rust-lang · Mark-Simulacrum · Jun 2, 2025 · Jun 2, 2025
diff --git a/triage/2025-06-02.md b/triage/2025-06-02.md
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+# 2025-06-02 Triage Log
+
+A fairly busy week, with lots of changes to performance. Most of the changes
+(at least in quantity of benchmarks) are attributable to an update of our PGO
+collection to newer benchmarks as part of the 2025 refresh.
+
+Triage done by **@simulacrum**.
+Revision range: [2805e1dc..2fc3deed](https://perf.rust-lang.org/?start=2805e1dc4c18ed4c84d161502c48da870c56f68a&end=2fc3deed9fcb8762ad57191e0195f06f7543e4a5&absolute=false&stat=instructions%3Au)
+
+**Summary**:
+
+| (instructions:u)                   | mean  | range           | count |
+|:----------------------------------:|:-----:|:---------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.7%  | [0.1%, 6.7%]    | 56    |
+| Regressions ❌ <br /> (secondary)  | 0.6%  | [0.1%, 1.3%]    | 69    |
+| Improvements ✅ <br /> (primary)   | -1.2% | [-8.9%, -0.1%]  | 133   |
+| Improvements ✅ <br /> (secondary) | -2.7% | [-20.2%, -0.1%] | 115   |
+| All ❌✅ (primary)                 | -0.6% | [-8.9%, 6.7%]   | 189   |
+
+2 Regressions, 6 Improvements, 13 Mixed; 9 of them in rollups
+40 artifact comparisons made in total
+
+#### Regressions
+
+Add some track_caller info to precondition panics [#129658](https://github.com/rust-lang/rust/pull/129658) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=45f256d9d7cffb66185c0bf1b8a864cba79db90c&end=be422939446d7c5b27ba98debb6b4b8d6a261f1a&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 1.0%  | [0.1%, 6.2%]   | 14    |
+| Regressions ❌ <br /> (secondary)  | 0.8%  | [0.4%, 1.2%]   | 4     |
+| Improvements ✅ <br /> (primary)   | -0.5% | [-0.5%, -0.5%] | 1     |
+| Improvements ✅ <br /> (secondary) | -     | -              | 0     |
+| All ❌✅ (primary)                 | 0.9%  | [-0.5%, 6.2%]  | 15    |
+
+Expected regressions to performance for better user experience. No further follow-up warranted.
+
+Rollup of 8 pull requests [#141753](https://github.com/rust-lang/rust/pull/141753) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=1c0849d8bae6f1af08a8b64a011b2ffa7c24f136&end=6de3a733122a82d9b3c3427c7ee16a1e1a022718&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.4%  | [0.1%, 1.4%]   | 78    |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.1%, 0.6%]   | 46    |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -0.2% | [-0.3%, -0.1%] | 3     |
+| All ❌✅ (primary)                 | 0.4%  | [0.1%, 1.4%]   | 78    |
+
+[#141407](https://github.com/rust-lang/rust/pull/141407) seems to be the
+primary cause of the impact. This is (in part) a correctness fix so marking the
+regression as triaged.
+
+#### Improvements
+
+Rollup of 8 pull requests [#141662](https://github.com/rust-lang/rust/pull/141662) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=c583fa6d8425dbb38fe5d1dbd007f9ca8e4aa128&end=45f256d9d7cffb66185c0bf1b8a864cba79db90c&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -1.3% | [-4.3%, -0.0%] | 56    |
+| All ❌✅ (primary)                 | -     | -              | 0     |
+
+Unclear cause. Probably not worth digging deeper as solely improvements and only in secondary benchmarks.
+
+Rollup of 8 pull requests [#141668](https://github.com/rust-lang/rust/pull/141668) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=be422939446d7c5b27ba98debb6b4b8d6a261f1a&end=04a67d5a0587ed98632f82c404ae20f9f0a51a1d&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.1%  | [0.1%, 0.1%]   | 2     |
+| Improvements ✅ <br /> (primary)   | -0.2% | [-0.2%, -0.1%] | 34    |
+| Improvements ✅ <br /> (secondary) | -0.2% | [-0.3%, -0.0%] | 19    |
+| All ❌✅ (primary)                 | -0.2% | [-0.2%, -0.1%] | 34    |
+
+Unclear cause. Probably not worth digging deeper as solely improvements and only in secondary benchmarks.
