ins derived its width as (_SIZE + 1) - pos, which is zero or negative when the
encoded msb is below pos: the following shift is then 32 or more, undefined, and
on x86 produces an all-ones mask that writes bits the JITs don't touch. Build
the mask from msb and shift it down instead, which is what the JITs do and can't
shift out of range. Hardware calls that encoding unpredictable, so consistency
is all that's wanted here.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SfY7iFJEjmRXf1XGrTs4MF
