feat: add #[repr(u64)] to Repr to optimize clone cost #6
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PureWhiteWu
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src/lib.rs
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| @@ -496,7 +491,7 @@ impl From<Cow<'static, str>> for FastStr { | |||
| } | |||
| } | |||
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| const INLINE_CAP: usize = 38; | |||
| const INLINE_CAP: usize = 24; | |||
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If you make Inline a separate struct (keep len: u8) and just slap #[repr(align(8))] on it you can have INLINE_CAP = 30 with same performance.
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Alternatively try reverting all changes and just adding #[repr(u64)] to Repr enum with INLINE_CAP = 30. This probably will have same effect, as it will change discriminant type from u8 to u64
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Hi, thank you very much!
This works! I will change this pr to use this method!
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Hi, I've tested again on amd64 and found that this method leads to the clone of Empty costs 8ns instead of 4ns. This method works fine on aarch64, but seems that it doesn't on amd64.
I've fixed this in 342bdc9
PureWhiteWu
changed the title
Motivation
Previously, I noticed the clone cost of
FastStris really high, for example, an emptyFastStrclone costs about 40ns on amd64 compared to about 4ns of a normalString.Solution
After some time of investigation, I found that this is because the
Repr::Inlinepart has really great affect on the performance. And after adding #[repr(u64)] toRepr, the performance boosts about 9x. But the root cause is still not clear.