Function fp8_act_quant_hopper_reject 

pub fn fp8_act_quant_hopper_reject(
    requested: bool,
    twin_present: bool,
    parent_present: bool,
    m: u32,
    k: u32,
    sm_count: u32,
) -> Option<&'static str>

Why the twin is NOT running — None means it is.

Pure, so the grammar is gradeable without a GPU or the process environment, and it NAMES the guard that refused: a perf path that asks to be enabled and silently is not measures as “no effect”.

parent_present is LOAD-BEARING and not defensive padding. A rejection hands the launch to Fp8ActQuant::shared, so with no parent to hand it to there is nothing to decline TO and the twin runs — which is exactly the pair native_fp8_act_quant_hopper_microtest builds to force each arm at every M it measures (shared: KernelHandle(0)). Without this branch the microtest’s Hopper arm would launch handle 0 at the six small-M arms whose numbers opened this lever.

It is tested ABOVE the lever as well as above the floor, deliberately: an operator’s ATLAS_FP8_ACT_QUANT_HOPPER=0 means “prefer the parent”, and on a pair that has none it must not come to mean “launch nothing”.