ignore head usages from ignored candidates #144991

lcnr · 2025-08-06T09:54:40Z

Fixes rust-lang/trait-system-refactor-initiative#210. The test now takes 0.8s to compile, which seems good enough to me. We are actually still walking the entire graph here, we're just avoiding unnecessary reruns.

The basic idea is that if we've only accessed a cycle head inside of a candidate which didn't impact the final result of our goal, we don't need to rerun that cycle head even if is the used provisional result differs from the final result.

We also use this information when rebasing goals over their cycle heads. If a goal doesn't actually depend on the result of that cycle head, rebasing always succeeds. However, we still need to make sure we track the fact that we relied on the cycle head at all to avoid query instability.

It is implemented by tracking the number of HeadUsages for every head while evaluating goals. We then also track the head usages while evaluating a single candidate, which the search graph returns as CandidateHeadUsages. If there is now an always applicable candidate candidate we know that all other candidates with that source did not matter. We then call fn ignore_candidate_head_usages to remove the usages while evaluating this single candidate from the total. If the final HeadUsages end up empty, we know that the result of this cycle head did not matter when evaluating its nested goals.

rustbot · 2025-08-06T09:54:45Z

r? @SparrowLii

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lcnr · 2025-08-06T09:54:54Z

r? @compiler-errors @BoxyUwU

rustbot · 2025-08-06T09:54:59Z

compiler-errors is not on the review rotation at the moment.
They may take a while to respond.

lcnr · 2025-08-06T10:43:42Z

@bors2 try @rust-timer queue

…=<try> ignore usages from ignored candidates

rust-bors · 2025-08-06T10:43:46Z

⌛ Trying commit d49499e with merge 5c09e88…

To cancel the try build, run the command @bors try cancel.

rust-bors · 2025-08-06T12:57:22Z

☀️ Try build successful (CI)
Build commit: 5c09e88 (5c09e883643532880d70eb19836fb01847e0e5f5, parent: dc0bae1db725fbba8524f195f74f680995fd549e)

rust-timer · 2025-08-06T14:26:18Z

Finished benchmarking commit (5c09e88): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.7%	[0.2%, 1.1%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.5%, secondary 2.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.0%, 3.1%]	2
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.0%, 3.1%]	2

Cycles

Results (secondary 0.4%)

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)	2.6%	[2.4%, 2.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.7%, -2.3%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 463.057s -> 466.661s (0.78%)
Artifact size: 377.38 MiB -> 377.62 MiB (0.06%)

lcnr · 2025-08-07T10:05:03Z

@bors2 try @rust-timer queue

rust-bors · 2025-08-07T10:05:06Z

⌛ Trying commit 2d42758 with merge 3a46227…

To cancel the try build, run the command @bors try cancel.

…=<try> ignore usages from ignored candidates

lcnr · 2025-08-12T09:38:24Z

@bors2 try @rust-timer queue

…=<try> ignore head usages from ignored candidates

rust-bors · 2025-08-12T11:51:29Z

☀️ Try build successful (CI)
Build commit: 546cb5f (546cb5ff81bf3b99c56bc90236840949825cc3c0, parent: a1531335fe2807715fff569904d99602022643a7)

rust-timer · 2025-08-12T13:03:42Z

Finished benchmarking commit (546cb5f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	2
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.3%]	3
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	2

Max RSS (memory usage)

Results (primary 1.3%, secondary 1.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	2
Regressions ❌ (secondary)	1.4%	[1.3%, 1.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	2

Cycles

Results (primary -2.4%, secondary -4.9%)

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)	-2.4%	[-2.7%, -2.0%]	3
Improvements ✅ (secondary)	-4.9%	[-4.9%, -4.9%]	1
All ❌✅ (primary)	-2.4%	[-2.7%, -2.0%]	3

Binary size

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

Bootstrap: 465.439s -> 465.953s (0.11%)
Artifact size: 377.34 MiB -> 377.51 MiB (0.05%)

BoxyUwU · 2025-08-14T14:17:26Z

r=me if CI passes

lcnr · 2025-08-14T14:43:10Z

@bors r=BoxyUwU

uwu

bors · 2025-08-14T14:43:12Z

📌 Commit 8afe306 has been approved by BoxyUwU

It is now in the queue for this repository.

…=BoxyUwU ignore head usages from ignored candidates Fixes rust-lang/trait-system-refactor-initiative#210. The test now takes 0.8s to compile, which seems good enough to me. We are actually still walking the entire graph here, we're just avoiding unnecessary reruns. The basic idea is that if we've only accessed a cycle head inside of a candidate which didn't impact the final result of our goal, we don't need to rerun that cycle head even if is the used provisional result differs from the final result. We also use this information when rebasing goals over their cycle heads. If a goal doesn't actually depend on the result of that cycle head, rebasing always succeeds. However, we still need to make sure we track the fact that we relied on the cycle head at all to avoid query instability. It is implemented by tracking the number of `HeadUsages` for every head while evaluating goals. We then also track the head usages while evaluating a single candidate, which the search graph returns as `CandidateHeadUsages`. If there is now an always applicable candidate candidate we know that all other candidates with that source did not matter. We then call `fn ignore_candidate_head_usages` to remove the usages while evaluating this single candidate from the total. If the final `HeadUsages` end up empty, we know that the result of this cycle head did not matter when evaluating its nested goals.

bors · 2025-08-15T12:35:13Z

⌛ Testing commit 8afe306 with merge c018ae5...

rustbot assigned SparrowLii Aug 6, 2025

lcnr marked this pull request as draft August 6, 2025 09:54

rustbot assigned compiler-errors and unassigned SparrowLii Aug 6, 2025