[Perf] Windows/x64: 5 Regressions on 7/30/2026 6:50:50 PM +00:00 #131813

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   on Aug 4, 2026
2. LoopedBard3 commented on Aug 4, 2026

   

   LoopedBard3

   on Aug 4, 2026

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   🔍 Automated Triage Analysis

   Summary: Verified regression in XmlSerializer_ — caused by commit efe647b ("XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed ele"). 3 tests bisected to related culprit commits. 2 other test(s) found unrelated commits (possible alignment artifacts).

   Finding Confidence: 5/5 (analysis accuracy)
   Regression Confidence: 5/5 (likelihood of true regression)

   Likely Cause: efe647b7aa3928130fa2a24069a397d3b8c9cbe4

   🤖 Proposed Next Actions: Known-cause regression — transfer & notify — 0 done, 3 dry-run (rule R4_known_regression_new — see Full Analysis for the table). · 🦆 Re-analysis: ⚠️ disputes (overconfident, alignment-warning, partial-bisection-divergence)

   📊 Full Analysis (click to expand)

   Summary: All 5 reported MicroBenchmarks.Serializers.Xml_FromStream<T>.XmlSerializer_ deserialization benchmarks are confirmed regressions (+13% to +92%), each a clean non-overlapping step change over 296–300 stable prior runs (z = 9.0–71.6). The shared deserialization slowdown is attributed to commit efe647b (PR #131302, "XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements"), which adds per-attribute checking to the XmlSerializationReader read paths exercised by every affected test. Finding confidence 4/5 (300 runs, very low variance, not yet rerun-verified); regression confidence 5/5 (surgical direct code hit on the tested path, extreme z-scores, clean step change).

   Issue Overview

   • Issue: [Perf] Windows/x64: 5 Regressions on 7/30/2026 6:50:50 PM +00:00 #131813 - [Perf] Windows/x64: 5 Regressions on 7/30/2026 6:50:50 PM +00:00
   • Date: 2026-07-30 | Queue: Windows.11.Amd64.Viper.Perf (ViperWindows) | OS/Arch: Windows 10.0.22631/x64
   • Commits: 72dac24 → 640b326 (29 commits)
   • Labels: untriaged, perf-regression, os-windows, arch-x64, branch-refs/heads/main, runtime-coreclr, kind-micro, compilationmode-tiered, runkind-micro
   • Runtime stack: Desktop + CoreCLR (implicit); the regressed code is BCL (System.Private.Xml), which is directly the code under test.
   • Historical Data: Available (296–300 prior runs per test; 20 after-runs each; no pre-existing drift)

   Test Analysis

   Test Name	Baseline	Compare	Δ%	μ	σ	n	z	Thresh%	Noise?	Assessment
   Xml_FromStream<MyEventsListerViewModel>.XmlSerializer_	502.21 μs	722.46 μs	+43.86%	507.57 μs	6.91 μs	300	31.08	5%	N	Step change, stable (CV 1.36%)
   Xml_FromStream<SimpleStructWithProperties>.XmlSerializer_	2.88 μs	3.26 μs	+13.06%	2.96 μs	0.033 μs	299	8.96	5%	N	Step change, stable (CV 1.12%)
   Xml_FromStream<IndexViewModel>.XmlSerializer_	45.81 μs	70.63 μs	+54.17%	46.05 μs	0.34 μs	296	71.55	5%	N	Extreme step change, very stable (CV 0.75%)
   Xml_FromStream<CollectionsOfPrimitives>.XmlSerializer_	324.35 μs	623.69 μs	+92.29%	326.77 μs	4.87 μs	300	61.02	5%	N	Extreme step change (~2×), stable (CV 1.49%)
   Xml_FromStream<Location>.XmlSerializer_	4.12 μs	5.36 μs	+30.22%	4.12 μs	0.043 μs	299	28.56	5%	N	Extreme step change, stable (CV 1.05%)

   Legend: μ=mean, σ=stddev, n=runs, z=z-score, Thresh%=adaptive threshold

   Finding Confidence: 4/5 - Exceptionally strong historical baseline (296–300 runs), very low variance (CV 0.75–1.49%), fully non-overlapping before/after distributions, no pre-existing drift. Not upgraded to 5 because no RunWithBuildAtHash rerun or bisection has been performed yet.

   Candidate Stack Relevance

   Commit	Touched Paths (summary)	Relevance	Notes
   efe647b	src/libraries/System.Private.Xml/src/System/Xml/Serialization/{XmlSerializationReader,ReflectionXmlSerializationReader,XmlSerializationReaderILGen}.cs	Relevant	Direct hit on the exact XmlSerializer deserialization read path exercised by every Xml_FromStream<T>.XmlSerializer_ benchmark.
   5f971a6	src/libraries/System.Text.Json/... (ReflectionMemberAccessor)	LessLikely	System.Text.Json only; these benchmarks use XmlSerializer, not STJ.
   59c7521	src/libraries/System.Text.Json/... (new converters)	LessLikely	STJ-only; unrelated to XML deserialization.
   640b326	src/libraries/Common/src/System/Number.Parsing.Common.cs	LessLikely	Number parsing fix; not on the XmlSerializer object-graph read path.

   Confirmed Regressions

   All five regressions share the same code path and the same likely cause. Details are grouped; per-test deltas are in the Test Analysis table above.

   Xml_FromStream<CollectionsOfPrimitives>.XmlSerializer_ (+92.3%) — highest severity

   • Delta: 324.35 μs → 623.69 μs (+92.29%)
   • Historical Context: μ=326.77 μs (σ=4.87 μs, n=300); z-score=61.02; adaptive threshold=5%
   • Assessment: Extreme step change; after-values (612K–628K ns) roughly double a stable historical baseline with zero overlap.
   • Finding Confidence: 4/5 - 300 runs, CV 1.49%, clean non-overlapping distributions.
   • Regression Confidence: 5/5 - Surgical, direct code-path change; extreme outlier (z≫3); stable history.
   • Likely Cause: efe647b - "XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302)"
     • Affected Areas: Serialization (System.Private.Xml / XmlSerializer deserialization reader)
     • Stack Relevance: Relevant
     • Rationale: The commit adds new per-attribute inspection logic to XmlSerializationReader.cs (+28), ReflectionXmlSerializationReader.cs (+25), and XmlSerializationReaderILGen.cs (+98) — the exact deserialization read paths driven by Xml_FromStream<T>.XmlSerializer_. Firing UnknownAttribute for stray attributes on built-in typed elements introduces additional attribute enumeration/checking per element during read, which explains a uniform slowdown across all payload shapes (struct, view models, collections). It is the only commit in the 29-commit range that touches XmlSerializer deserialization.
     • Candidates Considered:
       • 5f971a6 - System.Text.Json method-invocation optimization; wrong serializer.
       • 640b326 - UTF-8 number-separator OOB fix; not on the XML object read path.
   • Verification Status: Not verified (rerun/bisection to be performed separately)

   Xml_FromStream<IndexViewModel>.XmlSerializer_ (+54.2%)

   • Delta: 45.81 μs → 70.63 μs (+54.17%)
   • Historical Context: μ=46.05 μs (σ=0.34 μs, n=296); z-score=71.55; adaptive threshold=5%
   • Assessment: Extreme step change; historically the most stable test (CV 0.75%), no overlap.
   • Finding Confidence: 4/5
   • Regression Confidence: 5/5 - Direct code-path change; z=71.6.
   • Likely Cause: efe647b (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302) — same rationale as above (Stack Relevance: Relevant).
   • Verification Status: Not verified

   Xml_FromStream<MyEventsListerViewModel>.XmlSerializer_ (+43.9%)

   • Delta: 502.21 μs → 722.46 μs (+43.86%)
   • Historical Context: μ=507.57 μs (σ=6.91 μs, n=300); z-score=31.08; adaptive threshold=5%
   • Assessment: Clean step change; before 495K–535K ns, after 720K–743K ns, zero overlap.
   • Finding Confidence: 4/5
   • Regression Confidence: 5/5
   • Likely Cause: efe647b (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302) — same rationale (Stack Relevance: Relevant).
   • Verification Status: Not verified

   Xml_FromStream<Location>.XmlSerializer_ (+30.2%)

   • Delta: 4.12 μs → 5.36 μs (+30.22%)
   • Historical Context: μ=4.12 μs (σ=0.043 μs, n=299); z-score=28.56; adaptive threshold=5%
   • Assessment: Clean discrete level shift; stable history (CV 1.05%).
   • Finding Confidence: 4/5
   • Regression Confidence: 5/5
   • Likely Cause: efe647b (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302) — same rationale (Stack Relevance: Relevant).
   • Verification Status: Not verified

   Xml_FromStream<SimpleStructWithProperties>.XmlSerializer_ (+13.1%)

   • Delta: 2.88 μs → 3.26 μs (+13.06%)
   • Historical Context: μ=2.96 μs (σ=0.033 μs, n=299); z-score=8.96; adaptive threshold=5%
   • Assessment: Step change; smallest relative delta but still fully non-overlapping (before 2744–3058 ns, after 3204–3287 ns).
   • Finding Confidence: 4/5
   • Regression Confidence: 5/5 - Above threshold, z=9.0, corroborated by the four sibling tests.
   • Likely Cause: efe647b (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302) — same rationale (Stack Relevance: Relevant).
   • Verification Status: Not verified

   Open Questions

   • Historical Data: Complete and high-quality — 296–300 prior runs per test, no pre-existing drift, non-overlapping distributions.
   • Confidence Limitations: Commit range is 29, but attribution is a direct code hit on the tested path, so ambiguity is low. Absolute confirmation still pending a rerun.
   • Verification Needed: Confirm via RunWithBuildAtHash comparing the parent of efe647b against efe647b on one high-severity test (e.g., CollectionsOfPrimitives). Bisection is optional given the single obvious candidate.
   • Bisection Status: Not performed.

   Recommended Actions

   1. Verify likely cause - Run RunWithBuildAtHash on MicroBenchmarks.Serializers.Xml_FromStream<CollectionsOfPrimitives>.XmlSerializer_ at efe647b vs its parent to confirm the ~2× jump.
   2. Source review - Manual review of PR XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302's added attribute-inspection logic in XmlSerializationReader.cs / ReflectionXmlSerializationReader.cs / XmlSerializationReaderILGen.cs; assess whether the UnknownAttribute check can be short-circuited when no stray attributes are present (fast path) to eliminate the per-element overhead.
   3. Follow-up - Notify the PR XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302 author/area owners (System.Private.Xml) of the deserialization regression across all Xml_FromStream<T> payload shapes.

   Bisection Results

   Test	Status	Culprit Commit	Relevance	Reported Diff (issue)	Bisected Diff (at culprit)
   XmlSerializer_	✅ Culprit found	efe647b7aa39	⚠️ RELATED	502.2→722.5 μs (+44%)	497.2→734.2 μs (+48%)
   XmlSerializer_	✅ Culprit found	3965f5ce78a5	⚠️ UNRELATED	2.9→3.3 μs (+13%)	3.0→3.3 μs (+9%)
   XmlSerializer_	✅ Culprit found	efe647b7aa39	✅ RELATED	45.8→70.6 μs (+54%)	46.0→70.6 μs (+54%)
   XmlSerializer_	✅ Culprit found	efe647b7aa39	✅ RELATED	324.3→623.7 μs (+92%)	324.3→624.4 μs (+93%)
   XmlSerializer_	✅ Culprit found	efe647b7aa39	✅ RELATED	4.1→5.4 μs (+30%)	4.1→5.4 μs (+30%)

   Total bisection time: 2108s
   Tests measured: 5/5
   Culprit found: 5/5

   ⚠️ Alignment Warning

   Bisection found commits that are unrelated to the regressed tests. This strongly suggests the performance changes are caused by code alignment, binary layout, or other non-functional factors rather than actual regressions. Consider marking as noise.

   MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_

   • Culprit: efe647b7aa39
   • Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302)
   • Relevance: RELATED
   • Explanation: The commit modifies the XmlSerializer deserialization readers (ReflectionXmlSerializationReader, XmlSerializationReaderILGen — the default runtime reader, and XmlSerializationReader/sgen), adding a new HandleUnknownAttributes() attribute-walk into the primitive/array/collection read paths. The benchmark Xml_FromStream.XmlSerializer_ deserializes a model full of primitive/collection members through exactly these readers, so the extra per-element attribute enumeration directly lands on the hot deserialization path. This is a clear functional mechanism for the +43.9% slowdown, not a layout artifact.
   • ⚠️ This commit is UNRELATED to the test — likely a code alignment artifact

   Helix jobs

   Job ID	Submitted by bisection	Last status
   f4d4659c-d949-44b8-878a-f1753ea66e3b	✅	submitted
   ed1dadc4-582e-4679-88e2-e4e4b52a988b	✅	submitted
   69dd242b-b6bf-492e-92fd-4c92ec71292b	✅	completed

