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<title>GNU Compiler Collection (GCC) Internals: C Constraint Interface</title>
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<a name="C-Constraint-Interface"></a>
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<div class="header">
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<p>
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Previous: <a href="Define-Constraints.html#Define-Constraints" accesskey="p" rel="prev">Define Constraints</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</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="Testing-constraints-from-C"></a>
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<h4 class="subsection">16.8.8 Testing constraints from C</h4>
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<a name="index-testing-constraints"></a>
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<a name="index-constraints_002c-testing"></a>
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<p>It is occasionally useful to test a constraint from C code rather than
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implicitly via the constraint string in a <code>match_operand</code>. The
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generated file <samp>tm_p.h</samp> declares a few interfaces for working
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with constraints. At present these are defined for all constraints
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except <code>g</code> (which is equivalent to <code>general_operand</code>).
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</p>
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<p>Some valid constraint names are not valid C identifiers, so there is a
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mangling scheme for referring to them from C. Constraint names that
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do not contain angle brackets or underscores are left unchanged.
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Underscores are doubled, each ‘<samp><</samp>’ is replaced with ‘<samp>_l</samp>’, and
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each ‘<samp>></samp>’ with ‘<samp>_g</samp>’. Here are some examples:
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</p>
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<div class="example">
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<table>
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<tr><td><pre class="example"><strong>Original</strong></pre></td><td><pre class="example"><strong>Mangled</strong></pre></td></tr>
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<tr><td><pre class="example"><code>x</code></pre></td><td><pre class="example"><code>x</code></pre></td></tr>
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<tr><td><pre class="example"><code>P42x</code></pre></td><td><pre class="example"><code>P42x</code></pre></td></tr>
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<tr><td><pre class="example"><code>P4_x</code></pre></td><td><pre class="example"><code>P4__x</code></pre></td></tr>
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<tr><td><pre class="example"><code>P4>x</code></pre></td><td><pre class="example"><code>P4_gx</code></pre></td></tr>
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<tr><td><pre class="example"><code>P4>></code></pre></td><td><pre class="example"><code>P4_g_g</code></pre></td></tr>
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<tr><td><pre class="example"><code>P4_g></code></pre></td><td><pre class="example"><code>P4__g_g</code></pre></td></tr>
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</table>
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</div>
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<p>Throughout this section, the variable <var>c</var> is either a constraint
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in the abstract sense, or a constant from <code>enum constraint_num</code>;
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the variable <var>m</var> is a mangled constraint name (usually as part of
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a larger identifier).
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</p>
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<dl>
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<dt><a name="index-constraint_005fnum"></a>Enum: <strong>constraint_num</strong></dt>
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<dd><p>For each constraint except <code>g</code>, there is a corresponding
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enumeration constant: ‘<samp>CONSTRAINT_</samp>’ plus the mangled name of the
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constraint. Functions that take an <code>enum constraint_num</code> as an
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argument expect one of these constants.
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</p></dd></dl>
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<dl>
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<dt><a name="index-satisfies_005fconstraint_005f"></a>Function: <em>inline bool</em> <strong>satisfies_constraint_</strong> <em><var>m</var> (rtx <var>exp</var>)</em></dt>
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<dd><p>For each non-register constraint <var>m</var> except <code>g</code>, there is
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one of these functions; it returns <code>true</code> if <var>exp</var> satisfies the
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constraint. These functions are only visible if <samp>rtl.h</samp> was included
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before <samp>tm_p.h</samp>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-constraint_005fsatisfied_005fp"></a>Function: <em>bool</em> <strong>constraint_satisfied_p</strong> <em>(rtx <var>exp</var>, enum constraint_num <var>c</var>)</em></dt>
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<dd><p>Like the <code>satisfies_constraint_<var>m</var></code> functions, but the
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constraint to test is given as an argument, <var>c</var>. If <var>c</var>
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specifies a register constraint, this function will always return
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<code>false</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-reg_005fclass_005ffor_005fconstraint"></a>Function: <em>enum reg_class</em> <strong>reg_class_for_constraint</strong> <em>(enum constraint_num <var>c</var>)</em></dt>
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<dd><p>Returns the register class associated with <var>c</var>. If <var>c</var> is not
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a register constraint, or those registers are not available for the
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currently selected subtarget, returns <code>NO_REGS</code>.
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</p></dd></dl>
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<p>Here is an example use of <code>satisfies_constraint_<var>m</var></code>. In
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peephole optimizations (see <a href="Peephole-Definitions.html#Peephole-Definitions">Peephole Definitions</a>), operand
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constraint strings are ignored, so if there are relevant constraints,
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they must be tested in the C condition. In the example, the
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optimization is applied if operand 2 does <em>not</em> satisfy the
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‘<samp>K</samp>’ constraint. (This is a simplified version of a peephole
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definition from the i386 machine description.)
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_peephole2
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[(match_scratch:SI 3 "r")
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(set (match_operand:SI 0 "register_operand" "")
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(mult:SI (match_operand:SI 1 "memory_operand" "")
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(match_operand:SI 2 "immediate_operand" "")))]
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"!satisfies_constraint_K (operands[2])"
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[(set (match_dup 3) (match_dup 1))
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(set (match_dup 0) (mult:SI (match_dup 3) (match_dup 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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Previous: <a href="Define-Constraints.html#Define-Constraints" accesskey="p" rel="prev">Define Constraints</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</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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