Implement a faster sort algorithm
Hi everyone, this is my first PR.
I've made some changes to the standard sort algorithm, starting out with a few tweaks here and there, but in the end this endeavour became a complete rewrite of it.
#### Summary
Changes:
* Improved performance, especially on partially sorted inputs.
* Performs less comparisons on both random and partially sorted inputs.
* Decreased the size of temporary memory: the new sort allocates 4x less.
Benchmark:
```
name out1 ns/iter out2 ns/iter diff ns/iter diff %
slice::bench::sort_large_ascending 85,323 (937 MB/s) 8,970 (8918 MB/s) -76,353 -89.49%
slice::bench::sort_large_big_ascending 2,135,297 (599 MB/s) 355,955 (3595 MB/s) -1,779,342 -83.33%
slice::bench::sort_large_big_descending 2,266,402 (564 MB/s) 416,479 (3073 MB/s) -1,849,923 -81.62%
slice::bench::sort_large_big_random 3,053,031 (419 MB/s) 1,921,389 (666 MB/s) -1,131,642 -37.07%
slice::bench::sort_large_descending 313,181 (255 MB/s) 14,725 (5432 MB/s) -298,456 -95.30%
slice::bench::sort_large_mostly_ascending 287,706 (278 MB/s) 243,204 (328 MB/s) -44,502 -15.47%
slice::bench::sort_large_mostly_descending 415,078 (192 MB/s) 271,028 (295 MB/s) -144,050 -34.70%
slice::bench::sort_large_random 545,872 (146 MB/s) 521,559 (153 MB/s) -24,313 -4.45%
slice::bench::sort_large_random_expensive 30,321,770 (2 MB/s) 23,533,735 (3 MB/s) -6,788,035 -22.39%
slice::bench::sort_medium_ascending 616 (1298 MB/s) 155 (5161 MB/s) -461 -74.84%
slice::bench::sort_medium_descending 1,952 (409 MB/s) 202 (3960 MB/s) -1,750 -89.65%
slice::bench::sort_medium_random 3,646 (219 MB/s) 3,421 (233 MB/s) -225 -6.17%
slice::bench::sort_small_ascending 39 (2051 MB/s) 34 (2352 MB/s) -5 -12.82%
slice::bench::sort_small_big_ascending 96 (13333 MB/s) 96 (13333 MB/s) 0 0.00%
slice::bench::sort_small_big_descending 248 (5161 MB/s) 243 (5267 MB/s) -5 -2.02%
slice::bench::sort_small_big_random 501 (2554 MB/s) 490 (2612 MB/s) -11 -2.20%
slice::bench::sort_small_descending 95 (842 MB/s) 63 (1269 MB/s) -32 -33.68%
slice::bench::sort_small_random 372 (215 MB/s) 354 (225 MB/s) -18 -4.84%
```
#### Background
First, let me just do a quick brain dump to discuss what I learned along the way.
The official documentation says that the standard sort in Rust is a stable sort. This constraint is thus set in stone and immediately rules out many popular sorting algorithms. Essentially, the only algorithms we might even take into consideration are:
1. [Merge sort](https://en.wikipedia.org/wiki/Merge_sort)
2. [Block sort](https://en.wikipedia.org/wiki/Block_sort) (famous implementations are [WikiSort](https://github.com/BonzaiThePenguin/WikiSort) and [GrailSort](https://github.com/Mrrl/GrailSort))
3. [TimSort](https://en.wikipedia.org/wiki/Timsort)
Actually, all of those are just merge sort flavors. :) The current standard sort in Rust is a simple iterative merge sort. It has three problems. First, it's slow on partially sorted inputs (even though #29675 helped quite a bit). Second, it always makes around `log(n)` iterations copying the entire array between buffers, no matter what. Third, it allocates huge amounts of temporary memory (a buffer of size `2*n`, where `n` is the size of input).
The problem of auxilliary memory allocation is a tough one. Ideally, it would be best for our sort to allocate `O(1)` additional memory. This is what block sort (and it's variants) does. However, it's often very complicated (look at [this](https://github.com/BonzaiThePenguin/WikiSort/blob/master/WikiSort.cpp)) and even then performs rather poorly. The author of WikiSort claims good performance, but that must be taken with a grain of salt. It performs well in comparison to `std::stable_sort` in C++. It can even beat `std::sort` on partially sorted inputs, but on random inputs it's always far worse. My rule of thumb is: high performance, low memory overhead, stability - choose two.
TimSort is another option. It allocates a buffer of size `n/2`, which is not great, but acceptable. Performs extremelly well on partially sorted inputs. However, it seems pretty much all implementations suck on random inputs. I benchmarked implementations in [Rust](https://github.com/notriddle/rust-timsort), [C++](https://github.com/gfx/cpp-TimSort), and [D](fd518eb310/std/algorithm/sorting.d (L2062)). The results were a bit disappointing. It seems bad performance is due to complex galloping procedures in hot loops. Galloping noticeably improves performance on partially sorted inputs, but worsens it on random ones.
