Copy 1-/2-element arrays as scalars, not vectors #116479

scottmcm · 2023-10-06T08:21:48Z

For [T; 1] it's silly to copy as <1 x T> when we can just copy as T.
And treat [T; 2] as a scalar pair (like (T, T)) when copying it.

Inspired by #101210 (comment), which pointed out that Option<[u8; 1]> was codegenning worse than Option<u8>.

(I'm not sure why LLVM doesn't optimize out <1 x u8>, but might as well just not emit it in the first place in this codepath.)

rustbot · 2023-10-06T08:21:56Z

r? @davidtwco

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For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`. And treat `[T; 2]` as a scalar pair like `(T, T)` when copying it.

compiler-errors · 2023-10-06T15:42:25Z

@bors try @rust-timer queue

bors · 2023-10-06T15:43:34Z

⌛ Trying commit b5a9dd7 with merge b571f53...

Copy 1-/2-element arrays as scalars, not vectors For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`. And treat `[T; 2]` as a scalar pair (like `(T, T)`) when copying it. Inspired by rust-lang#101210 (comment), which pointed out that `Option<[u8; 1]>` was codegenning worse than `Option<u8>`. (I'm not sure *why* LLVM doesn't optimize out `<1 x u8>`, but might as well just not emit it in the first place in this codepath.)

compiler-errors · 2023-10-06T16:01:46Z

(don't expect perf to come back up with much but we'll see what it does to rustc at least)

bors · 2023-10-06T16:56:49Z

☀️ Try build successful - checks-actions
Build commit: b571f53 (b571f53b9ed2b2e638a502786e099948a6809c43)

erikdesjardins · 2023-10-06T20:38:46Z

compiler/rustc_codegen_llvm/src/type_of.rs

+                // (Having types like `<1 x u8>` is silly.)
+                let ety = element.llvm_type(cx);
+                return Some(cx.type_array(ety, *count));
+            } else if count.is_power_of_two() {


This is already inside if count.is_power_of_two(), did you mean to remove it from the outer condition?

Good point! I think I'd intended to remove it from the outer, but with the tests not actually failing I guess I never did.

@rustbot author

rust-timer · 2023-10-06T21:41:52Z

Finished benchmarking commit (b571f53): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.2%]	8
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.2%]	8

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	4.5%	[4.5%, 4.5%]	1
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.5%, -2.4%]	2
All ❌✅ (primary)	4.5%	[4.5%, 4.5%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 625.189s -> 622.203s (-0.48%)
Artifact size: 270.66 MiB -> 270.65 MiB (-0.00%)

Copy 1-element arrays as scalars, not vectors For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`. Inspired by rust-lang#101210 (comment), which pointed out that `Option<[u8; 1]>` was codegenning worse than `Option<u8>`. (I'm not sure *why* LLVM doesn't optimize out `<1 x u8>`, but might as well just not emit it in the first place in this codepath.) --- I think I bit off too much in rust-lang#116479; let me try just the scalar case first. r? `@ghost`

…rors Copy 1-element arrays as scalars, not vectors For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`. Inspired by rust-lang#101210 (comment), which pointed out that `Option<[u8; 1]>` was codegenning worse than `Option<u8>`. (I'm not sure *why* LLVM doesn't optimize out `<1 x u8>`, but might as well just not emit it in the first place in this codepath.) --- I think I bit off too much in rust-lang#116479; let me try just the scalar case first. r? `@ghost`

scottmcm · 2024-05-04T01:42:13Z

Given #123185, I think this is no longer a good idea.

rustbot assigned davidtwco Oct 6, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 6, 2023

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Copy 1-/2-element arrays as scalars, not vectors

b5a9dd7

For `[T; 1]` it's silly to copy as `<1 x T>` when we can just copy as `T`. And treat `[T; 2]` as a scalar pair like `(T, T)` when copying it.

scottmcm force-pushed the no-1-simd branch from 13b6eaf to b5a9dd7 Compare October 6, 2023 09:19

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 6, 2023

compiler-errors self-assigned this Oct 6, 2023

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erikdesjardins reviewed Oct 6, 2023

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scottmcm mentioned this pull request Oct 7, 2023

Copy 1-element arrays as scalars, not vectors #116510

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scottmcm marked this pull request as draft October 8, 2023 07:07

scottmcm closed this May 4, 2024

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Copy 1-/2-element arrays as scalars, not vectors #116479

Copy 1-/2-element arrays as scalars, not vectors #116479

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scottmcm commented May 4, 2024

Copy 1-/2-element arrays as scalars, not vectors #116479

Copy 1-/2-element arrays as scalars, not vectors #116479

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rust-timer commented Oct 6, 2023

Overall result: ❌ regressions - ACTION NEEDED

Instruction count

Max RSS (memory usage)

Cycles

Binary size

scottmcm commented May 4, 2024