Confidence-Gated Admission for Hardware Prefetching: When the Gate Matters More Than the Predictor

Abstract

Learned cache prefetchers are typically evaluated against classical predictors that always issue requests, confounding the prediction model with the admission policy. We disentangle these variables with matched controls: the same admission gate is applied to both a 257-parameter online MLP and a classical stride predictor. The neural advantage vanishes; the MLP is indistinguishable from gated stride on random traffic and slower on most regular streams. The gate itself is architecturally useful independent of the predictor: on twenty SPEC CPU2017 programs in native ChampSim, it removes 35% of prefetches and improves accuracy from 11% to 15%, but DRAM reads change by only 0.07% demonstrating that proxy metrics do not predict endpoint behavior. We prove gate-closed execution reproduces the no-prefetch baseline exactly. The gate matters more than the predictor, and better proxies do not imply better endpoints.