#### The new algorithm
Choosing the best algorithm is not easy. Plain merge sort is bad on partially sorted inputs. TimSort is bad on random inputs and block sort is even worse. However, if we take the main ideas from TimSort (intelligent merging strategy of sorted runs) and drop galloping, then we'll have great performance on random inputs and it won't be bad on partially sorted inputs either.
That is exactly what this new algorithm does. I can't call it TimSort, since it steals just a few of it's ideas. Complete TimSort would be a much more complex and elaborate implementation. In case we in the future figure out how to incorporate more of it's ideas into this implementation without crippling performance on random inputs, it's going to be very easy to extend. I also did several other minor improvements, like reworked insertion sort to make it faster.
There are also new, more thorough benchmarks and panic safety tests.
The final code is not terribly complex and has less unsafe code than I anticipated, but there's still plenty of it that should be carefully reviewed. I did my best at documenting non-obvious code.
I'd like to notify several people of this PR, since they might be interested and have useful insights:
1. @huonw because he wrote the [original merge sort](https://github.com/rust-lang/rust/pull/11064).
2. @alexcrichton because he was involved in multiple discussions of it.
3. @veddan because he wrote [introsort](https://github.com/veddan/rust-introsort) in Rust.
4. @notriddle because he wrote [TimSort](https://github.com/notriddle/rust-timsort) in Rust.
5. @bluss because he had an attempt at writing WikiSort in Rust.
6. @gnzlbg, @rkruppe, and @mark-i-m because they were involved in discussion #36318.
**P.S.** [quickersort](https://github.com/notriddle/quickersort) describes itself as being universally [faster](https://github.com/notriddle/quickersort/blob/master/perf.txt) than the standard sort, which is true. However, if this PR gets merged, things might [change](https://gist.github.com/stjepang/b9f0c3eaa0e1f1280b61b963dae19a30) a bit. ;)
rustbuild: Implement distcheck
This commit implements the `distcheck` target for rustbuild which is only ever
run on our nightly bots. This essentially just creates a tarball, un-tars it,
and then runs a full build, validating that the release tarballs do indeed have
everything they need to build Rust.
make `alloc` and `collections` compilable for thumbv6m-none-eabi
by cfging away `alloc::Arc` and changing OOM to abort for this target
r? @alexcrichton
cc @thejpster
This commit implements the `distcheck` target for rustbuild which is only ever
run on our nightly bots. This essentially just creates a tarball, un-tars it,
and then runs a full build, validating that the release tarballs do indeed have
everything they need to build Rust.
Since merge_sort is generic and collapse isn't, that means calls to
collapse won't be inlined. inlined. Therefore, we must stick an
`#[inline]` above `fn collapse`.
Specialization for Extend<&T> for vec
Specialize to use copy_from_slice when extending a Vec with &[T] where
T: Copy.
This specialization results in `.clone()` not being called in `extend_from_slice` and `extend` when the element is `Copy`.
Fixes#38021
rustbuild: Use src/rustc for assembled compilers
The `src/rustc` path is intended for assembling a compiler (e.g. the bare bones)
not actually compiling the whole compiler itself. This path was accidentally
getting hijacked to represent the whole compiler being compiled, so let's
redirect that elsewhere for that particular cargo project.
Closes#38039
annotate stricter lifetimes on LateLintPass methods to allow them to forward to a Visitor
this unblocks clippy (rustup blocked after #37918)
clippy has lots of lints that internally call an `intravisit::Visitor`, but the current lifetimes on `LateLintPass` methods conflicted with the required lifetimes (there was no connection between the HIR elements and the `TyCtxt`)
r? @Manishearth
This is a complete rewrite of the standard sort algorithm. The new algorithm
is a simplified variant of TimSort. In summary, the changes are:
* Improved performance, especially on partially sorted inputs.
* Performs less comparisons on both random and partially sorted inputs.
* Decreased the size of temporary memory: the new sort allocates 4x less.
Update book/ffi to use catch_unwind
r? @GuillaumeGomez
The doc mentioned to spawn a new thread instead of using catch_unwind, which has been the recommended way to catch panics for foreign function interfaces for a few releases now.
This commit fixes that.
reference: fix definition of :tt
The reference says that $x:tt matches "either side of the `=>` in macro_rules` which is technically true but completely uninformative. This changes that bullet point to what the book says (a single token or sequence of token trees inside brackets).
Warn when an import list is empty
For a given file
```rust
use std::*;
use std::{};
```
output the following warnings
```
warning: unused import: `use std::{};`, #[warn(unused_imports)] on by default
--> file.rs:2:1
|
2 | use std::{};
| ^^^^^^^^^^^^
warning: unused import: `std::*;`, #[warn(unused_imports)] on by default
--> file.rs:1:5
|
1 | use std::*;
| ^^^^^^^
```
r? @GuillaumeGomez
The doc mentioned to spawn a new thread instead of using catch_unwind, which has been the recommended way to catch panics for foreign function interfaces for a few releases now.
rustdoc: Sort lines in search index and implementors js
This means the files are generated deterministically even with rustdoc running in parallel.
