Module ablate 

Per-kernel-family SIMD ablation switch (measurement only, __ablate feature).

Why this exists

CpuLevel::Scalar / rav1d_set_cpu_flags_mask(0) does not disable the safe-SIMD kernels on aarch64: every safe_simd/*_arm.rs dispatcher gates on archmage::Arm64::summon() (a compile/run-time CPU token) and never reads rav1d_cpu_flags_mask. So there was no way to ask “which kernel family is responsible for this failing conformance vector?”.

This module adds one: a process-global bitmask of disabled families. A disabled dispatcher returns false (“not handled”) at its entry, so the caller falls through to the generic scalar reference. Ablating family X and re-running the corpus turns correlational evidence (“this kernel logs mismatches”) into causal evidence (“these N vectors’ MD5s are wrong because of X”).

Cost when the feature is off

is_off is #[inline(always)] and returns a const false when __ablate is not enabled, so every dispatcher’s guard folds away entirely. This is compile-time-gated for the same reason __simd_test is: no per-call atomic load on the hot path in a production build.

Scope

Measurement infrastructure. Never enable __ablate in a shipped build — ablating a family makes the decoder slower, not wrong (the scalar path is the reference), but the switch is process-global and racy by design.

Enums

Family

A dispatchable SIMD kernel family, at the granularity a fix would target.

Constants

ENABLED

Whether the ablation switch is compiled in.

Functions

activity_reset

Zero every counter (call between vectors to get per-vector numbers).

activity_snapshot

Per-family counts in Family::ALL order. All zero without __ablate.

is_off

Compiles to a constant false — every dispatcher’s guard folds away.

itx_shape_report

Header only: the counters do not exist without __ablate.

itx_shape_reset

Zero the census (call between vectors to get per-vector numbers).

note

Record that f processed units of work (pixels, unless noted).

note_itx_shape

Record one inverse-transform call: its size index, its bit depth, and whether a SIMD kernel took it (false = fell through to the reference).

set_disabled

Disable every family in families (and only those).