Use a lookup table in `#[derive(Debug)]` for fieldless enums by DaniPopes · Pull Request #114190 · rust-lang/rust

DaniPopes · 2023-07-28T21:34:07Z

Index into a constant array containing the Debug strings instead of matching directly.

Opening as draft since I don't know the performance implications of this, and would be nice to have a perf run done on this PR.

Closes #114106
Refs #114106 (comment), #106884

rustbot · 2023-07-28T21:34:15Z

r? @davidtwco

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compiler-errors · 2023-07-28T23:48:57Z

@bors try @rust-timer queue

bors · 2023-07-28T23:49:06Z

⌛ Trying commit 4d76b33 with merge fdbb5360b46d689c4d8ca16144ce40a45cb83099...

bors · 2023-07-29T01:02:03Z

☀️ Try build successful - checks-actions
Build commit: fdbb5360b46d689c4d8ca16144ce40a45cb83099 (fdbb5360b46d689c4d8ca16144ce40a45cb83099)

clubby789 · 2023-07-29T12:21:50Z

I haven't looked at the implementation yet but my previous attempt at this (#109615) might be of interest

rust-timer · 2023-07-29T13:54:43Z

Finished benchmarking commit (fdbb5360b46d689c4d8ca16144ce40a45cb83099): 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)	0.8%	[0.3%, 2.6%]	37
Regressions ❌ (secondary)	1.5%	[0.3%, 7.1%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	0.8%	[0.3%, 2.6%]	37

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)	3.9%	[0.5%, 11.7%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.0%	[-4.0%, -4.0%]	1
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	2.6%	[-4.0%, 11.7%]	6

Cycles

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)	1.8%	[0.9%, 2.7%]	7
Regressions ❌ (secondary)	2.0%	[1.3%, 2.9%]	5
Improvements ✅ (primary)	-3.1%	[-5.1%, -1.3%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-5.1%, 2.7%]	13

Binary size

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)	0.3%	[0.0%, 1.9%]	67
Regressions ❌ (secondary)	1.5%	[0.1%, 3.8%]	16
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.1%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[-0.4%, 1.9%]	72

Bootstrap: 649.861s -> 651.137s (0.20%)

Swatinem · 2023-07-30T12:55:41Z

+///             const { ["A", "B", "C"] }[match self {
+///                 Fieldless::A => 0usize,
+///                 Fieldless::B => 1usize,
+///                 Fieldless::C => 2usize,
+///             }],


I have been looking into a similar case recently as noted in #114106 (comment), and part of the problem might be the match statement itself.

For data-less enums with dense/contiguous discriminants, you should be able to just use something like *self as u32 to use the discriminant directly as an index instead of via the match roundtrip.

davidtwco · 2023-07-31T08:21:55Z

I don't think the perf results warrant landing this as-is, if you have other ideas that you'd like to try then we can re-run the benchmarking with those :)

DaniPopes · 2023-08-02T15:28:09Z

Closing given negative perf results and I don't really have any idea on how to improve this.

@ghost

Implement `Debug` for C-like enums with a concatenated string Fixes: #133945 Related to: #88793 #114106 Continuation of: #109615 #114190 This optimizes the expansion of `derive(Debug)` for C-like enums with 10 or more variants to: ```rust fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result { const __NAMES: &str = "AAAAABBBBCCDDDDDDDDEFFFFFFFFFFFFFGGGGGGHatsuneIIIIIIIJJJJJJJJJ"; const __OFFSET: [usize; 11] = [0usize, 5usize, 9usize, 11usize, 19usize, 20usize, 33usize, 39usize, 46usize, 53usize, 62usize]; let __d = ::core::intrinsics::discriminant_value(self) as usize; ::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES, &__OFFSET, __d) } ``` The number 10 was decided based on both [the rustc perf result](#155452 (comment)) and the results from the dedicated perf tool for this scenario: <img width="1920" height="1280" alt="chart-link" src="https://github.com/user-attachments/assets/1327ee63-ce05-482b-a1fd-c56e919250ea" /> <img width="1920" height="1280" alt="chart-obj" src="https://github.com/user-attachments/assets/1cb831d3-c9dc-469f-b87d-03cf633f8335" /> See https://codeberg.org/makai410/debug-fmt-perf for more details. r? @ghost (I want to see the perf first)

