On little-endian machines, the decoding of an oparg can be sped-up by using a single 16-bit pointer deference.
Current decoding:
leaq 2(%rcx), %rbp
movzbl -1(%rbp), %eax
movzbl -2(%rbp), %r14d
sall $8, %eax
addl %eax, %r14d
New decoding:
leaq 2(%rdx), %r12
movzwl -2(%r12), %r8d
The patch uses (unsigned short *) like the struct module does, but it could use uint16_t if necessary.
If next_instr can be advanced after the lookup rather than before, the generated code would be tighter still (removing the data dependency and shortening the movzwl instruction to drop the offset byte):
movzwl (%rdx), %r8d
leaq 2(%rdx), %rbp