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| <title>Using as: Xtensa Immediate Relaxation</title> |
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| <a name="Xtensa-Immediate-Relaxation"></a> |
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| 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="Other-Immediate-Field-Relaxation"></a> |
| <h4 class="subsubsection">9.52.4.3 Other Immediate Field Relaxation</h4> |
| <a name="index-immediate-fields_002c-relaxation"></a> |
| <a name="index-relaxation-of-immediate-fields"></a> |
| |
| <p>The assembler normally performs the following other relaxations. They |
| 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>). |
| </p> |
| <a name="index-MOVI-instructions_002c-relaxation"></a> |
| <a name="index-relaxation-of-MOVI-instructions"></a> |
| <p>The <code>MOVI</code> machine instruction can only materialize values in the |
| range from -2048 to 2047. Values outside this range are best |
| materialized with <code>L32R</code> instructions. Thus: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> movi a0, 100000 |
| </pre></div> |
| |
| <p>is assembled into the following machine code: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> .literal .L1, 100000 |
| l32r a0, .L1 |
| </pre></div> |
| |
| <a name="index-L8UI-instructions_002c-relaxation"></a> |
| <a name="index-L16SI-instructions_002c-relaxation"></a> |
| <a name="index-L16UI-instructions_002c-relaxation"></a> |
| <a name="index-L32I-instructions_002c-relaxation"></a> |
| <a name="index-relaxation-of-L8UI-instructions"></a> |
| <a name="index-relaxation-of-L16SI-instructions"></a> |
| <a name="index-relaxation-of-L16UI-instructions"></a> |
| <a name="index-relaxation-of-L32I-instructions"></a> |
| <p>The <code>L8UI</code> machine instruction can only be used with immediate |
| offsets in the range from 0 to 255. The <code>L16SI</code> and <code>L16UI</code> |
| machine instructions can only be used with offsets from 0 to 510. The |
| <code>L32I</code> machine instruction can only be used with offsets from 0 to |
| 1020. A load offset outside these ranges can be materialized with |
| an <code>L32R</code> instruction if the destination register of the load |
| is different than the source address register. For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> l32i a1, a0, 2040 |
| </pre></div> |
| |
| <p>is translated to: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> .literal .L1, 2040 |
| l32r a1, .L1 |
| </pre><pre class="smallexample"> add a1, a0, a1 |
| l32i a1, a1, 0 |
| </pre></div> |
| |
| <p>If the load destination and source address register are the same, an |
| out-of-range offset causes an error. |
| </p> |
| <a name="index-ADDI-instructions_002c-relaxation"></a> |
| <a name="index-relaxation-of-ADDI-instructions"></a> |
| <p>The Xtensa <code>ADDI</code> instruction only allows immediate operands in the |
| range from -128 to 127. There are a number of alternate instruction |
| sequences for the <code>ADDI</code> operation. First, if the |
| immediate is 0, the <code>ADDI</code> will be turned into a <code>MOV.N</code> |
| instruction (or the equivalent <code>OR</code> instruction if the code density |
| option is not available). If the <code>ADDI</code> immediate is outside of |
| the range -128 to 127, but inside the range -32896 to 32639, an |
| <code>ADDMI</code> instruction or <code>ADDMI</code>/<code>ADDI</code> sequence will be |
| used. Finally, if the immediate is outside of this range and a free |
| register is available, an <code>L32R</code>/<code>ADD</code> sequence will be used |
| with a literal allocated from the literal pool. |
| </p> |
| <p>For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> addi a5, a6, 0 |
| addi a5, a6, 512 |
| </pre><pre class="smallexample"> addi a5, a6, 513 |
| addi a5, a6, 50000 |
| </pre></div> |
| |
| <p>is assembled into the following: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> .literal .L1, 50000 |
| mov.n a5, a6 |
| </pre><pre class="smallexample"> addmi a5, a6, 0x200 |
| addmi a5, a6, 0x200 |
| addi a5, a5, 1 |
| </pre><pre class="smallexample"> l32r a5, .L1 |
| add a5, a6, a5 |
| </pre></div> |
| |
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| 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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