+Started a [PR build](https://github.com/rust-lang/rust/pull/141252#issuecomment-2930408560) to check one potential root cause.
+
+rustdoc: cleanups relating to allocations [#141573](https://github.com/rust-lang/rust/pull/141573) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=6de3a733122a82d9b3c3427c7ee16a1e1a022718&end=e6152cdf5b31bd844a4cc1049433859d54863602&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -0.4% | [-0.6%, -0.2%] | 8     |
+| Improvements ✅ <br /> (secondary) | -0.6% | [-0.6%, -0.6%] | 1     |
+| All ❌✅ (primary)                 | -0.4% | [-0.6%, -0.2%] | 8     |
+
+
+Add fast path for maybe-initializedness in liveness [#141667](https://github.com/rust-lang/rust/pull/141667) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=7a7bcbbcdbf2845164a94377d0e0efebb737ffd3&end=ec28ae9454139023117270985f114823d6570657&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range           | count |
+|:----------------------------------:|:-----:|:---------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -               | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.5%  | [0.5%, 0.5%]    | 1     |
+| Improvements ✅ <br /> (primary)   | -1.0% | [-10.6%, -0.2%] | 67    |
+| Improvements ✅ <br /> (secondary) | -0.5% | [-1.2%, -0.2%]  | 59    |
+| All ❌✅ (primary)                 | -1.0% | [-10.6%, -0.2%] | 67    |
+
+
+cstore: Use IndexSet as backing store for postorder dependencies [#141730](https://github.com/rust-lang/rust/pull/141730) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=13a4540908d25642c0fa0ab726095cf57bbe6824&end=ba55b7ce3cd931af7123e334647584f6fbdb3c20&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -2.3% | [-5.2%, -0.4%] | 11    |
+| All ❌✅ (primary)                 | -     | -              | 0     |
+
+Tweak fast path trait handling [#141731](https://github.com/rust-lang/rust/pull/141731) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=ba55b7ce3cd931af7123e334647584f6fbdb3c20&end=9b0268a43b0a5b58f50c96ded25364037c019a61&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -1.0% | [-2.4%, -0.2%] | 10    |
+| All ❌✅ (primary)                 | -     | -              | 0     |
+
+#### Mixed
+
+add additional `TypeFlags` fast paths [#141581](https://github.com/rust-lang/rust/pull/141581) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=ebe9b0060240953d721508ceb4d02a745efda88f&end=5f025f363df11c65bd31ade9fe6f48fd4f4239af&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.1%  | [0.1%, 0.2%]   | 8     |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.2%, 0.5%]   | 20    |
+| Improvements ✅ <br /> (primary)   | -0.3% | [-0.4%, -0.3%] | 5     |
+| Improvements ✅ <br /> (secondary) | -2.5% | [-5.7%, -0.2%] | 10    |
+| All ❌✅ (primary)                 | -0.0% | [-0.4%, 0.2%]  | 13    |
+
+Mostly no significant delta (averaging out to 0% change) but worth it in more pathological cases, so makes sense to land the change.
+
+Rollup of 16 pull requests [#141716](https://github.com/rust-lang/rust/pull/141716) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=5f025f363df11c65bd31ade9fe6f48fd4f4239af&end=38081f22c2d7380f272aa1d7fa9b935637701c2d&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.2%  | [0.2%, 0.2%]   | 3     |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.3%, 0.3%]   | 1     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -0.1% | [-0.1%, -0.1%] | 1     |
+| All ❌✅ (primary)                 | 0.2%  | [0.2%, 0.2%]   | 3     |
+
+Correctness fix in wfcheck ([#136429](https://github.com/rust-lang/rust/pull/136429)). No further investigation needed.
