Tiny Vecs are dumb.
Currently, if you repeatedly push to an empty vector, the capacity
growth sequence is 0, 1, 2, 4, 8, 16, etc. This commit changes the
relevant code (the "amortized" growth strategy) to skip 1 and 2, instead
using 0, 4, 8, 16, etc. (You can still get a capacity of 1 or 2 using
the "exact" growth strategy, e.g. via `reserve_exact()`.)
This idea (along with the phrase "tiny Vecs are dumb") comes from the
"doubling" growth strategy that was removed from `RawVec` in #72013.
That strategy was barely ever used -- only when a `VecDeque` was grown,
oddly enough -- which is why it was removed in #72013.
(Fun fact: until just a few days ago, I thought the "doubling" strategy
was used for repeated push case. In other words, this commit makes
`Vec`s behave the way I always thought they behaved.)
This change reduces the number of allocations done by rustc itself by
10% or more. It speeds up rustc, and will also speed up any other Rust
program that uses `Vec`s a lot.
In theory, the change could increase memory usage, but in practice it
doesn't. It would be an unusual program where very small `Vec`s having a
capacity of 4 rather than 1 or 2 would make a difference. You'd need a
*lot* of very small `Vec`s, and/or some very small `Vec`s with very
large elements.
r? @Amanieu
impl From<Cow> for Box, Rc, and Arc
These forward `Borrowed`/`Owned` values to existing `From` impls.
- `Box<T>` is a fundamental type, so it would be a breaking change to add a blanket impl. Therefore, `From<Cow>` is only implemented for `[T]`, `str`, `CStr`, `OsStr`, and `Path`.
- For `Rc<T>` and `Arc<T>`, `From<Cow>` is implemented for everything that implements `From` the borrowed and owned types separately.
Currently, if you repeatedly push to an empty vector, the capacity
growth sequence is 0, 1, 2, 4, 8, 16, etc. This commit changes the
relevant code (the "amortized" growth strategy) to skip 1 and 2 in most
cases, instead using 0, 4, 8, 16, etc. (You can still get a capacity of
1 or 2 using the "exact" growth strategy, e.g. via `reserve_exact()`.)
This idea (along with the phrase "tiny Vecs are dumb") comes from the
"doubling" growth strategy that was removed from `RawVec` in #72013.
That strategy was barely ever used -- only when a `VecDeque` was grown,
oddly enough -- which is why it was removed in #72013.
(Fun fact: until just a few days ago, I thought the "doubling" strategy
was used for repeated push case. In other words, this commit makes
`Vec`s behave the way I always thought they behaved.)
This change reduces the number of allocations done by rustc itself by
10% or more. It speeds up rustc, and will also speed up any other Rust
program that uses `Vec`s a lot.
make abort intrinsic safe, and correct its documentation
Turns out `std::process::abort` is not the same as the intrinsic, the comment was just wrong. Quoting from the unix implementation:
```
// On Unix-like platforms, libc::abort will unregister signal handlers
// including the SIGABRT handler, preventing the abort from being blocked, and
// fclose streams, with the side effect of flushing them so libc buffered
// output will be printed. Additionally the shell will generally print a more
// understandable error message like "Abort trap" rather than "Illegal
// instruction" that intrinsics::abort would cause, as intrinsics::abort is
// implemented as an illegal instruction.
```
Use min_specialization in liballoc
- Remove a type parameter from `[A]RcFromIter`.
- Remove an implementation of `[A]RcFromIter` that didn't actually
specialize anything.
- Remove unused implementation of `IsZero` for `Option<&mut T>`.
- Change specializations of `[A]RcEqIdent` to use a marker trait version
of `Eq`.
- Remove `BTreeClone`. I couldn't find a way to make this work with
`min_specialization`.
- Add `rustc_unsafe_specialization_marker` to `Copy` and `TrustedLen`.
After this only libcore is the only standard library crate using `feature(specialization)`.
cc #31844
Make `RawVec::grow` mostly non-generic.
