Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

reuse RHS allocation for vec.extend(vec.into_iter()) when they do not fit into the LHS #77496

Closed
wants to merge 12 commits into from
Closed

reuse RHS allocation for vec.extend(vec.into_iter()) when they do not fit into the LHS #77496

Changes from 4 commits
Commits
Show all changes
12 commits
Select commit Hold shift + click to select a range
c5af975
reuse RHS allocation in vec.extend() when the LHS is empty
the8472 Oct 3, 2020
2409653
vec.extend() reuse RHS allocation if combined data does not fit into LHS
the8472 Oct 4, 2020
8a0a13a
add comment for IntoIter::move_to
the8472 Oct 4, 2020
0c443cf
rename function
the8472 Oct 5, 2020
153a916
add debug asserts
the8472 Oct 5, 2020
cb477d4
rename function
the8472 Oct 6, 2020
5e6abf1
add ascii illustration
the8472 Oct 6, 2020
567cd52
move self-replacement to separate line
the8472 Oct 6, 2020
1dbab48
memory-based ascii illustration
the8472 Oct 6, 2020
aa4a4ac
annotate used primitives in ascii illustration
the8472 Oct 7, 2020
1885f38
reserve() was not annotated with free
the8472 Oct 9, 2020
d460c85
fix debug_assert and improve associated comment
the8472 Oct 15, 2020
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
34 changes: 29 additions & 5 deletions library/alloc/src/vec.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2204,7 +2204,7 @@ impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
}

let mut vec = Vec::new();
iterator.move_to(&mut vec);
iterator.move_into(&mut vec);
vec
}
}
Expand Down Expand Up @@ -2386,22 +2386,46 @@ where
impl<T> SpecExtend<T, IntoIter<T>> for Vec<T> {
fn spec_extend(&mut self, iterator: IntoIter<T>) {
// Avoid reallocation if we can use iterator's storage instead. This requires 1 memcpy and 0-1 memmove
the8472 marked this conversation as resolved.
Show resolved Hide resolved
// while reallocation would require 1 alloc, 1-2 memcpy, 1-2 free
// while reallocation would require 1 alloc, 1-2 memcpy, 1-2 free.
//
// ## non-empty self, partially consumed iterator
the8472 marked this conversation as resolved.
Show resolved Hide resolved
//
// == step == == memory == == self == == iter / v ==
// 0123456789abcdef0123456789abcdef
// 0---------------1---------------
//
// [initial] AAAA_-----__BBB___-------------- Vec(0x00, 4, 5) IntoIter(0x0a, 0x0c, 0x0f, 8)
// into_vec AAAA_-----____BBB_-------------- Vec(0x00, 4, 5) Vec(0x0a, 7, 8)
// prepend _____-----AAAABBB_-------------- Vec(0x00, 0, 5) Vec(0x0a, 7, 8)
// *self = v ----------AAAABBB_-------------- Vec(0x0a, 7, 8)
//
// ## empty self, partially consumed iterator
//
// [initial] ____------__BBBB__-------------- Vec(0x00, 0, 4) IntoIter(0x0a, 0x0c, 0x10, 8)
// into_vec ____------BBBB____-------------- Vec(0x00, 0, 4) Vec(0x0a, 4, 8)
// *self = v ----------BBBB____-------------- Vec(0x0a, 4, 8)
//
// ## empty self, pristine iterator
//
// [initial] ----------BBBB____-------------- Vec(0x00, 0, 0) IntoIter(0x0a, 0x0a, 0x0e, 8)
// *self = v ----------BBBB____-------------- Vec(0x0a, 4, 8)
//
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Suggested change
//

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

That line was intentionally left blank.

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think following the rest of the codebase convention here would have this blank line not have the comment //, and just be empty.

if mem::size_of::<T>() > 0
&& self.capacity() - self.len() < iterator.len()
&& iterator.cap - iterator.len() >= self.len()
{
// Safety: we just checked that IntoIter has sufficient capacity to prepend our elements.
// Prepending will then fill the uninitialized prefix.
*self = unsafe {
let v = unsafe {
let mut v = iterator.into_vec_with_uninit_prefix(self.len() as isize);
ptr::copy_nonoverlapping(self.as_ptr(), v.as_mut_ptr(), self.len);
self.set_len(0);
the8472 marked this conversation as resolved.
Show resolved Hide resolved
v
};
*self = v;
return;
the8472 marked this conversation as resolved.
Show resolved Hide resolved
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Can we turn this into an else block instead of early returning?

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

We can, but I prefer early returns since the later part indicates the default approach in contrast to the special case above.

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I mentioned the same thing above #77496 (comment)

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I find splitting the branches here a bit easier to follow so you can tell that there's two paths we can take and don't have any way to fall through this one.

}
iterator.move_to(self);
iterator.move_into(self);
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

It would be cooler if the code are linked to the cool diagram.

Suggested change
iterator.move_into(self);
// Insufficient capacity
iterator.move_into(self);

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Well, yes and no.

With the optimization present that is indeed all it covers. But it is the general codepath that also works without the optimization. So I don't want to give the impression that it can only handle that case.

}
}

Expand Down Expand Up @@ -2959,7 +2983,7 @@ impl<T> IntoIter<T> {
}

/// Move remaining elements to the end of `dest`.
fn move_to(mut self, dest: &mut Vec<T>) {
fn move_into(mut self, dest: &mut Vec<T>) {
unsafe {
dest.append_elements(self.as_slice() as _);
}
Expand Down