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<a name="Define-Subst-Example"></a>
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<p>
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Next: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="n" rel="next">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="define_005fsubst-Example"></a>
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<h4 class="subsection">16.21.1 <code>define_subst</code> Example</h4>
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<a name="index-define_005fsubst-1"></a>
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<p>To illustrate how <code>define_subst</code> works, let us examine a simple
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template transformation.
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</p>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_subst_attr "add_clobber_name" "add_clobber_subst" "_noclobber" "_clobber")
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</pre></div>
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<p>Here ‘<samp>add_clobber_name</samp>’ is the attribute name,
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‘<samp>add_clobber_subst</samp>’ is the name of the corresponding
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<code>define_subst</code>, the third argument (‘<samp>_noclobber</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>_clobber</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>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>The result of the <code>define_subst</code> usage would look like the following:
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</p>
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<div class="smallexample">
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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></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="n" rel="next">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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