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modpowTest.FastReducer_AssertFailure_RegressionTest hanging/timing out #126212
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Reacted by Katelyn GaddThe hang seems to hit on 32-bit platforms (x86 and arm32) at this callstack:
System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__AddSelf+0x55 [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.AddSub.cs @ 67] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator_FastReducer__SubMod+0x3d [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.FastReducer.cs @ 115] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator_FastReducer__Reduce+0xad [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.FastReducer.cs @ 64] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__PowCore_7+0x73 [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs @ 854] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__PowCore_6+0x59 [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs @ 829] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__PowCoreBarrett+0x131 [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs @ 407] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__PowCore_3+0x68 [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs @ 383] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigIntegerCalculator__Pow_6+0x10f [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs @ 287] System_Runtime_Numerics_Tests!System_Runtime_Numerics_System_Numerics_BigInteger__ModPow+0x15a [/_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs @ 1145] System_Runtime_Numerics_Tests!System_Runtime_Numerics_Tests_System_Numerics_Tests_modpowTest__FastReducer_AssertFailure_RegressionTest+0x42 [/_/src/libraries/System.Runtime.Numerics/tests/BigInteger/modpow.cs @ 383]Reacted by Adam Sitnik- added a commit that references this issue
on Mar 27, 2026 Note
This analysis was generated by GitHub Copilot.
Root Cause Analysis
The most likely cause is PR #125799 ("Rewrite BigInteger internals to use native-width (nuint) limbs"), merged on March 26 — one day before this issue was opened.
That PR is a large rewrite (29 files, thousands of lines) converting all BigInteger internals from
uint[]tonuint[]. On 64-bit platformsnuintis 64 bits (fewer limbs, faster), but on 32-bit platformsnuintis still 32 bits.Why it times out on 32-bit
The test isn't failing with an assertion — it's timing out (killed after running too long). The rewrite changed
FastReducer.SubModto include a new compensating loop:// Barrett's quotient estimate may overshoot by up to 2 while (CompareActual(left, right) < 0) { AddSelf(left, modulus); }
On 32-bit, there are twice as many limbs for the same number, so the multiply/divide/compare operations in the Barrett reduction inner loop are significantly more expensive. The test's large inputs likely push past the timeout threshold on 32-bit.
Timeline
- Mar 26: PR Rewrite BigInteger internals to use native-width (nuint) limbs #125799 merged (nuint rewrite)
- Mar 27: This issue opened — test timing out across 30+ unrelated PRs
- Mar 28: PR Disable FastReducer_AssertFailure_RegressionTest on 32-bit platforms #126216 disabled the test on 32-bit with
[ActiveIssue]
Status
The test is now disabled on 32-bit. The underlying perf regression on 32-bit hasn't been addressed — just the symptom suppressed.
- added 2 commits that reference this issue
on Mar 28, 2026 - locked and limited conversation to collaborators
on May 3, 2026 - added a commit that references this issue
on Jul 15, 2026
Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1355029&view=logs&jobId=f1f3e473-935f-5de7-3348-701fec448dfb
Build error leg or test failing: System.Numerics.Tests.modpowTest.FastReducer_AssertFailure_RegressionTest
Error Message
Fill the error message using step by step known issues guidance.
{ "ErrorMessage": ["System.Runtime.Numerics.Tests: [Long Running Test] 'System.Numerics.Tests.modpowTest.FastReducer_AssertFailure_RegressionTest'", "['System.Runtime.Numerics.Tests' END OF WORK ITEM LOG: Command timed out, and was killed]"], "BuildRetry": false, "ExcludeConsoleLog": false }Known issue validation
Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1355029
Error message validated:
[System.Runtime.Numerics.Tests: [Long Running Test] 'System.Numerics.Tests.modpowTest.FastReducer_AssertFailure_RegressionTest' ['System.Runtime.Numerics.Tests' END OF WORK ITEM LOG: Command timed out, and was killed]]Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 3/27/2026 5:29:45 PM UTC
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