What does the Buffer Adjust step in the early CCD-iSMSCTS flow actually do, and why is it needed specifically for SMSCTS-built clocks?

Short Answer

In the early CCD-iSMSCTS flow, CCD runs during initial_opto on iSMSCTS-synthesized clocks, generating level-friendly useful-skew offsets. The Buffer Adjust step (buffer_adjust), introduced in final_place before placement, adds or removes repeaters to physically match those useful-skew offsets -- because on an SMSCTS structure, an abstract latency offset has to be realized as actual repeater insertion/removal, not just a number applied post-hoc the way it might be on a simpler traditional tree.

Technical Explanation

• In the early CCD-iSMSCTS flow, CCD runs during initial_opto on iSMSCTS-synthesized clocks, generating SMSCTS-aware, level-friendly useful-skew offsets earlier than a traditional CCD flow would.
• The Buffer Adjust step (buffer_adjust) is introduced before final_place placement -- it adds or removes repeaters to physically match those useful-skew offsets.
• The reason this is needed specifically for SMSCTS clocks: on a structural multisource tree, an abstract latency offset has to be realized as actual repeater insertion/removal, not applied as a pure post-hoc timing number the way it might be on a simpler traditional tree.
• The early-CCD-support option (cts.multisource.integrated_subtree_synthesis_early_ccd_support) must be retained through the ENTIRE flow, from compile_fusion initial_opto all the way through clock_opt build_clock -- dropping it partway through breaks the chain of offset consistency this flow depends on.

Formula Or Decision Rule

Requires enabling compile.flow.enable_ccd, place_opt.flow.enable_ccd, compile.flow.enable_multisource_clock_trees, place_opt.flow.enable_multisource_clock_trees, and cts.multisource.integrated_subtree_synthesis_early_ccd_support -- ALL retained through the full flow from compile_fusion initial_opto through clock_opt build_clock.

What To Check

• Warning sign: useful-skew offsets computed during initial_opto don't seem to be reflected in the final built clock tree.
• Inspect: check whether cts.multisource.integrated_subtree_synthesis_early_ccd_support was actually retained through every subsequent stage, or dropped partway through the flow.
• Correct: re-enable it consistently from compile_fusion initial_opto through clock_opt build_clock and re-run, since a partial application breaks the offset chain.

Healthy, Suspicious & Hard-stop Results

• Expected: useful-skew offsets computed early in the flow are still reflected in the final built tree structure at clock_opt build_clock.
• Investigate: offsets computed during initial_opto don't match what's observed in the final tree -- check every stage in between for whether early-CCD-support was actually retained continuously.
• Stop: the early-CCD-iSMSCTS options are enabled inconsistently (on for some stages, off for others) across a single flow run -- that's not a supported partial configuration, per the documented continuity requirement.

What The Interviewer Is Testing

Whether you know this flow requires multiple options enabled TOGETHER and RETAINED CONTINUOUSLY, not a single flag you can toggle on for one stage only.