optimize vec.extend(slice.to_vec()), take 2

[the8472]

Redoing #130998
It was reverted in #151150 due to flakiness. I have traced this to layout randomization perturbing the test (the failure reproduces locally with layout randomization), which is now excluded.

[rustbot]

r? @Mark-Simulacrum

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[tbu-]

Does it make sense to add an optimization that breaks with layout randomization?

[the8472]

Layout randomization is primarily for debugging (it pessimizes struct layouts after all), and the optimization is nice-to-have so I think it's ok.
At worst it indicates that the optimization is brittle, but even with randomization it still worked for the Vec->Vec case, just the String->String case broke for some reason, not sure why.

[Mark-Simulacrum]

@bors r+ rollup=iffy

[rust-bors]

📌 Commit 2b8f4a5 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: Mark-Simulacrum
Duration: 3h 20m 56s
Pushing 873d468 to main...

[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 38c7129 (parent) -> 873d468 (this PR)

Test differences

Show 201 test diffs

Stage 1

• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> pass (J2)
• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> ignore (ignored when randomizing layouts) (J3)
• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> ignore (ignored when the LLVM version 20.1.2 is older than 21.0.0) (J5)

Stage 2

• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> pass (J0)
• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> ignore (ignored when the LLVM version 20.1.8 is older than 21.0.0) (J1)
• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> ignore (ignored when the LLVM version 20.1.2 is older than 21.0.0) (J4)
• [codegen] tests/codegen-llvm/lib-optimizations/append-elements.rs: [missing] -> ignore (ignored when randomizing layouts) (J6)

Additionally, 194 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: aarch64-apple, aarch64-gnu, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-gnu, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J1: aarch64-gnu-llvm-20-1
• J2: pr-check-2
• J3: x86_64-gnu-llvm-21-3
• J4: x86_64-gnu-llvm-20-2
• J5: x86_64-gnu-llvm-20-3
• J6: x86_64-gnu-llvm-21-2

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 873d4682c7d285540b8f28bfe637006cef8918a6 --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: 4766.0s -> 3378.1s (-29.1%)
2. x86_64-rust-for-linux: 2429.6s -> 2695.0s (+10.9%)
3. dist-x86_64-apple: 6596.3s -> 7265.6s (+10.1%)
4. dist-i686-linux: 5877.2s -> 6432.7s (+9.5%)
5. x86_64-gnu-llvm-21-1: 3831.1s -> 4140.6s (+8.1%)
6. i686-gnu-2: 4901.4s -> 5279.1s (+7.7%)
7. aarch64-gnu-llvm-20-2: 2915.3s -> 2700.0s (-7.4%)
8. x86_64-gnu-tools: 3086.7s -> 3308.8s (+7.2%)
9. aarch64-gnu-llvm-20-1: 3396.8s -> 3174.9s (-6.5%)
10. arm-android: 5721.3s -> 6086.4s (+6.4%)

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 (873d468): 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)	0.6%	[0.3%, 1.0%]	6
Regressions ❌ (secondary)	0.3%	[0.0%, 0.7%]	4
Improvements ✅ (primary)	-0.6%	[-0.7%, -0.5%]	2
Improvements ✅ (secondary)	-0.5%	[-4.2%, -0.0%]	12
All ❌✅ (primary)	0.3%	[-0.7%, 1.0%]	8

Max RSS (memory usage)

Results (primary 0.2%, secondary -3.7%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[2.1%, 3.8%]	3
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-2.9%	[-3.4%, -2.4%]	3
Improvements ✅ (secondary)	-5.0%	[-5.7%, -4.3%]	5
All ❌✅ (primary)	0.2%	[-3.4%, 3.8%]	6

Cycles

Results (primary -7.5%, secondary -1.7%)

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)	4.5%	[4.3%, 4.6%]	2
Improvements ✅ (primary)	-7.5%	[-13.9%, -0.7%]	14
Improvements ✅ (secondary)	-4.2%	[-7.6%, -2.2%]	5
All ❌✅ (primary)	-7.5%	[-13.9%, -0.7%]	14

Binary size

Results (primary -0.0%, 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)	0.1%	[0.0%, 0.2%]	19
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.0%]	9
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	2
All ❌✅ (primary)	-0.0%	[-0.6%, 0.2%]	28

Bootstrap: 474.192s -> 474.052s (-0.03%)
Artifact size: 383.57 MiB -> 383.68 MiB (0.03%)

[Kobzol]

The results are a wash overall.

@rustbot label: +perf-regression-triaged