Make root vars more stable

[compiler-errors]

Never resolve a ty/ct vid to a higher vid as its root. This should make the optimization in #141500 more "stable" when there are a lot of vars flying around.

r? @ghost

[compiler-errors]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit e1567df with merge 55fb0af…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 55fb0af (55fb0af4ed7d14a8bca0f3c87248c6c66fcde13b)

[rust-timer]

Finished benchmarking commit (55fb0af): comparison URL.

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

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 the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	2
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-0.6%	[-1.3%, -0.1%]	8
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Max RSS (memory usage)

Results (primary -1.1%, secondary 1.8%)

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)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	1.8%	[1.1%, 2.5%]	2
Improvements ✅ (primary)	-2.4%	[-4.1%, -0.7%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-4.1%, 1.5%]	3

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: 750.971s -> 751.025s (0.01%)
Artifact size: 371.78 MiB -> 371.71 MiB (-0.02%)

[compiler-errors]

Let's try this again

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit e1567df with merge 8c7ffa9…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8c7ffa9 (8c7ffa9414d872a8693f2144760144310f152c7f)

[rust-timer]

Finished benchmarking commit (8c7ffa9): comparison URL.

Overall result: ✅ improvements - no 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.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-1.5%, -0.4%]	14
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 3.1%, secondary -5.8%)

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)	3.1%	[2.0%, 4.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.8%	[-8.9%, -2.7%]	2
All ❌✅ (primary)	3.1%	[2.0%, 4.3%]	2

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: 751.176s -> 750.384s (-0.11%)
Artifact size: 372.27 MiB -> 372.21 MiB (-0.02%)

[compiler-errors]

I don't see why we shouldn't land this, since I kinda like the invariant that we always equate a two vars with the root being its lowest vid. But it's also basically useless today, so I could see us tabling this for the future too.

r? lcnr

[lcnr]

it's still a minor performance improvements and I agree that this change is desirable regardless of perf.

It also feels better wrt to fudging and what not

@bors r+ rollup=never

[bors]

📌 Commit e1567df has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit e1567df with merge 2b0274c...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 2b0274c to master...

[github-actions]

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 1c04750 (parent) -> 2b0274c (this PR)

Test differences

No test diffs found

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 2b0274c71dba0e24370ebf65593da450e2e91868 --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

1. dist-apple-various: 7994.5s -> 6762.3s (-15.4%)
2. mingw-check-tidy: 75.9s -> 66.6s (-12.2%)
3. aarch64-gnu: 6304.3s -> 6799.1s (7.8%)
4. mingw-check-1: 1835.7s -> 1968.4s (7.2%)
5. x86_64-apple-1: 6861.4s -> 7286.4s (6.2%)
6. armhf-gnu: 4779.0s -> 5021.3s (5.1%)
7. dist-x86_64-musl: 7357.6s -> 7010.5s (-4.7%)
8. dist-i686-mingw: 7754.7s -> 8114.6s (4.6%)
9. i686-gnu-1: 7884.8s -> 8247.0s (4.6%)
10. dist-loongarch64-musl: 4776.3s -> 4984.1s (4.4%)

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]

Finished benchmarking commit (2b0274c): comparison URL.

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

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)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-1.2%, -0.4%]	9
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	1

Max RSS (memory usage)

Results (primary -1.5%, secondary 4.1%)

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)	6.6%	[3.6%, 9.7%]	2
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-0.9%	[-0.9%, -0.9%]	1
All ❌✅ (primary)	-1.5%	[-1.5%, -1.5%]	1

Cycles

Results (secondary -2.7%)

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
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 756.123s -> 754.155s (-0.26%)
Artifact size: 372.14 MiB -> 372.17 MiB (0.01%)