[perf] Specialize common `(1, 1)` case for arg unification. by obi1kenobi · Pull Request #158865 · rust-lang/rust

obi1kenobi · 2026-07-06T15:23:01Z

While DeepRejectCtxt::args_may_unify_inner might be called with different numbers of obligation and impl args, the (1, 1) length case is very common: single-generic-param ADTs, trait references whose only generic is Self, etc.

Offer a fast path for that simple case, so that we avoid invoking iter::zip(...).all(predicate) iterator machinery given that it isn't necessary there.

We considered, and ultimately rejected, adding a separate specialization for the (0, 0) case since benchmarks indicated it wasn't cost-effective.

r? @oli-obk

AI disclosure: The optimization opportunity here was discovered as part of a systematic probe for missed optimizations utilizing both traditional and AI tools. The code here was initially prototyped and vetted by AI tools, followed by additional manual work. I secured approval in advance from the designated reviewer. I stand behind the quality of the code I'm submitting, and I vouch it's as good or better compared to if I had written every line by my own hand.

obi1kenobi · 2026-07-06T15:24:24Z

cc @Kobzol for perf run

lqd · 2026-07-06T15:24:42Z

@bors try @rust-timer queue

[perf] Specialize common `(1, 1)` case for arg unification.

rust-bors · 2026-07-06T17:32:54Z

☀️ Try build successful (CI)
Build commit: 08a2c83 (08a2c83fd14ebabf401ff0d6a14c0b48bbee9e1f)
Base parent: 3c00c96 (3c00c96d3af4d5b5e101e56cc161a608b21366ee)

rust-timer · 2026-07-07T00:44:15Z

Finished benchmarking commit (08a2c83): comparison URL.

Overall result: ✅ 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
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.3%]	7
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.3%]	3
All ❌✅ (primary)	-0.3%	[-0.4%, -0.3%]	7

Max RSS (memory usage)

Results (primary -0.6%)

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)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Cycles

Results (secondary 3.4%)

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)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 491.018s -> 489.64s (-0.28%)
Artifact size: 388.45 MiB -> 388.47 MiB (0.01%)

oli-obk · 2026-07-07T14:29:22Z

@bors try @rust-timer queue

[perf] Specialize common `(1, 1)` case for arg unification.

rust-bors · 2026-07-07T16:43:32Z

☀️ Try build successful (CI)
Build commit: 160aab0 (160aab06f8078e8ed680442a5f19b945262e0347)
Base parent: f10db29 (f10db292a3733b5c67c8da8c7661195ff4b05774)

rust-timer · 2026-07-07T18:10:10Z

Finished benchmarking commit (160aab0): comparison URL.

Overall result: ❌✅ regressions and 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.

@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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.2%]	4
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (primary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-3.0%, 2.9%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 487.206s -> 495.006s (1.60%)
Artifact size: 388.97 MiB -> 388.97 MiB (0.00%)

obi1kenobi · 2026-07-08T03:18:01Z

Having the (0, 0) case looks worse to me: we get an extra 0.06% with the new solver — a very small number compared to the significance threshold — in exchange for regressions elsewhere.

I'm planning to remove it from this PR. If I've misread the data, I'm happy to be told so and can put it back :)

oli-obk

View changes since this review

rust-bors · 2026-07-08T15:43:58Z

📌 Commit f09561b has been approved by oli-obk

It is now in the queue for this repository.

@oli-obk

[perf] Specialize common `(1, 1)` case for arg unification. While `DeepRejectCtxt::args_may_unify_inner` might be called with different numbers of obligation and impl args, the `(1, 1)` length case is very common: single-generic-param ADTs, trait references whose only generic is `Self`, etc. Offer a fast path for that simple case, so that we avoid invoking `iter::zip(...).all(predicate)` iterator machinery given that it isn't necessary there. We considered, and ultimately rejected, adding a separate specialization for the `(0, 0)` case since benchmarks indicated it wasn't cost-effective. r? @oli-obk **AI disclosure:** The optimization opportunity here was discovered as part of a systematic probe for missed optimizations utilizing both traditional and AI tools. The code here was initially prototyped and vetted by AI tools, followed by additional manual work. I secured approval in advance from the designated reviewer. I stand behind the quality of the code I'm submitting, and I vouch it's as good or better compared to if I had written every line by my own hand.

JonathanBrouwer · 2026-07-08T17:02:18Z

@bors yield

rust-bors · 2026-07-08T17:02:21Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/28960483198

The next pull request likely to be tested is #158958.

