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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 ‘<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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