154 lines
6.1 KiB
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154 lines
6.1 KiB
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<title>Using as: Xtensa Jump Relaxation</title>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Xtensa-Jump-Relaxation"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="Jump-Relaxation"></a>
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<h4 class="subsubsection">9.52.4.3 Jump Relaxation</h4>
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<a name="index-relaxation-of-jump-instructions"></a>
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<a name="index-jump-instructions_002c-relaxation"></a>
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<p>Jump instruction may require relaxation because the Xtensa jump instruction
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(<code>J</code>) provide a PC-relative offset of only 128 Kbytes in either
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direction. One option is to use jump long (<code>J.L</code>) instruction, which
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depending on jump distance may be assembled as jump (<code>J</code>) or indirect
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jump (<code>JX</code>). However it needs a free register. When there’s no spare
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register it is possible to plant intermediate jump sites (trampolines)
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between the jump instruction and its target. These sites may be located in
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areas unreachable by normal code execution flow, in that case they only
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contain intermediate jumps, or they may be inserted in the middle of code
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block, in which case there’s an additional jump from the beginning of the
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trampoline to the instruction past its end. So, for example:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> j 1f
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...
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retw
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...
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mov a10, a2
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call8 func
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...
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1:
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...
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</pre></div>
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<p>might be relaxed to:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> j .L0_TR_1
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...
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retw
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.L0_TR_1:
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j 1f
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...
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mov a10, a2
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call8 func
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...
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1:
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...
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</pre></div>
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<p>or to:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> j .L0_TR_1
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...
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retw
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...
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mov a10, a2
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j .L0_TR_0
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.L0_TR_1:
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j 1f
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.L0_TR_0:
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call8 func
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...
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1:
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...
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</pre></div>
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<p>The Xtensa assempler uses trampolines with jump around only when it cannot
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find suitable unreachable trampoline. There may be multiple trampolines
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between the jump instruction and its target.
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</p>
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<p>This relaxation does not apply to jumps to undefined symbols, assuming they
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will reach their targets once resolved.
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</p>
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<p>Jump relaxation is enabled by default because it does not affect code size
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or performance while the code itself is small. This relaxation may be
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disabled completely with ‘<samp>--no-trampolines</samp>’ or ‘<samp>--no-transform</samp>’
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command-line options (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>).
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</p>
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<hr>
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<div class="header">
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<p>
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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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</div>
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