21.7. 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 Section 21.12 Specifying CPU Architecture)

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