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perf: Use for_each in Vec::extend #68046

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perf: Use for_each in Vec::extend #68046

wants to merge 8 commits into from

Commits on Feb 6, 2020

  1. Add extend_chain bench

    @Marwes
    Marwes authored and Markus Westerlind committed Feb 6, 2020
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  2. More chain in extend benchmark

    Markus Westerlind committed Feb 6, 2020
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  3. Use a more complicated chain in benchmarks

    Markus Westerlind committed Feb 6, 2020
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  4. perf: Use for_each in Vec::extend 

    `for_each` are specialized for iterators such as `chain` allowing for
faster iteration than a normal `for/while` loop.

Note that since this only checks `size_hint` once at the start it may
end up needing to call `reserve` more in the case that `size_hint`
returns a larger and more accurate lower bound during iteration.

This could maybe be alleviated with an implementation closure like the current
one but the extra complexity will likely end up harming the normal case
of an accurate or 0 (think `filter`) lower bound.

```rust
while let Some(element) = iterator.next() {
    let (lower, _) = iterator.size_hint();
    self.reserve(lower.saturating_add(1));
    unsafe {
        let len = self.len();
        ptr::write(self.get_unchecked_mut(len), element);
        // NB can't overflow since we would have had to alloc the address space
        self.set_len(len + 1);
    }

    iterator.by_ref().take(self.capacity()).for_each(|element| {
        unsafe {
            let len = self.len();
            ptr::write(self.get_unchecked_mut(len), element);
            // NB can't overflow since we would have had to alloc the address space
            self.set_len(len + 1);
        }
    });
}

// OR

let (lower, _) = iterator.size_hint();
self.reserve(lower);
loop {
    let result = iterator.by_ref().try_for_each(|element| {
        if self.len() == self.capacity() {
            return Err(element);
        }
        unsafe {
            let len = self.len();
            ptr::write(self.get_unchecked_mut(len), element);
            // NB can't overflow since we would have had to alloc the address space
            self.set_len(len + 1);
        }
        Ok(())
    });

    match result {
        Ok(()) => break,
        Err(element) => {
            let (lower, _) = iterator.size_hint();
            self.reserve(lower.saturating_add(1));
            self.push(element);
        }
    }
}
```

Closes rust-lang#63340
    @Marwes
    Marwes authored and Markus Westerlind committed Feb 6, 2020
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  5. Use try_fold in Vec::extend 

    Should put less stress on LLVM since there are less closures passed
around and lets us refine how much we reserve with `size_hint` if the
first guess is too low.
    Markus Westerlind committed Feb 6, 2020
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  6. No by_ref

    Markus Westerlind committed Feb 6, 2020
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  7. Avoid a closure in Vec::extend_desugared

    Markus Westerlind committed Feb 6, 2020
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  8. Massage extend_desugared to reduce the compiletime impact

    Markus Westerlind committed Feb 6, 2020
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