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<h4 class="subsection">9.15.8 Handling of Jump Instructions</h4>
<p><a name="index-jump-optimization_002c-i386-1111"></a><a name="index-i386-jump-optimization-1112"></a><a name="index-jump-optimization_002c-x86_002d64-1113"></a><a name="index-x86_002d64-jump-optimization-1114"></a>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 ‘<samp><span class="samp">data16</span></samp>’ instruction prefix), since the 80386
insists upon masking ‘<samp><span class="samp">%eip</span></samp>’ to 16 bits after the word displacement
is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)
<p>Note that the ‘<samp><span class="samp">jcxz</span></samp>’, ‘<samp><span class="samp">jecxz</span></samp>’, ‘<samp><span class="samp">loop</span></samp>’, ‘<samp><span class="samp">loopz</span></samp>’,
‘<samp><span class="samp">loope</span></samp>’, ‘<samp><span class="samp">loopnz</span></samp>’ and ‘<samp><span class="samp">loopne</span></samp>’ instructions only come in byte
displacements, so that if you use these instructions (<code>gcc</code> 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 ‘<samp><span class="samp">jcxz foo</span></samp>’
to
<pre class="smallexample"> jcxz cx_zero
jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero:
</pre>
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