`cargo-llvm-lines` shows that, in various benchmarks, `RawVec::grow` is
instantiated 10s or 100s of times and accounts for 1-8% of lines of
generated LLVM IR.
This commit moves most of `RawVec::grow` into a separate function that
isn't parameterized by `T`, which means it doesn't need to be
instantiated many times. This reduces compile time significantly.
r? @ghost
The amortized case is much more common than the exact case, and it is
typically instantiated many times.
Also, we can put a chunk of the code into a function that isn't generic
over T, which reduces the amount of LLVM IR generated quite a lot,
improving compile times.
It's only used once, for `VecDeque`, and can easily be replaced by
something else. The commit changes `grow_if_necessary` to `grow` to
avoid some small regressions caused by changed inlining.
The commit also removes `Strategy::Double`, and streamlines the
remaining variants of `Strategy`.
It's a compile time win on some benchmarks because the many
instantations of `RawVec::grow` are a little smaller.
Btreemap iter intertwined
3 commits:
1. Introduced benchmarks for `BTreeMap::iter()`. Benchmarks named `iter_20` were of the whole iteration process, so I renamed them. Also the benchmarks of `range` that I wrote earlier weren't very good. I included an (awkwardly named) one that compares `iter()` to `range(..)` on the same set, because the contrast is surprising:
```
name ns/iter
btree::map::range_unbounded_unbounded 28,176
btree::map::range_unbounded_vs_iter 89,369
```
Both dig up the same pair of leaf edges. `range(..)` also checks that some keys are correctly ordered, the only thing `iter()` does more is to copy the map's length.
2. Slightly refactoring the code to what I find more readable (not in chronological order of discovery), boosts performance:
```
>cargo-benchcmp.exe benchcmp a1 a2 --threshold 5
name a1 ns/iter a2 ns/iter diff ns/iter diff % speedup
btree::map::find_rand_100 18 17 -1 -5.56% x 1.06
btree::map::first_and_last_10k 64 71 7 10.94% x 0.90
btree::map::iter_0 2,939 2,209 -730 -24.84% x 1.33
btree::map::iter_1 6,845 2,696 -4,149 -60.61% x 2.54
btree::map::iter_100 8,556 3,672 -4,884 -57.08% x 2.33
btree::map::iter_10k 9,292 5,884 -3,408 -36.68% x 1.58
btree::map::iter_1m 10,268 6,510 -3,758 -36.60% x 1.58
btree::map::iteration_mut_100000 478,575 453,050 -25,525 -5.33% x 1.06
btree::map::range_unbounded_unbounded 28,176 36,169 7,993 28.37% x 0.78
btree::map::range_unbounded_vs_iter 89,369 38,290 -51,079 -57.16% x 2.33
btree::set::clone_100_and_remove_all 4,801 4,245 -556 -11.58% x 1.13
btree::set::clone_10k_and_remove_all 529,450 496,030 -33,420 -6.31% x 1.07
```
But you can tell from the `range_unbounded_*` lines that, despite an unwarranted, vengeful attack on the range_unbounded_unbounded benchmark, this change still doesn't allow `iter()` to catch up with `range(..)`.
3. I guess that `range(..)` copes so well because it intertwines the leftmost and rightmost descend towards leaf edges, doing the two root node accesses close together, perhaps exploiting a CPU's internal pipelining? So the third commit distils a version of `range_search` (which we can't use directly because of the `Ord` bound), and we get another boost:
```
cargo-benchcmp.exe benchcmp a2 a3 --threshold 5
name a2 ns/iter a3 ns/iter diff ns/iter diff % speedup
btree::map::first_and_last_100 40 43 3 7.50% x 0.93
btree::map::first_and_last_10k 71 64 -7 -9.86% x 1.11
btree::map::iter_0 2,209 1,719 -490 -22.18% x 1.29
btree::map::iter_1 2,696 2,205 -491 -18.21% x 1.22
btree::map::iter_100 3,672 2,943 -729 -19.85% x 1.25
btree::map::iter_10k 5,884 3,929 -1,955 -33.23% x 1.50
btree::map::iter_1m 6,510 5,532 -978 -15.02% x 1.18
btree::map::iteration_mut_100000 453,050 476,667 23,617 5.21% x 0.95
btree::map::range_included_excluded 405,075 371,297 -33,778 -8.34% x 1.09
btree::map::range_included_included 427,577 397,440 -30,137 -7.05% x 1.08
btree::map::range_unbounded_unbounded 36,169 28,175 -7,994 -22.10% x 1.28
btree::map::range_unbounded_vs_iter 38,290 30,838 -7,452 -19.46% x 1.24
```
But I think this is just fake news from the microbenchmarking media. `iter()` is still trying to catch up with `range(..)`. And we can sure do without another function. So I would skip this 3rd commit.
