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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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| <a name="define_005fsubst-Example"></a> |
| <h4 class="subsection">16.21.1 <code>define_subst</code> Example</h4> |
| <a name="index-define_005fsubst-1"></a> |
| |
| <p>To illustrate how <code>define_subst</code> works, let us examine a simple |
| template transformation. |
| </p> |
| <p>Suppose there are two kinds of instructions: one that touches flags and |
| the other that does not. The instructions of the second type could be |
| generated with the following <code>define_subst</code>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_subst "add_clobber_subst" |
| [(set (match_operand:SI 0 "" "") |
| (match_operand:SI 1 "" ""))] |
| "" |
| [(set (match_dup 0) |
| (match_dup 1)) |
| (clobber (reg:CC FLAGS_REG))] |
| </pre></div> |
| |
| <p>This <code>define_subst</code> can be applied to any RTL pattern containing |
| <code>set</code> of mode SI and generates a copy with clobber when it is |
| applied. |
| </p> |
| <p>Assume there is an RTL template for a <code>max</code> instruction to be used |
| in <code>define_subst</code> mentioned above: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "maxsi" |
| [(set (match_operand:SI 0 "register_operand" "=r") |
| (max:SI |
| (match_operand:SI 1 "register_operand" "r") |
| (match_operand:SI 2 "register_operand" "r")))] |
| "" |
| "max\t{%2, %1, %0|%0, %1, %2}" |
| […]) |
| </pre></div> |
| |
| <p>To mark the RTL template for <code>define_subst</code> application, |
| subst-attributes are used. They should be declared in advance: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_subst_attr "add_clobber_name" "add_clobber_subst" "_noclobber" "_clobber") |
| </pre></div> |
| |
| <p>Here ‘<samp>add_clobber_name</samp>’ is the attribute name, |
| ‘<samp>add_clobber_subst</samp>’ is the name of the corresponding |
| <code>define_subst</code>, the third argument (‘<samp>_noclobber</samp>’) is the |
| attribute value that would be substituted into the unchanged version of |
| the source RTL template, and the last argument (‘<samp>_clobber</samp>’) is the |
| value that would be substituted into the second, transformed, |
| version of the RTL template. |
| </p> |
| <p>Once the subst-attribute has been defined, it should be used in RTL |
| templates which need to be processed by the <code>define_subst</code>. So, |
| the original RTL template should be changed: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "maxsi<add_clobber_name>" |
| [(set (match_operand:SI 0 "register_operand" "=r") |
| (max:SI |
| (match_operand:SI 1 "register_operand" "r") |
| (match_operand:SI 2 "register_operand" "r")))] |
| "" |
| "max\t{%2, %1, %0|%0, %1, %2}" |
| […]) |
| </pre></div> |
| |
| <p>The result of the <code>define_subst</code> usage would look like the following: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "maxsi_noclobber" |
| [(set (match_operand:SI 0 "register_operand" "=r") |
| (max:SI |
| (match_operand:SI 1 "register_operand" "r") |
| (match_operand:SI 2 "register_operand" "r")))] |
| "" |
| "max\t{%2, %1, %0|%0, %1, %2}" |
| […]) |
| (define_insn "maxsi_clobber" |
| [(set (match_operand:SI 0 "register_operand" "=r") |
| (max:SI |
| (match_operand:SI 1 "register_operand" "r") |
| (match_operand:SI 2 "register_operand" "r"))) |
| (clobber (reg:CC FLAGS_REG))] |
| "" |
| "max\t{%2, %1, %0|%0, %1, %2}" |
| […]) |
| </pre></div> |
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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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