match: Use an aggregate equality comparison for constant array/slice patterns by jakubadamw · Pull Request #155216 · rust-lang/rust

jakubadamw · 2026-04-12T21:59:41Z

When every element in an array or slice pattern is a constant and there is no .. subpattern, the match builder will now emit a single call to PartialEq::eq instead of comparing each element from the value one by one against the respective constant in the pattern.

This drastically reduces the number of MIR basic blocks for large constant-array matches – e.g. a 64-element [u8; 64] match previously generated 64 separate comparison blocks and now generates just one PartialEq::eq call that LLVM can lower to a memcmp(). The optimisation is gated on having at least two constant elements, meaning single-element arrays will still use a plain scalar comparison.

Example:

const FOO: [u8; 64] = *b"0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";

pub fn foo(x: &[u8; 64]) -> bool {
    // Before: 64 basic blocks, one per byte.
    // After:  a single `PartialEq::eq()` call.
    matches!(x, &FOO)
}

Closes #103073.
Closes #110870.

rustbot · 2026-04-12T21:59:50Z

Some changes occurred in match lowering

cc @Nadrieril

rustbot · 2026-04-12T21:59:52Z

r? @JonathanBrouwer

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JonathanBrouwer · 2026-04-13T18:15:32Z

@bors try @rust-timer queue

match: Use an aggregate equality comparison for constant array/slice patterns when possible

rust-bors · 2026-04-13T20:24:30Z

☀️ Try build successful (CI)
Build commit: 5a6dba6 (5a6dba60b1150a8b57fc739b0829fa4a65c5b8b3, parent: 14196dbfa3eb7c30195251eac092b1b86c8a2d84)

rust-timer · 2026-04-13T21:13:34Z

Finished benchmarking commit (5a6dba6): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

Results (primary -0.0%, secondary 0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.0%, 2.8%]	2
Regressions ❌ (secondary)	2.7%	[1.2%, 5.0%]	3
Improvements ✅ (primary)	-3.9%	[-3.9%, -3.9%]	1
Improvements ✅ (secondary)	-1.8%	[-2.2%, -1.6%]	3
All ❌✅ (primary)	-0.0%	[-3.9%, 2.8%]	3

Cycles

Results (primary -2.3%, secondary 14.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	14.4%	[2.4%, 26.4%]	2
Improvements ✅ (primary)	-2.3%	[-3.1%, -1.7%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-3.1%, -1.7%]	4

Binary size

Results (primary -0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.1%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.2%, 0.1%]	5

Bootstrap: 491.114s -> 490.988s (-0.03%)
Artifact size: 394.23 MiB -> 394.33 MiB (0.03%)

JonathanBrouwer · 2026-04-15T19:44:15Z

@rustbot reroll
Not familiar enough with this code to comfortably review this, but from a quick glance this looks great, thanks <3

dianne · 2026-04-17T21:38:39Z

+            if let PatKind::Constant { value } = pat.kind {
+                Some(ty::Const::new_value(tcx, value.valtree, value.ty))
+            } else {
+                None
+            }


It might also be worth reconstructing aggregate constants for arrays/slices of arrays of constants, etc.? I'm not a specialization expert, but it looks like arrays of bytewise-comparable things are also bytewise-comparable, at least for common array lengths¹. Since array and slice equality are specialized based on their element types' bytewise-comparability, we should be able to get better codegen for nested array patterns too (as long as the inner arrays are of one of those common lengths), I think?

View changes since the review

Footnotes

https://github.com/rust-lang/rust/blob/f29256dd1420dc681bf4956e3012ffe9eccdc7e7/library/core/src/cmp/bytewise.rs#L74-L85 ↩

@dianne, interesting. I’ll look into this next! 🙂

dianne · 2026-04-17T21:49:51Z

+                // When there is no `..`, all elements are constants, and
+                // there are at least two of them, collapse the individual
+                // element subpairs into a single aggregate comparison that
+                // is performed after the length check.
+                if slice.is_none()


An additional possibility: even if there is a .., the comparisons for the sub-slices before and after the .. could be done via aggregate equality when applicable. Credit to #121540, which I think did this?

