Rewrite Iterator::position default impl

[marthadev]

Storing the accumulating value outside the fold in an attempt to improve code generation has shown speedups on various handwritten benchmarks, see discussion at #119551.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

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[the8472]

@bors try @rust-timer queue

[bors]

⌛ Trying commit fd33955 with merge 5139266...

[bors]

☀️ Try build successful - checks-actions
Build commit: 5139266 (51392669857c5149d54ed2da5cb1628a5197d7d4)

[rust-timer]

Finished benchmarking commit (5139266): 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 a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	1

Max RSS (memory usage)

Results

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)	3.4%	[3.4%, 3.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-4.7%, -2.6%]	2
All ❌✅ (primary)	3.4%	[3.4%, 3.4%]	1

Cycles

Results

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.8%, -2.0%]	5
All ❌✅ (primary)	-	-	0

Binary size

Results

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

Bootstrap: 670.544s -> 670.096s (-0.07%)
Artifact size: 311.06 MiB -> 311.06 MiB (-0.00%)

[the8472]

Nice, no regressions and your benchmarks look good too.

Are you willing to try your hand at writing a codegen test based on the llvm ir difference #119551 (comment) ?
It's optional, I'd approve the PR without that too.

[marthadev]

Are you willing to try your hand at writing a codegen test based on the llvm ir difference #119551 (comment) ? It's optional, I'd approve the PR without that too.

I have no experience in writing codegen tests, and I sadly don't really have the time to learn about it right now. I will look into manually retesting when there are relevant changes to the ControlFlow enum, but for now this is all I can do.

[the8472]

Ok, that's fine. Can you squash your commits since the first one doesn't build?

[marthadev]

Ok, that's fine. Can you squash your commits since the first one doesn't build?

Should be fine now

[the8472]

Thanks.

@bors r+

[bors]

📌 Commit 0e9c3d9 has been approved by the8472

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0e9c3d9 with merge 9a56082...

[bors]

💥 Test timed out

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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

[the8472]

The apple builder was hanging while building LLVM, that should be unrelated to this change.

@bors retry

[bors]

⌛ Testing commit 0e9c3d9 with merge fde0e98...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing fde0e98 to master...

[rust-timer]

Finished benchmarking commit (fde0e98): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	1

Max RSS (memory usage)

Results

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)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.8%, -3.0%]	2
All ❌✅ (primary)	2.9%	[2.9%, 2.9%]	1

Cycles

Results

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)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.0%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.1%	[-8.1%, -8.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results

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

Bootstrap: 668.367s -> 667.404s (-0.14%)
Artifact size: 308.49 MiB -> 308.54 MiB (0.02%)