Pass --compact-trace to CBMC for concrete playback - #4680
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feliperodri merged 1 commit intoJul 29, 2026
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Concrete playback disables --slice-formula (traces from sliced formulas contain arbitrary values, model-checking#1288) and requests a full trace. On contract-heavy harnesses the resulting JSON trace is dominated by the values of hidden instrumentation steps: for the write_bytes harness from model-checking/verify-rust-std the trace weighs 427 MB, of which Kani consumes only the kani::any_raw_* return-value assignments (57 KB) - everything else is built, serialized and parsed for nothing, accounting for more than half of the end-to-end playback time. Pass --compact-trace in playback mode: CBMC's compact trace drops hidden steps and actual-parameter assignments while retaining regular assignments - a strict superset of what the playback parser reads. With a CBMC that honors the option in JSON mode, the trace shrinks to 3 MB and end-to-end playback time halves (46s to 23s), producing an identical generated unit test. The CBMC-side change is diffblue/cbmc#9135; CBMC versions that do not honor --compact-trace with --json-ui accept and ignore the option, so this change is compatible with the currently pinned CBMC and becomes effective once the CBMC pin reaches a release containing that fix. Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>
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Pull request overview
This change optimizes Kani’s concrete playback workflow by reducing the size (and parsing cost) of CBMC JSON traces, which in turn improves end-to-end playback performance on contract-heavy harnesses.
Changes:
- In concrete playback mode, pass
--compact-traceto CBMC alongside--traceto drop hidden/instrumentation-heavy steps from the trace while retaining the assignments concrete playback consumes. - Document the rationale and compatibility expectations with CBMC versions that accept but may ignore
--compact-traceunder--json-ui.
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feliperodri
approved these changes
Jul 29, 2026
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Concrete playback disables --slice-formula (traces from sliced formulas contain arbitrary values, #1288) and requests a full trace. On contract-heavy harnesses the resulting JSON trace is dominated by the values of hidden instrumentation steps: for the write_bytes harness from model-checking/verify-rust-std the trace weighs 427 MB, of which Kani consumes only the kani::any_raw_* return-value assignments (57 KB)
Pass --compact-trace in playback mode: CBMC's compact trace drops hidden steps and actual-parameter assignments while retaining regular assignments - a strict superset of what the playback parser reads. With a CBMC that honors the option in JSON mode, the trace shrinks to 3 MB and end-to-end playback time halves (46s to 23s), producing an identical generated unit test. The CBMC-side change is diffblue/cbmc#9135; CBMC versions that do not honor --compact-trace with --json-ui accept and ignore the option, so this change is compatible with the currently pinned CBMC and becomes effective once the CBMC pin reaches a release containing that fix.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.