Edit: assuming prefixes and suffixes are typically small and hand-written, it's probably not worth the trouble to use aggregate equality for them.

Even if only handling the case with no .., it might be worth moving the special-casing into prefix_slice_suffix to share it between PatKind::Slice and PatKind::Array, since that's where the commonalities live.

Edit: after prefix_slice_suffix's cleanup in #154943, I don't think it makes much sense to put this in there. I still think the logic for deciding whether to use aggregate equality is complex enough that it could be worth factoring out, but that's probably not the way to do it.

View changes since the review

jackh726 · 2026-04-23T18:43:22Z

@dianne do you want to take over review here?

dianne

I don't have much context on the ctfe or const traits side of things to evaluate the approach. cc @oli-obk maybe? It feels like we have to make some sort of compromise here to avoid calling anything const-unstable. Possibly we could block on const_cmp stabilizing, or possibly we could have some workaround until then if we want to land this first.

I think I can handle the technical review, at least. Tentatively, r? me (though feel free to steal the assignment if that'd be easier ^^)

View changes since this review

…n't get captured by this logic

…` features

…ng it Following the review feedback, `const_to_pat()` now records the original constant value on the array and slice pattern nodes it expands, and match lowering reads that value back instead of attempting to reconstruct an aggregate constant from the individual element subpatterns. This has two consequences. First, hand-written array and slice patterns are no longer collapsed into aggregate comparisons; the user's intent to match element by element is respected before the MIR boundary. Only patterns that were expanded from an actual constant (a named constant or a byte-string literal) use the aggregate `PartialEq::eq` comparison, and for those the semantics of matching against a constant and comparing with `PartialEq::eq` coincide. Second, nested constant arrays now benefit from the aggregate comparison as well, since the recorded value covers the whole constant, whereas the reconstruction required every immediate element of the pattern to be a leaf constant.

Give the `PartialEq::eq` call emitted for constant array/slice patterns `UnwindAction::Unreachable` instead of an unwind edge. The built-in `PartialEq` implementations for arrays and slices can be trusted not to panic, and the unwind edge would not be harmless: since the aggregate comparison replaces a series of `SwitchInt` tests that could never unwind, the extra edge would make borrow-checking stricter about the drop order in unwinding code, turning previously accepted programs into errors. The string equality tests keep their unwind edge, as they have always had one.

…regates too

- Drop the extra length test for constant slices; `PartialEq::eq` already checks the length. - Make an expanded constant on a non-array, non-slice pattern a bug. - Add a FIXME to lift the const-context restriction with `const_cmp`. - Justify keeping `AGGREGATE_EQ_MIN_LEN` in its doc comment. - Note in `core` that the bytewise `PartialEq` specialisations must not panic, and test that matches resolve to them.

Call the `raw_eq` and `compare_bytes` intrinsics directly instead of `PartialEq::eq`. Add a MIR test for a constant below `AGGREGATE_EQ_MIN_LEN`. Remove the codegen-units test and the comments in `core`.

rustbot · 2026-11-07T13:41:58Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

- Nest the aggregate slice comparison under a slice length test, so that patterns of the same length share one length check. - Move the array and slice aggregate comparisons into their own helpers. - Assert that the intrinsics called for aggregate comparisons cannot unwind. - Correct the comments on which intrinsic is not const-stable. - Check in the MIR test that the scrutinee is not indexed, including before the first `raw_eq`. - Check that the fallback `switchInt`s index into the scrutinee. - Add MIR tests for a short slice, a multi-arm slice match and wider slice elements. - Describe the length threshold in the MIR test without naming the constant. - Add a `#![no_core]` MIR test for crates without the intrinsic lang items. - Add run-time tests for multi-arm and wider-element slice matches.

jakubadamw · 2026-11-07T13:55:52Z

@rustbot label -S-waiting-on-author +S-waiting-on-review

@dianne thank you for another round of review. Hopefully we’re getting there!
I wonder, this may deserve one more perf run after all the recent rounds?

