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<h4 class="subsection">9.13.9 Handling of Jump Instructions</h4>

<p><a name="index-jump-optimization_002c-i386-950"></a><a name="index-i386-jump-optimization-951"></a><a name="index-jump-optimization_002c-x86_002d64-952"></a><a name="index-x86_002d64-jump-optimization-953"></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 &lsquo;<samp><span class="samp">data16</span></samp>&rsquo; instruction prefix), since the 80386
insists upon masking &lsquo;<samp><span class="samp">%eip</span></samp>&rsquo; 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 &lsquo;<samp><span class="samp">jcxz</span></samp>&rsquo;, &lsquo;<samp><span class="samp">jecxz</span></samp>&rsquo;, &lsquo;<samp><span class="samp">loop</span></samp>&rsquo;, &lsquo;<samp><span class="samp">loopz</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">loope</span></samp>&rsquo;, &lsquo;<samp><span class="samp">loopnz</span></samp>&rsquo; and &lsquo;<samp><span class="samp">loopne</span></samp>&rsquo; 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&amp;T
80386 assembler tries to get around this problem by expanding &lsquo;<samp><span class="samp">jcxz foo</span></samp>&rsquo;
to

<pre class="smallexample">              jcxz cx_zero
              jmp cx_nonzero
     cx_zero: jmp foo
     cx_nonzero:
</pre>
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