Add liballoc impl SpecFromElem for i8 #73529

pickfire · 2020-06-20T03:12:24Z

Speedup vec![1_i8; N] for non-zero element.

Before
test do_bench_from_elem_i8 ... bench: 130 ns/iter (+/- 7) = 61 MB/s
test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 4) = 66 MB/s
After
test do_bench_from_elem_i8 ... bench: 123 ns/iter (+/- 7) = 65 MB/s
test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 5) = 66 MB/s

No speed difference if element is already zero.

#[bench]
fn do_bench_from_elem_i8(b: &mut Bencher) {
    b.bytes = 8 as u64;
    b.iter(|| {
        let dst = ve::vec![10_i8; 100];
        assert_eq!(dst.len(), 100);
        assert!(dst.iter().all(|x| *x == 10));
    })
}

As suggested by @cuviper
https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/SpecForElem.20for.20other.20integers

r? @cuviper
CC @joshtriplett

Edit: Wow, I just realized both reviewers are Josh.

rust-highfive · 2020-06-20T03:19:22Z

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---
    Checking alloc v0.0.0 (/checkout/src/liballoc)
error[E0053]: method `from_elem` has an incompatible type for trait
    --> src/liballoc/vec.rs:1808:41
     |
1795 |     fn from_elem(elem: Self, n: usize) -> Vec<Self>;
     |                                           --------- type in trait
...
1808 |     fn from_elem(elem: i8, n: usize) -> Vec<u8> {
     |                                         ^^^^^^^ expected `i8`, found `u8`
     = note: expected fn pointer `fn(_, _) -> vec::Vec<i8>`
                found fn pointer `fn(_, _) -> vec::Vec<u8>`

error[E0308]: mismatched types
error[E0308]: mismatched types
    --> src/liballoc/vec.rs:1814:46
     |
1814 |             ptr::write_bytes(v.as_mut_ptr(), elem, n);
     |                                              ^^^^ expected `u8`, found `i8`
help: you can convert an `i8` to `u8` and panic if the converted value wouldn't fit
     |
     |
1814 |             ptr::write_bytes(v.as_mut_ptr(), elem.try_into().unwrap(), n);

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0053, E0308.
---
  local time: Sat Jun 20 03:17:31 UTC 2020
  network time: Sat, 20 Jun 2020 03:17:31 GMT
== end clock drift check ==

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rust-highfive · 2020-06-20T03:26:59Z

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---
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error[E0053]: method `from_elem` has an incompatible type for trait
    --> src/liballoc/vec.rs:1808:41
     |
1795 |     fn from_elem(elem: Self, n: usize) -> Vec<Self>;
     |                                           --------- type in trait
...
1808 |     fn from_elem(elem: i8, n: usize) -> Vec<u8> {
     |                                         ^^^^^^^ expected `i8`, found `u8`
     = note: expected fn pointer `fn(_, _) -> vec::Vec<i8>`
                found fn pointer `fn(_, _) -> vec::Vec<u8>`

    Checking rustc-demangle v0.1.16
---
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Step 7/7 : ENV SCRIPT python3 ../x.py test src/tools/expand-yaml-anchors &&            python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu &&            python3 ../x.py build --stage 0 src/tools/build-manifest &&            python3 ../x.py test --stage 0 src/tools/compiletest &&            python3 ../x.py test src/tools/tidy &&            python3 ../x.py doc --stage 0 src/libstd &&            /scripts/validate-toolstate.sh
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---
    Checking alloc v0.0.0 (/checkout/src/liballoc)
error[E0053]: method `from_elem` has an incompatible type for trait
    --> src/liballoc/vec.rs:1808:41
     |
1795 |     fn from_elem(elem: Self, n: usize) -> Vec<Self>;
     |                                           --------- type in trait
...
1808 |     fn from_elem(elem: i8, n: usize) -> Vec<u8> {
     |                                         ^^^^^^^ expected `i8`, found `u8`
     = note: expected fn pointer `fn(_, _) -> vec::Vec<i8>`
                found fn pointer `fn(_, _) -> vec::Vec<u8>`

    Checking cfg-if v0.1.10
---
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== end clock drift check ==

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@cuviper

Speedup vec![1_i8; N] for non-zero element. Before test do_bench_from_elem_i8 ... bench: 130 ns/iter (+/- 7) = 61 MB/s test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 4) = 66 MB/s After test do_bench_from_elem_i8 ... bench: 123 ns/iter (+/- 7) = 65 MB/s test do_bench_from_elem_u8 ... bench: 121 ns/iter (+/- 5) = 66 MB/s No speed difference if element is already zero. #[bench] fn do_bench_from_elem_i8(b: &mut Bencher) { b.bytes = 8 as u64; b.iter(|| { let dst = ve::vec![10_i8; 100]; assert_eq!(dst.len(), 100); assert!(dst.iter().all(|x| *x == 10)); }) } As suggested by @cuviper https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/SpecForElem.20for.20other.20integers

cuviper · 2020-06-26T18:14:00Z

@bors r+

bors · 2020-06-26T18:14:02Z

📌 Commit f66bcc5 has been approved by cuviper

@ghost

…arth Rollup of 12 pull requests Successful merges: - rust-lang#72771 (Warn if linking to a private item) - rust-lang#72937 (Fortanix SGX target libunwind build process changes) - rust-lang#73485 (Perform obligation deduplication to avoid buggy `ExistentialMismatch`) - rust-lang#73529 (Add liballoc impl SpecFromElem for i8) - rust-lang#73579 (add missing doc links) - rust-lang#73627 (Shortcuts for min/max on double-ended BTreeMap/BTreeSet iterators) - rust-lang#73691 (Bootstrap: detect Windows based on sys.platform) - rust-lang#73694 (Document the Self keyword) - rust-lang#73718 (Document the super keyword) - rust-lang#73728 (Document some invariants correctly/more) - rust-lang#73738 (Remove irrelevant comment) - rust-lang#73765 (Remove blank line) Failed merges: r? @ghost

pickfire · 2020-06-27T05:27:54Z

I don't know offhand how to get the compiler to generate a rep stosw (or d or q). (And in any case, it would need benchmarking to ensure that it's actually faster.)
I don't know if that would be a performance win, though it might be worth benchmarking (via a cfg-guarded specialization).

I believe we should add some TODO there from @joshtriplett for the other stuff but since I am not sure how to get those working.

rust-highfive assigned cuviper Jun 20, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 20, 2020

pickfire force-pushed the liballoc-specfromelem-i8 branch from 8f84b30 to be8ae19 Compare June 20, 2020 03:21

pickfire force-pushed the liballoc-specfromelem-i8 branch from be8ae19 to fa9555c Compare June 20, 2020 03:46

pickfire force-pushed the liballoc-specfromelem-i8 branch from fa9555c to f66bcc5 Compare June 20, 2020 03:55

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 26, 2020

Manishearth mentioned this pull request Jun 26, 2020

Rollup of 12 pull requests #73779

Merged

bors merged commit 23b0776 into rust-lang:master Jun 27, 2020

pickfire deleted the liballoc-specfromelem-i8 branch June 27, 2020 06:46

cuviper added this to the 1.46 milestone May 2, 2024

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