A single NaN or +Inf logit silently broke the default sampling path. +Inf
became `mx`, then `expf((Inf-mx))`/`expf((NaN-mx))` is NaN, the softmax sum went
NaN, every probability went NaN — and dist_sample's fallback loop
`if(g_pbuf[i]>0)` is false for NaN at every index, so it returned 0. Every
subsequent token: 0. No error, no warning. @KingIcyCreamProjects found it.
dist_build now takes `mx` over finite logits only, gives a non-finite logit
probability 0, and when the distribution is unusable (no finite logit, or a
non-finite/zero sum) collapses to a delta on the finite argmax and warns ONCE
on stderr — degraded, but a valid token and a visible cause, never a silent
stream of zeros. The finite argmax uses the index found during the mx pass
(robust even when lo[0] itself is NaN, where argmax_v would wrongly return 0).
tests/test_sample_nan.c: healthy logits still sample correctly; NaN/+Inf
injected at lo[0], the middle, and the last position all pick the finite
argmax; an all-non-finite vocab leaves no NaN in the buffer and doesn't crash.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JustVugg