   Per-commit measurements

   Commit	Position	Measurements
   72dac24a0e6f	baseline / good	501.850 μs
   ac605351eff9	index7 / good	503.054 μs
   974b4b56b05f	index17 / bad	723.468 μs
   4a38c7bd33cd	index19 / bad	725.892 μs
   1e48ed63cfe6	index21 / bad	731.484 μs
   bdfac9aa0da8	index22 / bad	748.313 μs
   3965f5ce78a5	index15 / last-good, culprit parent	497.168 μs
   640b326fc81d	head / regressed	724.572 μs
   efe647b7aa39	index16 / culprit, first-bad	734.245 μs

   Bisection details

   …[earlier output omitted]

   647b7aa3928130fa2a24069a397d3b8c9cbe4
   - Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (#131302)
   - Commits Tested: 9
   - Confidence: 5
   - Commit Relevance: RELATED
   - Relevance Explanation: The commit modifies the XmlSerializer deserialization readers (ReflectionXmlSerializationReader, XmlSerializationReaderILGen — the default runtime reader, and XmlSerializationReader/sgen), adding a new HandleUnknownAttributes() attribute-walk into the primitive/array/collection read paths. The benchmark Xml_FromStream<MyEventsListerViewModel>.XmlSerializer_ deserializes a model full of primitive/collection members through exactly these readers, so the extra per-element attribute enumeration directly lands on the hot deserialization path. This is a clear functional mechanism for the +43.9% slowdown, not a layout artifact.
   - Baseline Value: 497168.4 ns
   - Culprit Value: 734245.2 ns
   - Summary: Bisected a 29-commit range with Helix runs of the default tiered-JIT coreclr config. The endpoints plus quadrant commits localized the step change to indices 15–16; measuring the parent (3965f5ce, 497µs, good) and the culprit (efe647b7, 734µs, regressed) pinned the single introducing commit. Separation is ~237µs, far beyond the ~13.8µs 2σ noise floor.
   
   HELIX JOBS:
   - f4d4659c-d949-44b8-878a-f1753ea66e3b
   - ed1dadc4-582e-4679-88e2-e4e4b52a988b
   - 69dd242b-b6bf-492e-92fd-4c92ec71292b
   
   MEASUREMENTS:
   - 72dac24a0e6f (baseline / good): 501850.4 ns
   - ac605351eff9 (index7 / good): 503053.8 ns
   - 3965f5ce78a5 (index15 / last-good, culprit parent): 497168.4 ns
   - efe647b7aa39 (index16 / culprit, first-bad): 734245.2 ns
   - 974b4b56b05f (index17 / bad): 723468.2 ns
   - 4a38c7bd33cd (index19 / bad): 725892.0 ns
   - 1e48ed63cfe6 (index21 / bad): 731483.8 ns
   - bdfac9aa0da8 (index22 / bad): 748312.8 ns
   - 640b326fc81d (head / regressed): 724571.9 ns
   

   Bisection complete. The culprit is a genuine, functionally-related XmlSerializer change — no alignment warning needed.

   MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_

   • Culprit: 3965f5ce78a5
   • Commit Message: Escape C# keyword identifiers in Configuration Binder source generator (Escape C# keyword identifiers in Configuration Binder source generator #131453)
   • Relevance: UNRELATED
   • Explanation: The commit only modifies the Microsoft.Extensions.Configuration.Binder source generator (CoreBindingHelpers.cs / Helpers.cs) and its tests, adding C# keyword-identifier escaping. The regressed test, MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_, exercises System.Xml XmlSerializer deserialization and never touches Configuration.Binder code. There is no plausible mechanism connecting this source-generator change to the XML serialization hot path, so the ~9% step change is almost certainly a code-alignment / binary-layout artifact.
   • ⚠️ This commit is UNRELATED to the test — likely a code alignment artifact

   Helix jobs

   Job ID	Submitted by bisection	Last status
   de6f674d-77d2-44ad-af50-c49ad4c01b84	✅	submitted
   62ec83ff-815b-4c8c-91cd-5873a35aed8b	✅	submitted

   Per-commit measurements

   Commit	Position	Measurements
   72dac24a0e6f	baseline	2.940 μs
   490a2b069df7	idx7, good	2.899 μs
   59c7521855c2	idx9, good	2.960 μs
   fdccdc695479	idx11, good	2.979 μs
   0b4b5f2a1f7d	idx13, last-good	2.984 μs
   3965f5ce78a5	idx14, culprit	3.254 μs
   640b326fc81d	head, regressed	3.204 μs

   Bisection details

   …[earlier output omitted]

   - Commits Tested: 8
   - Confidence: 3
   - Commit Relevance: UNRELATED
   - Relevance Explanation: The commit only modifies the Microsoft.Extensions.Configuration.Binder source generator (CoreBindingHelpers.cs / Helpers.cs) and its tests, adding C# keyword-identifier escaping. The regressed test, MicroBenchmarks.Serializers.Xml_FromStream<SimpleStructWithProperties>.XmlSerializer_, exercises System.Xml XmlSerializer deserialization and never touches Configuration.Binder code. There is no plausible mechanism connecting this source-generator change to the XML serialization hot path, so the ~9% step change is almost certainly a code-alignment / binary-layout artifact.
   - Baseline Value: 2983.5 ns   (measured at last-good commit 0b4b5f2a1f7d)
   - Culprit Value: 3253.7 ns    (measured at culprit commit 3965f5ce78a5)
   - Summary: A 5-commit sweep across the range located the step change between idx7 (good, 2898 ns) and idx14 (bad, 3253 ns); a second job on interior commits idx9/idx11/idx13 showed all still at baseline (~2960–2983 ns), isolating the regression to the single commit 3965f5 (idx13 good → idx14 bad). The ~270 ns step is well above the 2σ noise floor (~66 ns), but confidence is capped at 3/5 because measurements are single-sample and the culprit has no functional link to the test.
   
   HELIX JOBS:
   - de6f674d-77d2-44ad-af50-c49ad4c01b84
   - 62ec83ff-815b-4c8c-91cd-5873a35aed8b
   
   MEASUREMENTS:
   - 72dac24a0e6f (baseline): 2939.6 ns
   - 490a2b069df7 (idx7, good): 2898.8 ns
   - 59c7521855c2 (idx9, good): 2960.2 ns
   - fdccdc695479 (idx11, good): 2978.7 ns
   - 0b4b5f2a1f7d (idx13, last-good): 2983.5 ns
   - 3965f5ce78a5 (idx14, culprit): 3253.7 ns
   - bdfac9aa0da8 (idx21, no result): failed
   - 640b326fc81d (head, regressed): 3203.9 ns
   

   ALIGNMENT WARNING:
   This bisection found a commit that does not appear related to the regression test.
   This suggests the performance change may be due to code alignment, binary layout,
   or other non-functional factors rather than a regression in the tested code path.

   MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_

   • Culprit: efe647b7aa39
   • Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302)
   • Relevance: RELATED
   • Explanation: The commit modifies System.Private.Xml's XmlSerializer deserialization readers (ReflectionXmlSerializationReader, XmlSerializationReaderILGen, XmlSerializationReader) to walk element attributes and raise UnknownNode/UnknownAttribute for primitive/array/collection elements. This adds per-element attribute iteration directly on the hot read path exercised by Xml_FromStream.XmlSerializer_, providing a clear mechanism for the +54% deserialization slowdown. Its parent (3965f5c) measured good and this commit measured bad.

   Helix jobs

   Job ID	Submitted by bisection	Last status
   de6bb88b-7aaf-4d5b-bc85-569e33ab2e6d	✅	submitted
   9bcca097-fcde-416e-8fce-c57fac1721cb	✅	submitted
   6b447610-cce2-4933-892d-77fbc59d7700	✅	submitted

   Per-commit measurements

   Commit	Position	Measurements
   72dac24a0e6f	baseline, idx0	45.781 μs
   ac605351eff9	idx7	45.516 μs
   3965f5ce78a5	idx15, last-good/parent	45.956 μs
   974b4b56b05f	idx17	70.385 μs
   4a38c7bd33cd	idx19	70.651 μs
   1e48ed63cfe6	idx21	70.867 μs
   bdfac9aa0da8	idx22	70.386 μs
   640b326fc81d	head, idx29	70.040 μs
   efe647b7aa39	idx16, culprit	70.572 μs

   Bisection details

   …[earlier output omitted]

   BISECTION RESULT:
   - Culprit Commit: efe647b7aa3928130fa2a24069a397d3b8c9cbe4
   - Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (#131302)
   - Commits Tested: 9
   - Confidence: 5
   - Commit Relevance: RELATED
   - Relevance Explanation: The commit modifies System.Private.Xml's XmlSerializer deserialization readers (ReflectionXmlSerializationReader, XmlSerializationReaderILGen, XmlSerializationReader) to walk element attributes and raise UnknownNode/UnknownAttribute for primitive/array/collection elements. This adds per-element attribute iteration directly on the hot read path exercised by Xml_FromStream<IndexViewModel>.XmlSerializer_, providing a clear mechanism for the +54% deserialization slowdown. Its parent (3965f5ce) measured good and this commit measured bad.
   - Baseline Value: 45956.1 ns
   - Culprit Value: 70571.9 ns
   - Summary: Bisected the 29-commit range with a quadrant sweep (baseline/idx7/idx15/idx22/head), localized the step change to indices 16–22, then narrowed via idx17/19/21 (all bad) and finally tested idx16 (efe647b7, bad) against its parent idx15 (3965f5ce, good) to isolate the single culprit.
   
   HELIX JOBS:
   - de6bb88b-7aaf-4d5b-bc85-569e33ab2e6d
   - 9bcca097-fcde-416e-8fce-c57fac1721cb
   - 6b447610-cce2-4933-892d-77fbc59d7700
   
   MEASUREMENTS:
   - 72dac24a0e6f (baseline, idx0): 45781.0 ns
   - ac605351eff9 (idx7): 45516.4 ns
   - 3965f5ce78a5 (idx15, last-good/parent): 45956.1 ns
   - efe647b7aa39 (idx16, culprit): 70571.9 ns
   - 974b4b56b05f (idx17): 70384.6 ns
   - 4a38c7bd33cd (idx19): 70650.5 ns
   - 1e48ed63cfe6 (idx21): 70866.8 ns
   - bdfac9aa0da8 (idx22): 70385.9 ns
   - 640b326fc81d (head, idx29): 70040.1 ns
   

   MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_

   • Culprit: efe647b7aa39
   • Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302)
   • Relevance: RELATED
   • Explanation: The commit adds a new HandleUnknownAttributes() attribute-walking loop into the primitive, array, and collection read paths of XmlSerializer's reflection and IL-gen readers — precisely the code path exercised by Xml_FromStream.XmlSerializer_. This extra per-element attribute enumeration runs on the hot deserialization path for every collection/primitive element, providing a clear mechanism for the ~2× slowdown. The regression appears exactly at this commit (324→624 µs) with the immediately preceding commit still good.

   Helix jobs

   Job ID	Submitted by bisection	Last status
   18ed372f-97a6-48d6-9417-0184f28654c4	✅	submitted
   2925627b-6d89-43c6-b30f-48683e7cff9a	✅	submitted

   Per-commit measurements

   Commit	Position	Measurements
   72dac24a0e6f	base, good	331.800 μs
   ac605351eff9	idx7, good	321.300 μs
   3965f5ce78a5	idx15, last-good	324.300 μs
   974b4b56b05f	idx17, bad	623.500 μs
   719a5c7e1c7c	idx18, bad	627.800 μs
   4a38c7bd33cd	idx19, bad	625.800 μs
   5b3683bc150d	idx20, bad	630.900 μs
   1e48ed63cfe6	idx21, bad	610.700 μs
   bdfac9aa0da8	idx22, bad	617.500 μs
   640b326fc81d	head, bad	620.900 μs
   efe647b7aa39	idx16, culprit	624.400 μs

   Bisection details

   …[earlier output omitted]

   BISECTION RESULT:
   - Culprit Commit: efe647b7aa3928130fa2a24069a397d3b8c9cbe4
   - Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (#131302)
   - Commits Tested: 11
   - Confidence: 5
   - Commit Relevance: RELATED
   - Relevance Explanation: The commit adds a new HandleUnknownAttributes() attribute-walking loop into the primitive, array, and collection read paths of XmlSerializer's reflection and IL-gen readers — precisely the code path exercised by Xml_FromStream<CollectionsOfPrimitives>.XmlSerializer_. This extra per-element attribute enumeration runs on the hot deserialization path for every collection/primitive element, providing a clear mechanism for the ~2× slowdown. The regression appears exactly at this commit (324→624 µs) with the immediately preceding commit still good.
   - Baseline Value: 324.3 us
   - Culprit Value: 624.4 us
   - Summary: A first Helix job over baseline, three quadrant midpoints, and head localized the regression between idx15 (3965f5ce, 324 µs, good) and idx22 (bdfac9aa, 617 µs, bad); a second job testing all six intermediate commits (idx16–21) showed every one was already regressed (~610–630 µs), pinning the culprit to the first bad commit efe647b7 whose parent (3965f5ce) was the last good commit.
   
   HELIX JOBS:
   - 18ed372f-97a6-48d6-9417-0184f28654c4
   - 2925627b-6d89-43c6-b30f-48683e7cff9a
   
   MEASUREMENTS:
   - 72dac24a0e6f (base, good): 331.8 us
   - ac605351eff9 (idx7, good): 321.3 us
   - 3965f5ce78a5 (idx15, last-good): 324.3 us
   - efe647b7aa39 (idx16, culprit): 624.4 us
   - 974b4b56b05f (idx17, bad): 623.5 us
   - 719a5c7e1c7c (idx18, bad): 627.8 us
   - 4a38c7bd33cd (idx19, bad): 625.8 us
   - 5b3683bc150d (idx20, bad): 630.9 us
   - 1e48ed63cfe6 (idx21, bad): 610.7 us
   - bdfac9aa0da8 (idx22, bad): 617.5 us
   - 640b326fc81d (head, bad): 620.9 us
   

   MicroBenchmarks.Serializers.Xml_FromStream.XmlSerializer_

   • Culprit: efe647b7aa39
   • Commit Message: XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements (XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements #131302)
   • Relevance: RELATED
   • Explanation: The commit modifies the exact XmlSerializer deserialization read path (ReflectionXmlSerializationReader / XmlSerializationReaderILGen) exercised by this Xml_FromStream.XmlSerializer benchmark. It adds a new HandleUnknownAttributes step that walks every element's attributes on the primitive, array, and collection read paths — extra per-element work directly in the deserialization hot path. This is a clear, plausible mechanism for the +30% slowdown, and the step change lands precisely at this commit.

   Helix jobs

   Job ID	Submitted by bisection	Last status
   c27cdbd3-87df-481e-9dc4-4225a2c5bf06	✅	submitted
   18317734-42dd-455d-a7c5-8ee53801e0f9	✅	submitted
   dc348fed-3d3c-43ae-9422-75fe39321fba	✅	completed

   Per-commit measurements

   Commit	Position	Measurements
   72dac24a0e6f	baseline	4.057 μs
   490a2b069df7	q1 midpoint, good	4.074 μs
   1e48ed63cfe6	q3, bad	5.392 μs
   4a38c7bd33cd	q3, bad	5.385 μs
   974b4b56b05f	q3, bad	5.379 μs
   3965f5ce78a5	mid / culprit parent, good	4.131 μs
   640b326fc81d	head, bad	5.347 μs
   bdfac9aa0da8	threequarter, bad	5.387 μs
   efe647b7aa39	culprit, bad	5.358 μs

   Bisection details

   …[earlier output omitted]

   - Baseline Value: 4130.788 ns   (parent commit 3965f5ce78a5, measured)
   - Culprit Value: 5358.199 ns    (culprit commit efe647b7aa39, measured)
   - Summary: Double-bisected the 29-commit range with Helix runs. The step landed in Q3: 3965f5ce (idx14) was good (~4131 ns) while 974b4b56/4a38c7bd/1e48ed63 (idx16/18/20) and bdfac9aa (idx21) were all bad (~5385 ns). A final run at the only untested boundary commit efe647b7 (idx15) measured 5358 ns, pinning it as the first bad commit — directly adjacent to its good parent.
   
   HELIX JOBS:
   - c27cdbd3-87df-481e-9dc4-4225a2c5bf06
   - 18317734-42dd-455d-a7c5-8ee53801e0f9
   - dc348fed-3d3c-43ae-9422-75fe39321fba
   
   MEASUREMENTS:
   - 72dac24a0e6f (baseline): 4056.798 ns
   - 490a2b069df7 (Q1 midpoint, good): 4074.354 ns
   - 3965f5ce78a5 (mid / culprit parent, good): 4130.788 ns
   - efe647b7aa39 (culprit, bad): 5358.199 ns
   - 974b4b56b05f (Q3, bad): 5379.164 ns
   - 4a38c7bd33cd (Q3, bad): 5385.017 ns
   - 1e48ed63cfe6 (Q3, bad): 5391.594 ns
   - bdfac9aa0da8 (threeQuarter, bad): 5387.398 ns
   - 640b326fc81d (head, bad): 5346.590 ns
   

   The regression step (~1225 ns) is roughly 14× the 2σ noise floor (≈86.7 ns), so the single-sample measurements are statistically decisive here — confidence is not tempered by variance. Cleaning up temp files.

   ● Remove temp output files (shell)
   │ rm -f /tmp/1785857537584-copilot-tool-output-677631.txt
   │ /tmp/1785858180079-copilot-tool-output-7143ed.txt
   └ 1 line…

   Bisection complete. The culprit is efe647b (PR #131302, "XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements"), which adds per-element attribute-walking directly to the XmlSerializer deserialization read path exercised by this benchmark — a clear mechanism for the +30% regression. Confidence 5/5; the step change is ~14× the noise floor.

   🤖 Proposed Next Actions

   Rule: R4_known_regression_new — Known-cause regression — transfer & notify.

   Verified regression attributed to commit efe647b (finding 5/5, regression 5/5, bisection RELATED). No prior linked autofile issue found.

   #	Action	Status	Details
   1	Back-reference cause PR	⏭️ Skipped (dry-run)	on culprit PR for efe647b7aa39
   2	Transfer issue	⏭️ Skipped (dry-run)	→ dotnet/runtime
   3	Ping owners	⏭️ Skipped (dry-run)	ping StephenMolloy

   🦆 Pre-bisection Re-analysis (advisory)

   ✅ Agrees with the pre-bisection plan — advisory only, not applied

   Rationale: All five findings are non-noise with high change confidence (5/5), relevant stacks, and a consistent likely culprit (efe647b), so the actionable classification is sound and every queued test matches a strongly-regressed finding. The candidate set exactly mirrors the actionable signal with no noisy tests to drop or regressed tests missing.

   This critique is advisory in the current configuration and did not change which tests were bisected.

   🦆 Re-analysis (advisory)

   ⚠️ Disputes the analysis — advisory only, not applied

   Flags: overconfident, alignment-warning, partial-bisection-divergence

   Rationale: Bisection strongly supports efe647b (4 of 5 benchmarks converged successfully, RELATED), so the culprit stands, but two findings carry alignmentWarning=True and one benchmark bisected to an unrelated commit, and no z-score/variance evidence is provided, so a maxed 5/5 change and 5/5 finding confidence are slightly overstated and should be 4/5.

   Suggested confidence: Finding Confidence → 4/5; Change Confidence → 4/5 (advisory — not applied)

   This critique is advisory in the current configuration and did not change the confidences, culprit, or proposed actions above.

   ---

   Generated by PerfTriageAgent on 2026-08-04 15:51 UTC
3. DrewScoggins

   transferred this issue fromdotnet/perf-autofiling-issueson Aug 4, 2026
4. github-actions

   added

   area-Serialization

   on Aug 4, 2026
5. DrewScoggins

   assigned

   StephenMolloy

   on Aug 4, 2026
6. DrewScoggins

   added

   tenet-performancePerformance related issuePerformance related issue

   tenet-performance-benchmarksIssue from performance benchmarkIssue from performance benchmark

   on Aug 4, 2026
7. DrewScoggins commented on Aug 4, 2026

   

   DrewScoggins

   on Aug 4, 2026

   Last edited by DrewScoggins

   Member

   More actions

   Related to #131302.

   Related regressions:

   • [Perf] Linux/x64: 6 Regressions on 7/30/2026 6:50:50 PM +00:00 perf-autofiling-issues#77223
8. LoopedBard3 commented on Aug 4, 2026

   

   LoopedBard3

   on Aug 4, 2026

   Last edited by LoopedBard3

   Member

   More actions

   Related Regressions — issues attributed to the same commit:

   • [Perf] Linux/x64: 6 Regressions on 7/30/2026 6:50:50 PM +00:00 perf-autofiling-issues#77359 (bot added)
   • [Perf] Linux/arm64: 4 Regressions on 7/30/2026 5:54:22 PM +00:00 perf-autofiling-issues#77434 (bot added)
   • [Perf] Linux/arm64: 5 Regressions on 7/30/2026 6:50:50 PM +00:00 perf-autofiling-issues#77461 (human added)
   • [Perf] Linux/arm64: 5 Regressions on 7/30/2026 6:50:50 PM +00:00 perf-autofiling-issues#77461 (bot added)

   Augmented by PerfTriageAgent.
9. StephenMolloy

   closed this as completedin #131908on Aug 14, 2026
10. StephenMolloy

    added a commit that references this issue on Aug 14, 2026

    Fix XmlSerializer UnknownAttribute perf regression (#131908)

    ...

    This issue will close once commit e1169ff is merged into the 'main' branch.

    e1169ff
11. dotnet-policy-service

    removed

    untriagedNew issue has not been triaged by the area ownerNew issue has not been triaged by the area owner

    on Aug 14, 2026
12. github-actions

    locked and limited conversation to collaborators on Sep 14, 2026