Slightly optimize reading of source files

[Kobzol]

Discussed on Zulip (https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/rustc.20uses.20silly.20amount.20of.20syscalls.20for.20file.20IO/with/536015625.

Do not open/close each source file twice when reading it. We still stat it twice though.

[Kobzol]

@bors try @rust-timer queue

[Kobzol]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 95ae8eb (95ae8eb3c562954cebf4450e01020a53bc83a274, parent: ee361e8fca1c30e13e7a31cc82b64c045339d3a8)

[rust-timer]

Finished benchmarking commit (95ae8eb): comparison URL.

Overall result: ✅ improvements - no action needed

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.

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

Max RSS (memory usage)

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

Cycles

Results (secondary 4.0%)

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)	4.0%	[4.0%, 4.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 467.59s -> 467.992s (0.09%)
Artifact size: 378.37 MiB -> 378.32 MiB (-0.01%)

[rustbot]

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Kobzol]

No regressions, and the change is rather miniscule, so could be worth to land it.

I took a look at the other inefficiencies (first we check if file exists and only then we load it), but it wasn't trivial to modify it without regressing error messages.

r? @the8472

[the8472]

@bors r+

[bors]

📌 Commit 3145c06 has been approved by the8472

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3145c06 with merge ace9a74...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing ace9a74 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 d327d65 (parent) -> ace9a74 (this PR)

Test differences

Show 4 test diffs

4 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 ace9a74442aec13c75532d34e15d97428056866d --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-android: 2086.8s -> 2571.4s (23.2%)
2. x86_64-gnu-nopt: 7385.7s -> 8765.9s (18.7%)
3. dist-aarch64-apple: 5533.4s -> 6291.5s (13.7%)
4. x86_64-rust-for-linux: 2706.6s -> 3024.0s (11.7%)
5. dist-x86_64-apple: 7762.4s -> 6950.2s (-10.5%)
6. aarch64-gnu: 7672.0s -> 6912.7s (-9.9%)
7. aarch64-msvc-2: 4773.6s -> 5245.3s (9.9%)
8. dist-apple-various: 4860.2s -> 5306.9s (9.2%)
9. aarch64-gnu-llvm-19-2: 2422.0s -> 2586.0s (6.8%)
10. i686-gnu-1: 8082.8s -> 8599.4s (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 (ace9a74): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -2.9%, secondary 3.1%)

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)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-2.9%	[-5.2%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.9%	[-5.2%, -0.6%]	2

Cycles

Results (secondary 1.5%)

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)	1.5%	[1.5%, 1.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 466.213s -> 468.198s (0.43%)
Artifact size: 378.38 MiB -> 378.40 MiB (0.00%)