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Assume the returned value in .filter(…).count() #149495

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@scottmcm scottmcm commented Dec 1, 2025

Similar to how this helps in slice::Iter::position, LLVM sometimes loses track of how high this can get, so for TrustedLen iterators tell it what the upper bound is.

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 1, 2025
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Assume the returned value in .filter(…).count()
6bd9d76 
Similar to how this helps in `slice::Iter::position`, LLVM sometimes loses track of how high this can get, so for `TrustedLen` iterators tell it what the upper bound is.
@scottmcm scottmcm force-pushed the assume-filter-count branch from df4a074 to 6bd9d76 Compare December 2, 2025 05:32
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Mark-Simulacrum commented Dec 4, 2025

@bors try @rust-timer queue

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rust-bors bot added a commit that referenced this pull request Dec 4, 2025
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Auto merge of #149495 - scottmcm:assume-filter-count, r=<try>
359130d 
Assume the returned value in `.filter(…).count()`
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 4, 2025
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rust-bors bot commented Dec 4, 2025

☀️ Try build successful (CI)
Build commit: 359130d (359130dd192de66fdeefa2ab307f414171d29beb, parent: 83e49b75e7daf827e4390ae0ccbcb0d0e2c96493)

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rust-timer commented Dec 4, 2025

Finished benchmarking commit (359130d): comparison URL.

Overall result: ❌✅ regressions and 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)		 0.1% [0.1%, 0.1%] 1
Improvements ✅
(primary)		 -0.7% [-0.7%, -0.7%] 1
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) -0.7% [-0.7%, -0.7%] 1

Max RSS (memory usage)

Results (primary 0.7%, secondary -0.8%)

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

mean range count
Regressions ❌
(primary)		 2.7% [0.9%, 3.8%] 3
Regressions ❌
(secondary)		 0.6% [0.6%, 0.6%] 1
Improvements ✅
(primary)		 -2.4% [-3.9%, -0.8%] 2
Improvements ✅
(secondary)		 -2.2% [-2.2%, -2.2%] 1
All ❌✅ (primary) 0.7% [-3.9%, 3.8%] 5

Cycles

Results (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)		 - - 0
Regressions ❌
(secondary)		 2.8% [2.8%, 2.8%] 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: 469.131s -> 468.538s (-0.13%)
Artifact size: 386.75 MiB -> 386.72 MiB (-0.01%)

@rustbot rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 4, 2025
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Mark-Simulacrum commented Dec 5, 2025

r=me given that perf seems neutral, so it's probably not too expensive (though I don't know how often we hit this in benchmarks)

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