cts: fix infinite loop in HTree bisection when sink count reaches 1 - #11146
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cts: fix infinite loop in HTree bisection when sink count reaches 1#11146oharboe wants to merge 1 commit into
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With max_wl set, once sinks per sub-region reaches 1 the only remaining stop criteria were hpwl < max_wl (CTS-0038) or hpwl not improving (CTS-0054). If hpwl keeps improving asymptotically above max_wl, the loop bisects up to clock_tree_max_depth (100) with per-level cost doubling, i.e. a practical hang. Break once sinks <= 1: further bisection cannot reduce fanout. Reviewer notes: - All .ok diffs change only the stop-criterion message line; buffer counts and tree structure are unchanged, so no QoR change in the existing suite. - Trade-off: under max_wl this stops even if hpwl is still improving toward max_wl; a long final wire is left to repair_clock_nets. - Check is placed after computeLevelTopology, matching where the old criteria fired. CTS-0136 is a new message ID. Reported in ascenium/issues/50; demonstrated by max_wl_two_sinks. Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
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This pull request introduces an early stop criterion in HTreeBuilder::run() when the number of sinks per sub-region is less than or equal to 1, logging the event with a new message ID CTS-0136 and breaking the loop. Multiple test expectation files (.ok files) have been updated to reflect this change. There are no review comments, so I have no feedback to provide.
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@luis201420 @maliberty Shooting from the hip here... I got this from letting agy loose on a confidential test case with an infinite loop. Is this plausible? |
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With max_wl set, once sinks per sub-region reaches 1 the only remaining stop criteria were hpwl < max_wl (CTS-0038) or hpwl not improving (CTS-0054). If hpwl keeps improving asymptotically above max_wl, the loop bisects up to clock_tree_max_depth (100) with per-level cost doubling, i.e. a practical hang. Break once sinks <= 1: further bisection cannot reduce fanout.
Reviewer notes:
Reported in ascenium/issues/50; demonstrated by max_wl_two_sinks.
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