Fixes the first part of #30220.
mk: Switch rustbuild to the default build system
This commit switches the default build system for Rust from the makefiles to
rustbuild. The rustbuild build system has been in development for almost a year
now and has become quite mature over time. This commit is an implementation of
the proposal on [internals] which slates deletion of the makefiles on
2017-02-02.
[internals]: https://internals.rust-lang.org/t/proposal-for-promoting-rustbuild-to-official-status/4368
This commit also updates various documentation in `README.md`,
`CONTRIBUTING.md`, `src/bootstrap/README.md`, and throughout the source code of
rustbuild itself.
This commit switches the default build system for Rust from the makefiles to
rustbuild. The rustbuild build system has been in development for almost a year
now and has become quite mature over time. This commit is an implementation of
the proposal on [internals] which slates deletion of the makefiles on
2016-01-02.
[internals]: https://internals.rust-lang.org/t/proposal-for-promoting-rustbuild-to-official-status/4368
This commit also updates various documentation in `README.md`,
`CONTRIBUTING.md`, `src/bootstrap/README.md`, and throughout the source code of
rustbuild itself.
Closes#37858
Forward more ExactSizeIterator methods and `is_empty` edits
- Forward ExactSizeIterator methods in more places, like `&mut I` and `Box<I>` iterator impls.
- Improve `VecDeque::is_empty` itself (see commit 4)
- All the collections iterators now have `len` or `is_empty` forwarded if doing so is a benefit. In the remaining cases, they already use a simple size hint (using something like a stored `usize` value), which is sufficient for the default implementation of len and is_empty.
Remove redundant assertion near is_char_boundary
Follow-up from #38056. `is_char_boundary` already checks for `idx <= len`, so, an extra assertion is redundant.
Remove Self: Sized from Iterator::nth
It is an unnecessary restriction; nth neither needs self to be sized
nor needs to be exempted from the trait object.
It increases the utility of the nth method, because type specific
implementations are available through `&mut I` or through an iterator
trait object.
It is a backwards compatible change due to the special cases of the
`where Self: Sized` bound; it was already optional to include this bound
in `Iterator` implementations.
Smarter HashMap/HashSet pre-allocation for extend/from_iter
HashMap/HashSet from_iter and extend are making totally different assumptions.
A more balanced decision may allocate half the lower hint (rounding up). For "well defined" iterators this effectively limits the worst case to two resizes (the initial reserve + one resize).
cc #36579
cc @bluss
Simplify calling find_implied_output_region.
@nnethercote added the optimization that find_implied_output_region
takes a closure as an optimization in #37014, but passing an iterator is
simpler, and more ergonomic for callers.
Don't apply msvc link opts for non-opt build
`/OPT:REF,ICF` sometimes takes lots of time. It makes no sense to apply them when doing debug build. MSVC's linker by default disables these optimizations when `/DEBUG` is specified, unless they are explicitly passed.
@nnethercote added the optimization that find_implied_output_region
takes a closure as an optimization in #37014, but passing an iterator is
simpler, and more ergonomic for callers.
Refactor ty::FnSig to contain a &'tcx Slice<Ty<'tcx>>
We refactor this in order to achieve the following wins:
- Decrease the size of `FnSig` (`Vec` + `bool`: 32, `&Slice` + `bool`: 24).
- Potentially decrease total allocated memory due to arena-allocating `FnSig` inputs/output; since they are allocated in the type list arena, other users of type lists can reuse the same allocation for an equivalent type list.
- Remove the last part of the type system which needs drop glue (#37965 removed the other remaining part). This makes arenas containing `FnSig` faster to drop (since we don't need to drop a Vec for each one), and makes reusing them without clearing/dropping potentially possible.
r? @eddyb
configure: only req CMake if we're building LLVM
CMake is only necessary if LLVM is going to be built and not in any
other case.
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
For a given file
```rust
use std::*;
use std::{};
```
output the following warnings
```
warning: unused import: `use std::{};`, #[warn(unused_imports)] on by default
--> file.rs:2:1
|
2 | use std::{};
| ^^^^^^^^^^^^
warning: unused import: `std::*;`, #[warn(unused_imports)] on by default
--> file.rs:1:5
|
1 | use std::*;
| ^^^^^^^
```
Point arg num mismatch errors back to their definition
This PR updates the arg num errors (like E0061) to point back at the function definition where they were defined.
Before:
```
error[E0061]: this function takes 2 parameters but 1 parameter was supplied
--> E0061.rs:18:7
|
18 | f(0);
| ^
|
= note: the following parameter types were expected:
= note: u16, &str
```
Now:
```
error[E0061]: this function takes 2 parameters but 1 parameter was supplied
--> E0061.rs:18:7
|
11 | fn f(a: u16, b: &str) {}
| ------------------------ defined here
...
18 | f(0);
| ^ expected 2 parameters
```
This is an incremental improvement. We probably want to underline only the function name and also have support for functions defined in crates outside of the current crate.
r? @nikomatsakis