+
+Bump rustc-perf and update PGO crates [#141490](https://github.com/rust-lang/rust/pull/141490) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=13718eb788622ef8c998650451174570230d2971&end=1bbd62e547ba5cc08ccb44c27def3d33195d2dd5&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.3%  | [0.1%, 2.0%]   | 46    |
+| Regressions ❌ <br /> (secondary)  | 0.7%  | [0.1%, 3.2%]   | 110   |
+| Improvements ✅ <br /> (primary)   | -0.4% | [-0.9%, -0.1%] | 72    |
+| Improvements ✅ <br /> (secondary) | -0.9% | [-4.8%, -0.2%] | 62    |
+| All ❌✅ (primary)                 | -0.1% | [-0.9%, 2.0%]  | 118   |
+
+Broad changes due to many benchmark crates getting updated, no particular outcome expected.
+
+Rollup of 11 pull requests [#141739](https://github.com/rust-lang/rust/pull/141739) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=1bbd62e547ba5cc08ccb44c27def3d33195d2dd5&end=1ac1950c337039add1a83113ed6d1bd64bcb1142&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.1%  | [0.1%, 0.1%]   | 4     |
+| Regressions ❌ <br /> (secondary)  | 0.5%  | [0.4%, 0.6%]   | 2     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -0.4% | [-0.5%, -0.4%] | 2     |
+| All ❌✅ (primary)                 | 0.1%  | [0.1%, 0.1%]   | 4     |
+
+Nothing obvious in one cachegrind diff, not worth digging deeper.
+
+Make some assertions in solver into debug assertions [#141651](https://github.com/rust-lang/rust/pull/141651) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=1ac1950c337039add1a83113ed6d1bd64bcb1142&end=1c0849d8bae6f1af08a8b64a011b2ffa7c24f136&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.4%  | [0.3%, 0.5%]   | 2     |
+| Improvements ✅ <br /> (primary)   | -0.1% | [-0.1%, -0.1%] | 2     |
+| Improvements ✅ <br /> (secondary) | -0.8% | [-1.1%, -0.2%] | 11    |
+| All ❌✅ (primary)                 | -0.1% | [-0.1%, -0.1%] | 2     |
+
+Rollup of 5 pull requests [#141765](https://github.com/rust-lang/rust/pull/141765) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=e6152cdf5b31bd844a4cc1049433859d54863602&end=aa5832b142a2bfd322659d60f5d57e25d2a1bd5d&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.3%  | [0.3%, 0.3%]   | 6     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -     | -              | 0     |
+| Improvements ✅ <br /> (secondary) | -0.4% | [-0.4%, -0.4%] | 1     |
+| All ❌✅ (primary)                 | 0.3%  | [0.3%, 0.3%]   | 6     |
+
+rustc_codegen_llvm: use `threadlocal.address` intrinsic to access TLS [#139385](https://github.com/rust-lang/rust/pull/139385) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=aa5832b142a2bfd322659d60f5d57e25d2a1bd5d&end=15825b7161f8bd6a3482211fbf6727a52aa1166b&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.3%, 0.3%]   | 1     |
+| Improvements ✅ <br /> (primary)   | -0.7% | [-2.0%, -0.2%] | 22    |
+| Improvements ✅ <br /> (secondary) | -1.4% | [-1.5%, -1.4%] | 3     |
+| All ❌✅ (primary)                 | -0.7% | [-2.0%, -0.2%] | 22    |
+
+Expected speed up to thread local access.
