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86 lines
4.5 KiB
HTML
86 lines
4.5 KiB
HTML
4 years ago
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<title>Constraints - GNU Compiler Collection (GCC) Internals</title>
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Predicates.html#Predicates" title="Predicates">
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<link rel="next" href="Standard-Names.html#Standard-Names" title="Standard Names">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Constraints"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Standard-Names.html#Standard-Names">Standard Names</a>,
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Previous: <a rel="previous" accesskey="p" href="Predicates.html#Predicates">Predicates</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.8 Operand Constraints</h3>
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<p><a name="index-operand-constraints-3304"></a><a name="index-constraints-3305"></a>
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Each <code>match_operand</code> in an instruction pattern can specify
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constraints for the operands allowed. The constraints allow you to
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fine-tune matching within the set of operands allowed by the
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predicate.
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<p>Constraints can say whether
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an operand may be in a register, and which kinds of register; whether the
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operand can be a memory reference, and which kinds of address; whether the
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operand may be an immediate constant, and which possible values it may
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have. Constraints can also require two operands to match.
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Side-effects aren't allowed in operands of inline <code>asm</code>, unless
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‘<samp><span class="samp"><</span></samp>’ or ‘<samp><span class="samp">></span></samp>’ constraints are used, because there is no guarantee
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that the side-effects will happen exactly once in an instruction that can update
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the addressing register.
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<ul class="menu">
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<li><a accesskey="1" href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>: Basic use of constraints.
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<li><a accesskey="2" href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>: When an insn has two alternative constraint-patterns.
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<li><a accesskey="3" href="Class-Preferences.html#Class-Preferences">Class Preferences</a>: Constraints guide which hard register to put things in.
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<li><a accesskey="4" href="Modifiers.html#Modifiers">Modifiers</a>: More precise control over effects of constraints.
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<li><a accesskey="5" href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>: Existing constraints for some particular machines.
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<li><a accesskey="6" href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>: Disable insn alternatives using attributes.
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<li><a accesskey="7" href="Define-Constraints.html#Define-Constraints">Define Constraints</a>: How to define machine-specific constraints.
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<li><a accesskey="8" href="C-Constraint-Interface.html#C-Constraint-Interface">C Constraint Interface</a>: How to test constraints from C code.
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</ul>
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</body></html>
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