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| <title>Using the GNU Compiler Collection (GCC): Constraints</title> |
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| <a name="Constraints-for-asm-Operands"></a> |
| <h3 class="section">6.42 Constraints for <code>asm</code> Operands</h3> |
| <a name="index-operand-constraints_002c-asm"></a> |
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| <a name="index-asm-constraints"></a> |
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| <p>Here are specific details on what constraint letters you can use with |
| <code>asm</code> operands. |
| Constraints can say whether |
| an operand may be in a register, and which kinds of register; whether the |
| operand can be a memory reference, and which kinds of address; whether the |
| operand may be an immediate constant, and which possible values it may |
| have. Constraints can also require two operands to match. |
| Side-effects aren’t allowed in operands of inline <code>asm</code>, unless |
| ‘<samp><</samp>’ or ‘<samp>></samp>’ constraints are used, because there is no guarantee |
| that the side-effects will happen exactly once in an instruction that can update |
| the addressing register. |
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| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="Simple-Constraints.html#Simple-Constraints" accesskey="1">Simple Constraints</a>:</td><td> </td><td align="left" valign="top">Basic use of constraints. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="2">Multi-Alternative</a>:</td><td> </td><td align="left" valign="top">When an insn has two alternative constraint-patterns. |
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| <tr><td align="left" valign="top">• <a href="Modifiers.html#Modifiers" accesskey="3">Modifiers</a>:</td><td> </td><td align="left" valign="top">More precise control over effects of constraints. |
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| <tr><td align="left" valign="top">• <a href="Machine-Constraints.html#Machine-Constraints" accesskey="4">Machine Constraints</a>:</td><td> </td><td align="left" valign="top">Special constraints for some particular machines. |
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