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135 lines
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135 lines
5.6 KiB
HTML
4 years ago
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<title>Define Subst Example - 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="Define-Subst.html#Define-Subst" title="Define Subst">
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<link rel="next" href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" title="Define Subst Pattern Matching">
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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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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Define-Subst-Example"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching">Define Subst Pattern Matching</a>,
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Up: <a rel="up" accesskey="u" href="Define-Subst.html#Define-Subst">Define Subst</a>
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<hr>
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</div>
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<h4 class="subsection">16.21.1 <code>define_subst</code> Example</h4>
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<p><a name="index-define_005fsubst-3825"></a>
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To illustrate how <code>define_subst</code> works, let us examine a simple
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template transformation.
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<p>Suppose there are two kinds of instructions: one that touches flags and
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the other that does not. The instructions of the second type could be
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generated with the following <code>define_subst</code>:
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<pre class="smallexample"> (define_subst "add_clobber_subst"
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[(set (match_operand:SI 0 "" "")
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(match_operand:SI 1 "" ""))]
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""
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[(set (match_dup 0)
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(match_dup 1))
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(clobber (reg:CC FLAGS_REG))]
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</pre>
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<p>This <code>define_subst</code> can be applied to any RTL pattern containing
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<code>set</code> of mode SI and generates a copy with clobber when it is
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applied.
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<p>Assume there is an RTL template for a <code>max</code> instruction to be used
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in <code>define_subst</code> mentioned above:
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<pre class="smallexample"> (define_insn "maxsi"
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[(set (match_operand:SI 0 "register_operand" "=r")
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(max:SI
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(match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))]
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""
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"max\t{%2, %1, %0|%0, %1, %2}"
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[...])
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</pre>
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<p>To mark the RTL template for <code>define_subst</code> application,
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subst-attributes are used. They should be declared in advance:
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<pre class="smallexample"> (define_subst_attr "add_clobber_name" "add_clobber_subst" "_noclobber" "_clobber")
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</pre>
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<p>Here ‘<samp><span class="samp">add_clobber_name</span></samp>’ is the attribute name,
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‘<samp><span class="samp">add_clobber_subst</span></samp>’ is the name of the corresponding
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<code>define_subst</code>, the third argument (‘<samp><span class="samp">_noclobber</span></samp>’) is the
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attribute value that would be substituted into the unchanged version of
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the source RTL template, and the last argument (‘<samp><span class="samp">_clobber</span></samp>’) is the
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value that would be substituted into the second, transformed,
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version of the RTL template.
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<p>Once the subst-attribute has been defined, it should be used in RTL
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templates which need to be processed by the <code>define_subst</code>. So,
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the original RTL template should be changed:
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<pre class="smallexample"> (define_insn "maxsi<add_clobber_name>"
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[(set (match_operand:SI 0 "register_operand" "=r")
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(max:SI
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(match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))]
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""
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"max\t{%2, %1, %0|%0, %1, %2}"
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[...])
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</pre>
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<p>The result of the <code>define_subst</code> usage would look like the following:
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<pre class="smallexample"> (define_insn "maxsi_noclobber"
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[(set (match_operand:SI 0 "register_operand" "=r")
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(max:SI
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(match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))]
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""
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"max\t{%2, %1, %0|%0, %1, %2}"
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[...])
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(define_insn "maxsi_clobber"
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[(set (match_operand:SI 0 "register_operand" "=r")
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(max:SI
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(match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))
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(clobber (reg:CC FLAGS_REG))]
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""
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"max\t{%2, %1, %0|%0, %1, %2}"
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[...])
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</pre>
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</body></html>
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