The intersection distance offset in Cycles could have returned
a denormal floating point value for the input values of 0 (and
for the denormal input value).
This could lead to a situation when ray is unable to be advanced
when it hits an edge between two triangles: the intersection will
keep bouncing between two adjacent triangles. This is because the
ray->tmin is compared inclusively, and 0 >= <denormal zero>.
The solution is to return the smallest possible normalized floating
point value from the intersection_t_offset if the input is zero
or a denormal value (which is covered by the same t == 0 check).
This fix is hard to measure on the user level. The old code did
not cause any infinite traversal loop because of the way how the
integration is organized (some kernels offset ray.P, others check
for the number of bounces). It is possible that this fixes some
corner cases of noise: i.e. if some shadow rays falsefully were
considered occluded due to reached maximum number of bounces.
The actual problematic case was discovered during working on a
prototype which had an in-lined intersection loop with the ray
tmin offset.
Pull Request: https://projects.blender.org/blender/blender/pulls/107364
02b5c04b2b