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| <title>GNU Compiler Collection (GCC) Internals: C Constraint Interface</title> |
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| <a name="C-Constraint-Interface"></a> |
| <div class="header"> |
| <p> |
| 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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| <a name="Testing-constraints-from-C"></a> |
| <h4 class="subsection">16.8.8 Testing constraints from C</h4> |
| <a name="index-testing-constraints"></a> |
| <a name="index-constraints_002c-testing"></a> |
| |
| <p>It is occasionally useful to test a constraint from C code rather than |
| implicitly via the constraint string in a <code>match_operand</code>. The |
| generated file <samp>tm_p.h</samp> declares a few interfaces for working |
| with machine-specific constraints. None of these interfaces work with |
| the generic constraints described in <a href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>. This |
| may change in the future. |
| </p> |
| <p><strong>Warning:</strong> <samp>tm_p.h</samp> may declare other functions that |
| operate on constraints, besides the ones documented here. Do not use |
| those functions from machine-dependent code. They exist to implement |
| the old constraint interface that machine-independent components of |
| the compiler still expect. They will change or disappear in the |
| future. |
| </p> |
| <p>Some valid constraint names are not valid C identifiers, so there is a |
| mangling scheme for referring to them from C. Constraint names that |
| do not contain angle brackets or underscores are left unchanged. |
| Underscores are doubled, each ‘<samp><</samp>’ is replaced with ‘<samp>_l</samp>’, and |
| each ‘<samp>></samp>’ with ‘<samp>_g</samp>’. Here are some examples: |
| </p> |
| <div class="example"> |
| <table> |
| <tr><td><pre class="example"><strong>Original</strong></pre></td><td><pre class="example"><strong>Mangled</strong></pre></td></tr> |
| <tr><td><pre class="example"><code>x</code></pre></td><td><pre class="example"><code>x</code></pre></td></tr> |
| <tr><td><pre class="example"><code>P42x</code></pre></td><td><pre class="example"><code>P42x</code></pre></td></tr> |
| <tr><td><pre class="example"><code>P4_x</code></pre></td><td><pre class="example"><code>P4__x</code></pre></td></tr> |
| <tr><td><pre class="example"><code>P4>x</code></pre></td><td><pre class="example"><code>P4_gx</code></pre></td></tr> |
| <tr><td><pre class="example"><code>P4>></code></pre></td><td><pre class="example"><code>P4_g_g</code></pre></td></tr> |
| <tr><td><pre class="example"><code>P4_g></code></pre></td><td><pre class="example"><code>P4__g_g</code></pre></td></tr> |
| </table> |
| </div> |
| |
| <p>Throughout this section, the variable <var>c</var> is either a constraint |
| in the abstract sense, or a constant from <code>enum constraint_num</code>; |
| the variable <var>m</var> is a mangled constraint name (usually as part of |
| a larger identifier). |
| </p> |
| <dl> |
| <dt><a name="index-constraint_005fnum"></a>Enum: <strong>constraint_num</strong></dt> |
| <dd><p>For each machine-specific constraint, there is a corresponding |
| enumeration constant: ‘<samp>CONSTRAINT_</samp>’ plus the mangled name of the |
| constraint. Functions that take an <code>enum constraint_num</code> as an |
| argument expect one of these constants. |
| </p> |
| <p>Machine-independent constraints do not have associated constants. |
| This may change in the future. |
| </p></dd></dl> |
| |
| <dl> |
| <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> |
| <dd><p>For each machine-specific, non-register constraint <var>m</var>, there is |
| one of these functions; it returns <code>true</code> if <var>exp</var> satisfies the |
| constraint. These functions are only visible if <samp>rtl.h</samp> was included |
| before <samp>tm_p.h</samp>. |
| </p></dd></dl> |
| |
| <dl> |
| <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> |
| <dd><p>Like the <code>satisfies_constraint_<var>m</var></code> functions, but the |
| constraint to test is given as an argument, <var>c</var>. If <var>c</var> |
| specifies a register constraint, this function will always return |
| <code>false</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-regclass_005ffor_005fconstraint"></a>Function: <em>enum reg_class</em> <strong>regclass_for_constraint</strong> <em>(enum constraint_num <var>c</var>)</em></dt> |
| <dd><p>Returns the register class associated with <var>c</var>. If <var>c</var> is not |
| a register constraint, or those registers are not available for the |
| currently selected subtarget, returns <code>NO_REGS</code>. |
| </p></dd></dl> |
| |
| <p>Here is an example use of <code>satisfies_constraint_<var>m</var></code>. In |
| peephole optimizations (see <a href="Peephole-Definitions.html#Peephole-Definitions">Peephole Definitions</a>), operand |
| constraint strings are ignored, so if there are relevant constraints, |
| they must be tested in the C condition. In the example, the |
| optimization is applied if operand 2 does <em>not</em> satisfy the |
| ‘<samp>K</samp>’ constraint. (This is a simplified version of a peephole |
| definition from the i386 machine description.) |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_peephole2 |
| [(match_scratch:SI 3 "r") |
| (set (match_operand:SI 0 "register_operand" "") |
| (mult:SI (match_operand:SI 1 "memory_operand" "") |
| (match_operand:SI 2 "immediate_operand" "")))] |
| |
| "!satisfies_constraint_K (operands[2])" |
| |
| [(set (match_dup 3) (match_dup 1)) |
| (set (match_dup 0) (mult:SI (match_dup 3) (match_dup 2)))] |
| |
| "") |
| </pre></div> |
| |
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