15 not 14. The instruction-size limit of 15 bytes still applies to instructions with a REX prefix. Section 2.2.1, Intel® 64 and IA-32 Architectures Software De

15 not 14. The instruction-size limit of 15 bytes still applies to instructions with a REX prefix. Section 2.2.1, Intel® 64 and IA-32 Architectures Software Developer’s Manual Also, long instructions are infrequent, 96%+ are 7 bytes or less. That's static and dynamic is higher.