[UBSAN][TrackFindingTracklet] shift exponent -1 is negative

[smuzaffar]

UBSAN IBs show that we have runtime error [a] coming from if (newalpha >= (1 << (nalphabits - 1))) https://github.com/cms-sw/cmssw/blob/master/L1Trigger/TrackFindingTracklet/src/Stub.cc#L60 . This happens when https://github.com/cms-sw/cmssw/blob/master/L1Trigger/TrackFindingTracklet/src/Stub.cc#L32-L35 condition is false and nalphabits remain 0. @cms-sw/l1-l2 can you please review this code and provide a fix?

[a] https://cmssdt.cern.ch/SDT/jenkins-artifacts/ubsan_logs/CMSSW_14_2_X_2024-10-14-2300/logs/c8/c8317d776207b840ed29d1bc0b023c15/log

23634.0/step2:L1Trigger/TrackFindingTracklet/src/Stub.cc:60:22: runtime error: shift exponent -1 is negative
24034.0/step2:L1Trigger/TrackFindingTracklet/src/Stub.cc:60:22: runtime error: shift exponent -1 is negative
24834.911/step2:L1Trigger/TrackFindingTracklet/src/Stub.cc:60:22: runtime error: shift exponent -1 is negative
25234.0/step2:L1Trigger/TrackFindingTracklet/src/Stub.cc:60:22: runtime error: shift exponent -1 is negative
25634.0/step2:L1Trigger/TrackFindingTracklet/src/Stub.cc:60:22: runtime error: shift exponent -1 is negative

[smuzaffar]

assign L1Trigger/TrackFindingTracklet

[cmsbuild]

New categories assigned: l1

@aloeliger,@epalencia you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

cms-bot internal usage

[cmsbuild]

A new Issue was created by @smuzaffar.

@Dr15Jones, @antoniovilela, @makortel, @mandrenguyen, @rappoccio, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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[iarspider]

Also reported by LLVM analyzer: https://cmssdt.cern.ch/SDT/jenkins-artifacts/ib-static-analysis/CMSSW_14_2_X_2024-10-15-1400/el8_amd64_gcc12/llvm-analysis/report-4d0f82.html#EndPath

[aloeliger]

@BenjaminRS

[BenjaminRS]

I believe this code comes under the L1 Tracker group. In particular this piece of code was introduced in their PR#74 here. Perhaps @tomalin and @zdemirag can comment?

[tomalin]

Adding also @aehart

[aehart]

I have opened #46481 to address this error. As I mention there, this error and its fix have no effect on the end results because newalpha is never used when nalphabits is zero. So the fix is just to appease the UBSAN.