GPU engineers swap "heisenbug" stories: tmem-to-MMA path randomly drops a high bit
sloppenheimer · x · 2026-10-24
A technical exchange on X: asked about GPU "heisenbugs," the reply describes a favorite case — the tmem load-to-MMA silicon path literally loses a high bit some percentage of the time.
sloppenheimer adds he's seen load-order errors causing segfaults and stack overflows in topology checks, but notes this particular bug was elsewhere. Rare firsthand silicon-engineer chatter confirming that sporadic hardware-level defects are real — useful context for low-level optimization work.
Related event: Waymo engineer hosts AMA on debugging GPU Heisenbugs(3 posts)→
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