JonathanBrouwer · 2026-07-09T11:04:09Z

@bors p=6 scheduling

@oli-obk

[perf] Specialize common `(1, 1)` case for arg unification. While `DeepRejectCtxt::args_may_unify_inner` might be called with different numbers of obligation and impl args, the `(1, 1)` length case is very common: single-generic-param ADTs, trait references whose only generic is `Self`, etc. Offer a fast path for that simple case, so that we avoid invoking `iter::zip(...).all(predicate)` iterator machinery given that it isn't necessary there. We considered, and ultimately rejected, adding a separate specialization for the `(0, 0)` case since benchmarks indicated it wasn't cost-effective. r? @oli-obk **AI disclosure:** The optimization opportunity here was discovered as part of a systematic probe for missed optimizations utilizing both traditional and AI tools. The code here was initially prototyped and vetted by AI tools, followed by additional manual work. I secured approval in advance from the designated reviewer. I stand behind the quality of the code I'm submitting, and I vouch it's as good or better compared to if I had written every line by my own hand.

rust-bors · 2026-07-09T12:48:47Z

💔 Test for dd4afd8 failed: CI. Failed job:

auto - x86_64-gnu-debug (web logs, enhanced plaintext logs)

rust-log-analyzer · 2026-07-09T12:48:59Z

The job x86_64-gnu-debug failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

oli-obk · 2026-07-09T13:09:50Z

@bors retry network failure

rust-bors · 2026-07-09T16:53:09Z

☀️ Test successful - CI
Approved by: oli-obk
Duration: 3h 21m 25s
Pushing 3664b37 to main...

github-actions · 2026-07-09T16:56:51Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 561ea2b (parent) -> 3664b37 (this PR)

Test differences

Show 2 test diffs

2 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 3664b37017c529cad0f0ed259769f15743c21a6a --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-gnu-distcheck: 1h 25m -> 2h 15m (+58.1%)
x86_64-gnu-gcc-core-tests: 14m 8s -> 6m 18s (-55.4%)
x86_64-gnu-gcc: 1h 11m -> 38m 47s (-46.1%)
dist-i686-mingw: 2h -> 2h 43m (+35.5%)
dist-powerpc64le-linux-musl: 1h 8m -> 1h 30m (+33.4%)
dist-x86_64-mingw: 2h 32m -> 1h 42m (-32.7%)
dist-loongarch64-musl: 1h 40m -> 1h 10m (-29.8%)
i686-gnu-2: 1h 44m -> 1h 13m (-29.4%)
x86_64-msvc-1: 2h 38m -> 1h 55m (-27.0%)
i686-msvc-2: 2h 3m -> 1h 31m (-26.2%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-07-09T17:35:02Z

Finished benchmarking commit (3664b37): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.2%	[0.2%, 0.3%]	7
Improvements ✅ (primary)	-0.5%	[-0.7%, -0.2%]	8
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.1%]	6
All ❌✅ (primary)	-0.4%	[-0.7%, 0.2%]	9

Max RSS (memory usage)

Results (secondary 6.0%)

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)	6.0%	[6.0%, 6.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.9%, secondary 9.3%)

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)	15.5%	[14.4%, 16.6%]	2
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-2.9%	[-2.9%, -2.9%]	1

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	52
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	22
All ❌✅ (primary)	-0.1%	[-0.1%, -0.0%]	52

Bootstrap: 489.147s -> 487.967s (-0.24%)
Artifact size: 388.47 MiB -> 388.37 MiB (-0.03%)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 6, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 6, 2026

Auto merge of #158865 - obi1kenobi:pg/fast-reject-1-1-case, r=<try>

08a2c83

[perf] Specialize common `(1, 1)` case for arg unification.

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rust-bors Bot pushed a commit that referenced this pull request Jul 7, 2026

Auto merge of #158865 - obi1kenobi:pg/fast-reject-1-1-case, r=<try>

160aab0

[perf] Specialize common `(1, 1)` case for arg unification.

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 7, 2026

rustbot assigned oli-obk Jul 8, 2026

[perf] Specialize common (1, 1) case for arg unification.

f09561b

obi1kenobi force-pushed the pg/fast-reject-1-1-case branch from bb64fd1 to f09561b Compare July 8, 2026 03:35

obi1kenobi marked this pull request as ready for review July 8, 2026 03:48

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 8, 2026

oli-obk approved these changes Jul 8, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 9, 2026

rust-bors Bot merged commit 3664b37 into rust-lang:main Jul 9, 2026
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rustbot added this to the 1.99.0 milestone Jul 9, 2026

rustbot added the perf-regression Performance regression. label Jul 9, 2026

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Overall result: ✅ improvements - no action needed

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

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Test differences

Job duration changes

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Overall result: ❌✅ regressions and improvements - please read:

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