9.15.8 Handling of Jump Instructions

Jump instructions are always optimized to use the smallest possible displacements. This is accomplished by using byte (8-bit) displacement jumps whenever the target is sufficiently close. If a byte displacement is insufficient a long displacement is used. We do not support word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump instruction with the ‘data16’ instruction prefix), since the 80386 insists upon masking ‘%eip’ to 16 bits after the word displacement is added. (See also see i386-Arch)

Note that the ‘jcxz’, ‘jecxz’, ‘loop’, ‘loopz’, ‘loope’, ‘loopnz’ and ‘loopne’ instructions only come in byte displacements, so that if you use these instructions (gcc does not use them) you may get an error message (and incorrect code). The AT&T 80386 assembler tries to get around this problem by expanding ‘jcxz foo’ to

         jcxz cx_zero
         jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero: