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83 lines
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83 lines
3.5 KiB
HTML
4 years ago
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<title>Constant Attributes - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Insn-Attributes.html#Insn-Attributes" title="Insn Attributes">
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<link rel="prev" href="Insn-Lengths.html#Insn-Lengths" title="Insn Lengths">
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<link rel="next" href="Mnemonic-Attribute.html#Mnemonic-Attribute" title="Mnemonic Attribute">
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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="Constant-Attributes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>,
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Previous: <a rel="previous" accesskey="p" href="Insn-Lengths.html#Insn-Lengths">Insn Lengths</a>,
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Up: <a rel="up" accesskey="u" href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>
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<hr>
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</div>
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<h4 class="subsection">16.19.6 Constant Attributes</h4>
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<p><a name="index-constant-attributes-3776"></a>
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A special form of <code>define_attr</code>, where the expression for the
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default value is a <code>const</code> expression, indicates an attribute that
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is constant for a given run of the compiler. Constant attributes may be
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used to specify which variety of processor is used. For example,
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<pre class="smallexample"> (define_attr "cpu" "m88100,m88110,m88000"
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(const
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(cond [(symbol_ref "TARGET_88100") (const_string "m88100")
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(symbol_ref "TARGET_88110") (const_string "m88110")]
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(const_string "m88000"))))
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(define_attr "memory" "fast,slow"
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(const
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(if_then_else (symbol_ref "TARGET_FAST_MEM")
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(const_string "fast")
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(const_string "slow"))))
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</pre>
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<p>The routine generated for constant attributes has no parameters as it
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does not depend on any particular insn. RTL expressions used to define
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the value of a constant attribute may use the <code>symbol_ref</code> form,
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but may not use either the <code>match_operand</code> form or <code>eq_attr</code>
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forms involving insn attributes.
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</body></html>
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