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| <title>Using as: Xtensa Jump Relaxation</title> |
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| <a name="Xtensa-Jump-Relaxation"></a> |
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| Next: <a href="Xtensa-Immediate-Relaxation.html#Xtensa-Immediate-Relaxation" accesskey="n" rel="next">Xtensa Immediate Relaxation</a>, Previous: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="p" rel="prev">Xtensa Call Relaxation</a>, Up: <a href="Xtensa-Relaxation.html#Xtensa-Relaxation" accesskey="u" rel="up">Xtensa Relaxation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Jump-Relaxation"></a> |
| <h4 class="subsubsection">9.53.4.3 Jump Relaxation</h4> |
| <a name="index-relaxation-of-jump-instructions"></a> |
| <a name="index-jump-instructions_002c-relaxation"></a> |
| |
| <p>Jump instruction may require relaxation because the Xtensa jump instruction |
| (<code>J</code>) provide a PC-relative offset of only 128 Kbytes in either |
| direction. One option is to use jump long (<code>J.L</code>) instruction, which |
| depending on jump distance may be assembled as jump (<code>J</code>) or indirect |
| jump (<code>JX</code>). However it needs a free register. When there’s no spare |
| register it is possible to plant intermediate jump sites (trampolines) |
| between the jump instruction and its target. These sites may be located in |
| areas unreachable by normal code execution flow, in that case they only |
| contain intermediate jumps, or they may be inserted in the middle of code |
| block, in which case there’s an additional jump from the beginning of the |
| trampoline to the instruction past its end. So, for example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> j 1f |
| ... |
| retw |
| ... |
| mov a10, a2 |
| call8 func |
| ... |
| 1: |
| ... |
| </pre></div> |
| |
| <p>might be relaxed to: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> j .L0_TR_1 |
| ... |
| retw |
| .L0_TR_1: |
| j 1f |
| ... |
| mov a10, a2 |
| call8 func |
| ... |
| 1: |
| ... |
| </pre></div> |
| |
| <p>or to: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> j .L0_TR_1 |
| ... |
| retw |
| ... |
| mov a10, a2 |
| j .L0_TR_0 |
| .L0_TR_1: |
| j 1f |
| .L0_TR_0: |
| call8 func |
| ... |
| 1: |
| ... |
| </pre></div> |
| |
| <p>The Xtensa assempler uses trampolines with jump around only when it cannot |
| find suitable unreachable trampoline. There may be multiple trampolines |
| between the jump instruction and its target. |
| </p> |
| <p>This relaxation does not apply to jumps to undefined symbols, assuming they |
| will reach their targets once resolved. |
| </p> |
| <p>Jump relaxation is enabled by default because it does not affect code size |
| or performance while the code itself is small. This relaxation may be |
| disabled completely with ‘<samp>--no-trampolines</samp>’ or ‘<samp>--no-transform</samp>’ |
| command-line options (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>). |
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| Next: <a href="Xtensa-Immediate-Relaxation.html#Xtensa-Immediate-Relaxation" accesskey="n" rel="next">Xtensa Immediate Relaxation</a>, Previous: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="p" rel="prev">Xtensa Call Relaxation</a>, Up: <a href="Xtensa-Relaxation.html#Xtensa-Relaxation" accesskey="u" rel="up">Xtensa Relaxation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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