<html lang="en">
<head>
<title>C Constraint Interface - GNU Compiler Collection (GCC) Internals</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Constraints.html#Constraints" title="Constraints">
<link rel="prev" href="Define-Constraints.html#Define-Constraints" title="Define Constraints">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Funding Free Software'', the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
pre.display { font-family:inherit }
pre.format { font-family:inherit }
pre.smalldisplay { font-family:inherit; font-size:smaller }
pre.smallformat { font-family:inherit; font-size:smaller }
pre.smallexample { font-size:smaller }
pre.smalllisp { font-size:smaller }
span.sc { font-variant:small-caps }
span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<a name="C-Constraint-Interface"></a>
<p>
Previous: <a rel="previous" accesskey="p" href="Define-Constraints.html#Define-Constraints">Define Constraints</a>,
Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a>
<hr>
</div>
<h4 class="subsection">16.8.8 Testing constraints from C</h4>
<p><a name="index-testing-constraints-3371"></a><a name="index-constraints_002c-testing-3372"></a>
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><span class="file">tm_p.h</span></samp> declares a few interfaces for working
with constraints. At present these are defined for all constraints
except <code>g</code> (which is equivalent to <code>general_operand</code>).
<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><span class="samp"><</span></samp>’ is replaced with ‘<samp><span class="samp">_l</span></samp>’, and
each ‘<samp><span class="samp">></span></samp>’ with ‘<samp><span class="samp">_g</span></samp>’. Here are some examples:
<!-- the @c's prevent double blank lines in the printed manual. -->
<pre class="example">
<p><table summary=""><tr align="left"><td valign="top"><strong>Original</strong> </td><td valign="top"><strong>Mangled</strong> <br></td></tr><tr align="left"><td valign="top"><code>x</code> </td><td valign="top"><code>x</code> <br></td></tr><tr align="left"><td valign="top"><code>P42x</code> </td><td valign="top"><code>P42x</code> <br></td></tr><tr align="left"><td valign="top"><code>P4_x</code> </td><td valign="top"><code>P4__x</code> <br></td></tr><tr align="left"><td valign="top"><code>P4>x</code> </td><td valign="top"><code>P4_gx</code> <br></td></tr><tr align="left"><td valign="top"><code>P4>></code> </td><td valign="top"><code>P4_g_g</code> <br></td></tr><tr align="left"><td valign="top"><code>P4_g></code> </td><td valign="top"><code>P4__g_g</code>
<br></td></tr></table>
</pre>
<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).
<div class="defun">
— Enum: <b>constraint_num</b><var><a name="index-constraint_005fnum-3373"></a></var><br>
<blockquote><p>For each constraint except <code>g</code>, there is a corresponding
enumeration constant: ‘<samp><span class="samp">CONSTRAINT_</span></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></blockquote></div>
<div class="defun">
— Function: inline bool <b>satisfies_constraint_</b><var>m </var>(<var>rtx exp</var>)<var><a name="index-satisfies_005fconstraint_005f-3374"></a></var><br>
<blockquote><p>For each non-register constraint <var>m</var> except <code>g</code>, 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><span class="file">rtl.h</span></samp> was included
before <samp><span class="file">tm_p.h</span></samp>.
</p></blockquote></div>
<div class="defun">
— Function: bool <b>constraint_satisfied_p</b> (<var>rtx exp, enum constraint_num c</var>)<var><a name="index-constraint_005fsatisfied_005fp-3375"></a></var><br>
<blockquote><p>Like the <code>satisfies_constraint_</code><var>m</var> 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></blockquote></div>
<div class="defun">
— Function: enum reg_class <b>reg_class_for_constraint</b> (<var>enum constraint_num c</var>)<var><a name="index-reg_005fclass_005ffor_005fconstraint-3376"></a></var><br>
<blockquote><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></blockquote></div>
<p>Here is an example use of <code>satisfies_constraint_</code><var>m</var>. 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><span class="samp">K</span></samp>’ constraint. (This is a simplified version of a peephole
definition from the i386 machine description.)
<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>
</body></html>