r? @Mark-Simulacrum
Update btree_map::VacantEntry::insert docs to actually call insert
It looks like they were copied from the `or_insert` docs. This change
makes the example more like the hash_map::VacantEntry::insert docs.
The `remove_current` method only returns the inner `T` and deallocates the list node. This is unnecessary for move operations, where the element is going to be linked back into this (or even a different) `LinkedList`. The `remove_current_as_list` method avoids this by returning the unlinked list node as a new single-element `LinkedList` structure .
Vec drop and truncate: drop using raw slice *mut [T]
By creating a *mut [T] directly (without going through &mut [T]), avoid
questions of validity of the contents of the slice.
Consider the following risky code:
```rust
unsafe {
let mut v = Vec::<bool>::with_capacity(16);
v.set_len(16);
}
```
The intention is that with this change, we avoid one of the soundness
questions about the above snippet, because Vec::drop no longer
produces a mutable slice of the vector's contents.
r? @RalfJung
Add a function to turn Box<T> into Box<[T]>
Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
```rust
pub fn into_boxed_slice<T>(boxed: Box<T>) -> Box<[T]> {
unsafe {
let slice = slice::from_raw_parts_mut(Box::into_raw(boxed), 1);
Box::from_raw(slice)
}
}
```
The only thing that makes me a little uncomfortable is this line :
> The alignment of array types is greater or equal to the alignment of its element type
from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html
But then I see:
> The alignment of &T, &mut T, *const T and *mut T are the same, and are at least the word size.
> The alignment of &[T] is the word size.
from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation
So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)
- Remove a type parameter from `[A]RcFromIter`.
- Remove an implementation of `[A]RcFromIter` that didn't actually
specialize anything.
- Remove unused implementation of `IsZero` for `Option<&mut T>`.
- Change specializations of `[A]RcEqIdent` to use a marker trait version
of `Eq`.
- Remove `BTreeClone`. I couldn't find a way to make this work with
`min_specialization`.
- Add `rustc_unsafe_specialization_marker` to `Copy` and `TrustedLen`.
Fix stable(since) attribute for BTreeMap::remove_entry
Stabilized in #70712.
Maybe checking that the since attributes are added correctly should be automated through tidy? This is the third PR I'm opening that fixes a stable(since) attribute for something meant to be stabilized in 1.43 / 1.44 initially but then only stabilized in 1.45. (the other two are #71571, #71574)
Rollup of 7 pull requests
Successful merges:
- #69041 (proc_macro: Stabilize `Span::resolved_at` and `Span::located_at`)
- #69813 (Implement BitOr and BitOrAssign for the NonZero integer types)
- #70712 (stabilize BTreeMap::remove_entry)
- #71168 (Deprecate `{Box,Rc,Arc}::into_raw_non_null`)
- #71544 (Replace filter_map().next() calls with find_map())
- #71545 (Fix comment in docstring example for Error::kind)
- #71548 (Add missing Send and Sync impls for linked list Cursor and CursorMut.)
Failed merges:
r? @ghost
Add missing Send and Sync impls for linked list Cursor and CursorMut.
Someone pointed out these to me, and i think it's indeed reasonable to add those impl.
r? @Amanieu
Nicholas Nethercote