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| <title>Using as: Xtensa Branch Relaxation</title> |
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| <a name="Xtensa-Branch-Relaxation"></a> |
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| Next: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="n" rel="next">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="Conditional-Branch-Relaxation"></a> |
| <h4 class="subsubsection">9.52.4.1 Conditional Branch Relaxation</h4> |
| <a name="index-relaxation-of-branch-instructions"></a> |
| <a name="index-branch-instructions_002c-relaxation"></a> |
| |
| <p>When the target of a branch is too far away from the branch itself, |
| i.e., when the offset from the branch to the target is too large to fit |
| in the immediate field of the branch instruction, it may be necessary to |
| replace the branch with a branch around a jump. For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> beqz a2, L |
| </pre></div> |
| |
| <p>may result in: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> bnez.n a2, M |
| j L |
| M: |
| </pre></div> |
| |
| <p>(The <code>BNEZ.N</code> instruction would be used in this example only if the |
| density option is available. Otherwise, <code>BNEZ</code> would be used.) |
| </p> |
| <p>This relaxation works well because the unconditional jump instruction |
| has a much larger offset range than the various conditional branches. |
| However, an error will occur if a branch target is beyond the range of a |
| jump instruction. <code>as</code> cannot relax unconditional jumps. |
| Similarly, an error will occur if the original input contains an |
| unconditional jump to a target that is out of range. |
| </p> |
| <p>Branch relaxation is enabled by default. It can be disabled by using |
| underscore prefixes (see <a href="Xtensa-Opcodes.html#Xtensa-Opcodes">Opcode Names</a>), the |
| ‘<samp>--no-transform</samp>’ command-line option (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>), or the <code>no-transform</code> directive |
| (see <a href="Transform-Directive.html#Transform-Directive">transform</a>). |
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