[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?