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cstring: avoid excessive growth just to 0-terminate - #35871
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Based on following what happens in CString::new("string literal"):
1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.
If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.
Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.rust-highfive
commented
Aug 21, 2016
Contributor
(rust_highfive has picked a reviewer for you, use r? to override) |
retep998
commented
Aug 21, 2016
Contributor
The |
ContributorAuthor
That can be addressed by specialization (I imagine, |
alexcrichton
commented
Aug 21, 2016
Member
bors
commented
Aug 21, 2016
Collaborator
📌 Commit 876c02c has been approved by |
eddyb added a commit
to eddyb/rust
that referenced
this pull request
Aug 22, 2016
cstring: avoid excessive growth just to 0-terminate
Based on following what happens in CString::new("string literal"):
1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.
If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.
Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.
Addresses part of rust-lang#35838bors
commented
Aug 22, 2016
Collaborator
⌛ Testing commit 876c02c with merge 3c5a0fa... |
bors added a commit
that referenced
this pull request
Aug 22, 2016
cstring: avoid excessive growth just to 0-terminate
Based on following what happens in CString::new("string literal"):
1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.
If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.
Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.
Addresses part of #35838bors
commented
Aug 22, 2016
Collaborator
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Based on following what happens in CString::new("string literal"):
Into<Vec<u8>>, a Vec is allocated with capacity exactly equalto the string's input length.
v.push(0), the Vec is grown to twice capacity, since it was full.v.into_boxed_slice(), the Vec capacity is shrunk to fit the length again.If we use
.reserve_exact(1)just before the push, then we avoid thecapacity doubling that we're going to have to shrink anyway.
Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.
Addresses part of #35838