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