I don't immediately know how to use SIMD to scan for 24-bit wide zero terminator or for any character for that matter. 128/24 is 5 1/3 or 5.333... Alignment would mean a lot of slow operations to stitch data between memory loads.

How would you efficiently scan for a specific 24-bit character by using SIMD? Say you're not running on Haswell, but something older that's not as good with unaligned memory accesses?