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: