cf-performance skill: lessons and tools from the PR #1798 session#1799
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SKILL.md: - New "Removing a defensive copy" section: the defensive/cache-return copy is usually load-bearing (consumers mutate the result, so a boundary flip moves the copy rather than removing it); freeze-as-bug-finder to enumerate mutating consumers; when a freeze/assert flushes a cluster, suspect one shared copier bug before assuming pervasive aliasing; a benign latent aliasing bug may have no wrong-result symptom (needs a unit test or the enforcement that makes it observable). - Two new habits: measure the addressable fraction before building a subset-only optimization (with the static-counter + shutdown-hook + -D-toggle recipe), and "count != cost". - Deterministic-allocation section: settings=profile traces carry no TLAB attribution (use record-jfr.sh's JFC for that), and the deterministic reader is blind to pure-CPU/traversal wins; points at the new ab-measure.sh driver. Tools: - gen-sized-program.py gains a --shape flag (generic|vararg|deep-nesting| many-fields); vararg is what exposed the PR #1798 copier bug. Backward compatible. - ab-measure.sh: A/B driver that runs each program N times and reports median allocation and median wall for the currently-built side. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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