@ghost

Implement `Debug` for C-like enums with a concatenated string Fixes: rust-lang/rust#133945 Related to: rust-lang/rust#88793 rust-lang/rust#114106 Continuation of: rust-lang/rust#109615 rust-lang/rust#114190 This optimizes the expansion of `derive(Debug)` for C-like enums with 10 or more variants to: ```rust fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result { const __NAMES: &str = "AAAAABBBBCCDDDDDDDDEFFFFFFFFFFFFFGGGGGGHatsuneIIIIIIIJJJJJJJJJ"; const __OFFSET: [usize; 11] = [0usize, 5usize, 9usize, 11usize, 19usize, 20usize, 33usize, 39usize, 46usize, 53usize, 62usize]; let __d = ::core::intrinsics::discriminant_value(self) as usize; ::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES, &__OFFSET, __d) } ``` The number 10 was decided based on both [the rustc perf result](rust-lang/rust#155452 (comment)) and the results from the dedicated perf tool for this scenario: <img width="1920" height="1280" alt="chart-link" src="https://github.com/user-attachments/assets/1327ee63-ce05-482b-a1fd-c56e919250ea" /> <img width="1920" height="1280" alt="chart-obj" src="https://github.com/user-attachments/assets/1cb831d3-c9dc-469f-b87d-03cf633f8335" /> See https://codeberg.org/makai410/debug-fmt-perf for more details. r? @ghost (I want to see the perf first)

@ghost

Implement `Debug` for C-like enums with a concatenated string Fixes: rust-lang/rust#133945 Related to: rust-lang/rust#88793 rust-lang/rust#114106 Continuation of: rust-lang/rust#109615 rust-lang/rust#114190 This optimizes the expansion of `derive(Debug)` for C-like enums with 10 or more variants to: ```rust fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result { const __NAMES: &str = "AAAAABBBBCCDDDDDDDDEFFFFFFFFFFFFFGGGGGGHatsuneIIIIIIIJJJJJJJJJ"; const __OFFSET: [usize; 11] = [0usize, 5usize, 9usize, 11usize, 19usize, 20usize, 33usize, 39usize, 46usize, 53usize, 62usize]; let __d = ::core::intrinsics::discriminant_value(self) as usize; ::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES, &__OFFSET, __d) } ``` The number 10 was decided based on both [the rustc perf result](rust-lang/rust#155452 (comment)) and the results from the dedicated perf tool for this scenario: <img width="1920" height="1280" alt="chart-link" src="https://github.com/user-attachments/assets/1327ee63-ce05-482b-a1fd-c56e919250ea" /> <img width="1920" height="1280" alt="chart-obj" src="https://github.com/user-attachments/assets/1cb831d3-c9dc-469f-b87d-03cf633f8335" /> See https://codeberg.org/makai410/debug-fmt-perf for more details. r? @ghost (I want to see the perf first)

Use a lookup table in #[derive(Debug)] for field-less enums

4d76b33

rustbot assigned davidtwco Jul 28, 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 Jul 28, 2023

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 29, 2023

Swatinem reviewed Jul 30, 2023

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DaniPopes closed this Aug 2, 2023

makai410 mentioned this pull request Apr 17, 2026

Implement Debug for C-like enums with a concatenated string #155452

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Use a lookup table in `#[derive(Debug)]` for fieldless enums - #114190

Use a lookup table in `#[derive(Debug)]` for fieldless enums#114190
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Overall result: ❌ regressions - ACTION NEEDED

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