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| <h5 class="subsubsection">9.55.2.1 Opcode Names</h5> |
| |
| <p><a name="index-Xtensa-opcode-names-2621"></a><a name="index-opcode-names_002c-Xtensa-2622"></a> |
| See the <cite>Xtensa Instruction Set Architecture (ISA) Reference |
| Manual</cite> for a complete list of opcodes and descriptions of their |
| semantics. |
| |
| <p><a name="index-g_t_005f-opcode-prefix-2623"></a>If an opcode name is prefixed with an underscore character (‘<samp><span class="samp">_</span></samp>’), |
| <samp><span class="command">as</span></samp> will not transform that instruction in any way. The |
| underscore prefix disables both optimization (see <a href="Xtensa-Optimizations.html#Xtensa-Optimizations">Xtensa Optimizations</a>) and relaxation (see <a href="Xtensa-Relaxation.html#Xtensa-Relaxation">Xtensa Relaxation</a>) for that particular instruction. Only |
| use the underscore prefix when it is essential to select the exact |
| opcode produced by the assembler. Using this feature unnecessarily |
| makes the code less efficient by disabling assembler optimization and |
| less flexible by disabling relaxation. |
| |
| <p>Note that this special handling of underscore prefixes only applies to |
| Xtensa opcodes, not to either built-in macros or user-defined macros. |
| When an underscore prefix is used with a macro (e.g., <code>_MOV</code>), it |
| refers to a different macro. The assembler generally provides built-in |
| macros both with and without the underscore prefix, where the underscore |
| versions behave as if the underscore carries through to the instructions |
| in the macros. For example, <code>_MOV</code> may expand to <code>_MOV.N</code>. |
| |
| <p>The underscore prefix only applies to individual instructions, not to |
| series of instructions. For example, if a series of instructions have |
| underscore prefixes, the assembler will not transform the individual |
| instructions, but it may insert other instructions between them (e.g., |
| to align a <code>LOOP</code> instruction). To prevent the assembler from |
| modifying a series of instructions as a whole, use the |
| <code>no-transform</code> directive. See <a href="Transform-Directive.html#Transform-Directive">transform</a>. |
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