wip perf analysis - #27
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Jul 18, 2026
sampleArgDeep (the differential harness's arg synthesizer) ignored a generic
custom type's type ARGS: it read `Option<'a>`'s definition and sampled `'a` as
the fallback Int64, so a param typed `Option<String>` got `Some(42)` and the
whole call failed to type-check. That's ~a fifth of the known-good set stuck in
`cf` (compiled fine, unprovable) purely because the harness couldn't build a
valid input.
Fix: substituteType binds each custom type's declared params to its actual type
args before descending, so `Option<String>` samples `Some("hello")`. Depth- and
node-budget-bounded (maxSampleDepth/sampleNodeBudget) so wide nested records don't
build an arg that makes the proof crawl.
On a 389-fn sample this collapses `cf` ~77 -> ~22 and pushes proven matches up
accordingly. It also surfaces 2 real miscompiles that were never exercisable
before (task #32: first numeric field of a JSON-serialized record compiles to
garbage float bits) — which is the point: more of the tree becomes provable, and
the bugs it finds were always there.
Not free: fns whose interpreter execution is genuinely slow now hit the per-fn
timeout as harness-drop instead of failing fast at arg-synth. The sweep's per-fn
retry isolates them so they don't hang the run, but a real fix wants interpreter
cancellation (task #27).
Claude-Session: https://claude.ai/code/session_015dcGWPp734R2y8UP3Qtva1
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