+
+Rollup of 8 pull requests [#141791](https://github.com/rust-lang/rust/pull/141791) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=70b3f4666e24ce22fc32f5e357dbcf85d3254e63&end=0b00e6847b576656d80d7d1e845100fe1663ce0e&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 2.9%  | [2.9%, 2.9%]   | 1     |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.3%, 0.3%]   | 1     |
+| Improvements ✅ <br /> (primary)   | -1.6% | [-1.6%, -1.6%] | 1     |
+| Improvements ✅ <br /> (secondary) | -0.4% | [-0.4%, -0.3%] | 2     |
+| All ❌✅ (primary)                 | 0.7%  | [-1.6%, 2.9%]  | 2     |
+
+https://github.com/rust-lang/rust/pull/141765#issuecomment-2930545642
+
+Revert "increase perf of charsearcher for single ascii characters" [#141678](https://github.com/rust-lang/rust/pull/141678) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=ec28ae9454139023117270985f114823d6570657&end=e0d014a3dffbb3f0575cfbeb0f480c5080c4d018&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 1.9%  | [1.6%, 2.2%]   | 2     |
+| Regressions ❌ <br /> (secondary)  | -     | -              | 0     |
+| Improvements ✅ <br /> (primary)   | -1.4% | [-7.3%, -0.2%] | 36    |
+| Improvements ✅ <br /> (secondary) | -1.6% | [-4.2%, -0.3%] | 25    |
+| All ❌✅ (primary)                 | -1.2% | [-7.3%, 2.2%]  | 38    |
+
+This fixes a perf regression from a previous week.
+
+PGO new solver [#141453](https://github.com/rust-lang/rust/pull/141453) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=e0d014a3dffbb3f0575cfbeb0f480c5080c4d018&end=738c08b63c4f9e3ebdaec5eece7b6fbc354f6467&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range           | count |
+|:----------------------------------:|:-----:|:---------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | 0.3%  | [0.2%, 0.4%]    | 10    |
+| Regressions ❌ <br /> (secondary)  | 0.2%  | [0.2%, 0.3%]    | 6     |
+| Improvements ✅ <br /> (primary)   | -0.2% | [-0.2%, -0.2%]  | 1     |
+| Improvements ✅ <br /> (secondary) | -8.1% | [-15.0%, -3.3%] | 14    |
+| All ❌✅ (primary)                 | 0.3%  | [-0.2%, 0.4%]   | 11    |
+
+Expected improvements primarily to new solver benchmarks as we now collect PGO
+data. Some regressions are expected due to less precise PGO data for old solver
+patterns.
+
+`slice.get(i)` should use a slice projection in MIR, like `slice[i]` does [#139118](https://github.com/rust-lang/rust/pull/139118) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=4d08223c054cf5a56d9761ca925fd46ffebe7115&end=f0999ffdc4818e498949d3b1f2a0ce6be02a0436&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.3%  | [0.2%, 0.4%]   | 3     |
+| Improvements ✅ <br /> (primary)   | -1.2% | [-1.3%, -1.1%] | 2     |
+| Improvements ✅ <br /> (secondary) | -     | -              | 0     |
+| All ❌✅ (primary)                 | -1.2% | [-1.3%, -1.1%] | 2     |
+
+Within noise threshold for (new) regressed benchmark, otherwise see [#139118 (comment)](https://github.com/rust-lang/rust/pull/139118#issuecomment-2925635519).
+
+Merge coroutine obligation checking into borrowck parallel loop [#141773](https://github.com/rust-lang/rust/pull/141773) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=2fc3deed9fcb8762ad57191e0195f06f7543e4a5&end=91fad92585b2dafc52a074e502b2a6c1f093ca35&stat=instructions:u)
+
+| (instructions:u)                   | mean  | range          | count |
+|:----------------------------------:|:-----:|:--------------:|:-----:|
+| Regressions ❌ <br /> (primary)    | -     | -              | 0     |
+| Regressions ❌ <br /> (secondary)  | 0.4%  | [0.3%, 0.5%]   | 2     |
+| Improvements ✅ <br /> (primary)   | -2.9% | [-2.9%, -2.9%] | 1     |
+| Improvements ✅ <br /> (secondary) | -     | -              | 0     |
+| All ❌✅ (primary)                 | -2.9% | [-2.9%, -2.9%] | 1     |
+
+Not really measurable on (at least) perf, new -Zthreads=4 benchmark doesn't see
+any meaningful improvement from this.