which does not provide an accurate size_hint() in its Iterator implementation. This leads to quite a large number or reallocations. Providing a suitable size_hint() implementation reduces the time spent in item-bodies checking quite a bit. ``` crate | before | after | ~change -------|------------------------- core | 7.28s | 5.44s | -25% std | 2.07s | 1.88s | -9.2% syntax | 8.86s | 8.30s | -6.3% ```
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r? @jroesch (rust_highfive has picked a reviewer for you, use r? to override) |
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| fn size_hint(&self) -> (usize, Option<usize>) { | ||
| let size = self.vec.as_slice().len(); | ||
| (size, Some(size)) |
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This overestimates the number of elements in the iterator, doesn't it?
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No, it doesn't. The underlying vector is used in its entirety. If you check VecPerParamSpace::limits() one can see how it is split into three consecutive ranges.
(We discussed this on IRC, I'm just adding this comment for reference.)
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I now understand the iterator! It's fine. We cleared this up on irc.
Manishearth
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When foldings Substs, we map over VecPerParamSpace instances using EnumeratedItems which does not provide an accurate size_hint() in its Iterator implementation. This leads to quite a large number or reallocations. Providing a suitable size_hint() implementation reduces the time spent in item-bodies checking quite a bit. ``` crate | before | after | ~change -------|------------------------- core | 7.28s | 5.44s | -25% std | 2.07s | 1.88s | -9.2% syntax | 8.86s | 8.30s | -6.3% ```
bors
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When foldings Substs, we map over VecPerParamSpace instances using
EnumeratedItems which does not provide an accurate size_hint()
in its Iterator implementation. This leads to quite a large number or
reallocations. Providing a suitable size_hint() implementation reduces
the time spent in item-bodies checking quite a bit.