Nadrieril · 2026-11-07T15:17:46Z

Never hurts!

@bors try @rust-timer queue

match: Use an aggregate equality comparison for constant array/slice patterns

rust-bors · 2026-11-07T16:23:30Z

☀️ Try build successful (CI)
Build commit: 4c09b15 (4c09b1582280874f43344561ae8887376b676546)
Base parent: e30d778 (e30d778f3029b78bb9c2e0dbe11a3b7389b652c4)

rust-timer · 2026-11-07T18:25:38Z

Finished benchmarking commit (4c09b15): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@bors rollup=iffy rustc-perf
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

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

Max RSS (memory usage)

Results (primary -6.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-6.1%	[-6.1%, -6.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-6.1%	[-6.1%, -6.1%]	1

Cycles

Results (primary 2.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.5%	[1.8%, 3.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[1.8%, 3.1%]	2

Binary size

Results (primary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.1%]	15
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-0.5%, -0.1%]	15

Bootstrap: 484.651s -> 482.321s (-0.48%)
Artifact size: 406.22 MiB -> 406.20 MiB (-0.01%)

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 Apr 12, 2026

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jakubadamw marked this pull request as draft April 12, 2026 23:12

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jakubadamw marked this pull request as ready for review April 12, 2026 23:50

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rust-bors Bot pushed a commit that referenced this pull request Apr 13, 2026

Auto merge of #155216 - jakubadamw:issue-110870-103073, r=<try>

5a6dba6

match: Use an aggregate equality comparison for constant array/slice patterns when possible

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 13, 2026

rustbot assigned jackh726 and unassigned JonathanBrouwer Apr 15, 2026

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Nadrieril reviewed Apr 22, 2026

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Comment thread compiler/rustc_mir_build/src/builder/matches/match_pair.rs Resolved

dianne reviewed Apr 23, 2026

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Comment thread compiler/rustc_mir_build/src/builder/matches/test.rs Resolved

Comment thread compiler/rustc_mir_build/src/builder/matches/match_pair.rs Resolved

rustbot assigned dianne Apr 23, 2026

jakubadamw added 13 commits November 7, 2026 13:33

Update the tests to cover const contexts as well

4fd29eb

Raise the aggregate equality comparison threshold so simple arrays do…

4a27e24

…n't get captured by this logic

Merge the AggregateEq and StringEq test arms into a single case

57190cb

Add a missing space

fc0981a

Remove the special treatment of the const_cmp and `const_trait_impl…

124d951

…` features

Add a MIR test for a constant array with a custom element type

9b5bc46

Expand on why the aggregate comparison is assumed not to unwind

c1937a5

Note in the comment that exhaustiveness relies on PartialEq for agg…

7d581e4

…regates too

Restrict the aggregate comparison to bytewise-comparable element types

f438dea

Address review feedback

9c1a04e

Call the `raw_eq` and `compare_bytes` intrinsics directly instead of `PartialEq::eq`. Add a MIR test for a constant below `AGGREGATE_EQ_MIN_LEN`. Remove the codegen-units test and the comments in `core`.

jakubadamw force-pushed the issue-110870-103073 branch from e0ef1aa to b7e5b00 Compare November 7, 2026 13:41

jakubadamw force-pushed the issue-110870-103073 branch from b7e5b00 to 2e0b6a0 Compare November 7, 2026 13:54

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Auto merge of #155216 - jakubadamw:issue-110870-103073, r=<try>

4c09b15

match: Use an aggregate equality comparison for constant array/slice patterns

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rustbot removed perf-regression Performance regression. S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Nov 7, 2026

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Overall result: ❌✅ regressions and improvements - no action needed

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Overall result: ✅ improvements - no action needed

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