Fix for nalphabits == 0

[aehart]

PR description:

This PR fixes the runtime error seen in recent UBSAN IBs that was reported in #46403.

The fix is to add a short circuit to the offending conditional, so that when nalphabits is zero, the negative bit shift is never evaluated. This has no effect on the end results because newalpha is never used when nalphabits is zero.

Fixes #46403

PR validation:

I was able to reproduce the runtime error reported in #46403 within the 23634.0 workflow, and after adding this short circuit, the same workflow gives no error.

@tomalin

[cmsbuild]

cms-bot internal usage

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-46481/42336

[cmsbuild]

A new Pull Request was created by @aehart for master.

It involves the following packages:

• L1Trigger/TrackFindingTracklet (l1)

@aloeliger, @cmsbuild, @epalencia can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @erikbutz, @missirol, @mmusich, @skinnari this is something you requested to watch as well.
@antoniovilela, @mandrenguyen, @rappoccio, @sextonkennedy you are the release manager for this.

cms-bot commands are listed here

[smuzaffar]

test parameters:

• workflow = 23634.0

[smuzaffar]

please test

[smuzaffar]

please test for CMSSW_14_2_UBSAN_X

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4f04e0/42345/summary.html
COMMIT: 702ab37
CMSSW: CMSSW_14_2_UBSAN_X_2024-10-21-2300/el8_amd64_gcc12
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/46481/42345/install.sh to create a dev area with all the needed externals and cmssw changes.

The following merge commits were also included on top of IB + this PR after doing git cms-merge-topic:

• @SanghyunKo Adding unbiased superclusters to miniAOD and EGM nano #46423
• @tomalin Increase field width in MessageLogger Summary #46241
• @trackreco Tracking validation updates/improvements #46324
• @JunseokLee3609 Update new timing seeds for HI #46462

You can see more details here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4f04e0/42345/git-recent-commits.json
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4f04e0/42345/git-merge-result

[smuzaffar]

please test

UBSAN tests shows that runtime error is fixed now

[cmsbuild]

+1

Size: This PR adds an extra 20KB to repository
Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4f04e0/42348/summary.html
COMMIT: 702ab37
CMSSW: CMSSW_14_2_X_2024-10-22-2300/el8_amd64_gcc12
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/46481/42348/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 8 differences found in the comparisons
• DQMHistoTests: Total files compared: 47
• DQMHistoTests: Total histograms compared: 3665145
• DQMHistoTests: Total failures: 505
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3664620
• DQMHistoTests: Total skipped: 20
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 46 files compared)
• Checked 206 log files, 175 edm output root files, 47 DQM output files
• TriggerResults: no differences found

[aloeliger]

+l1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @sextonkennedy, @mandrenguyen, @antoniovilela, @rappoccio (and backports should be raised in the release meeting by the corresponding L2)

[mandrenguyen]

+1