The bucket search in topk_nary_search.comp started from the range
[0, 0xFF800000), which ends just below the ordered-uint mapping of +inf,
so +inf and NaN were never counted. A workgroup block with fewer than k
countable values left the ballot empty and the shader read uninitialized
shared state (hang/device lost on NVIDIA, wrong indices on AMD), and a few
+inf in a block were selected without being counted, dropping real top
values.
Map NaN to -inf on input, start from [0, 0xFFFFFFFF) so every value is
counted, and clamp the top bucket's end (2^32) instead of wrapping to 0.
The k = 1 path compared float bits as signed integers, which orders
negative values backwards; compare floats instead.
Add test_top_k_inf to test-backend-ops: negative values, fewer than k
+inf and many -inf, for k = 1, 10, 40.
Assisted-by: Claude Opus 5.5
gianni-cor
ff5888f999