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| <a name="Define-Constraints"></a> |
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| Next: <a href="C-Constraint-Interface.html#C-Constraint-Interface" accesskey="n" rel="next">C Constraint Interface</a>, Previous: <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives" accesskey="p" rel="prev">Disable Insn Alternatives</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> |
| <a name="Defining-Machine_002dSpecific-Constraints"></a> |
| <h4 class="subsection">16.8.7 Defining Machine-Specific Constraints</h4> |
| <a name="index-defining-constraints"></a> |
| <a name="index-constraints_002c-defining"></a> |
| |
| <p>Machine-specific constraints fall into two categories: register and |
| non-register constraints. Within the latter category, constraints |
| which allow subsets of all possible memory or address operands should |
| be specially marked, to give <code>reload</code> more information. |
| </p> |
| <p>Machine-specific constraints can be given names of arbitrary length, |
| but they must be entirely composed of letters, digits, underscores |
| (‘<samp>_</samp>’), and angle brackets (‘<samp>< ></samp>’). Like C identifiers, they |
| must begin with a letter or underscore. |
| </p> |
| <p>In order to avoid ambiguity in operand constraint strings, no |
| constraint can have a name that begins with any other constraint’s |
| name. For example, if <code>x</code> is defined as a constraint name, |
| <code>xy</code> may not be, and vice versa. As a consequence of this rule, |
| no constraint may begin with one of the generic constraint letters: |
| ‘<samp>E F V X g i m n o p r s</samp>’. |
| </p> |
| <p>Register constraints correspond directly to register classes. |
| See <a href="Register-Classes.html#Register-Classes">Register Classes</a>. There is thus not much flexibility in their |
| definitions. |
| </p> |
| <dl> |
| <dt><a name="index-define_005fregister_005fconstraint"></a>MD Expression: <strong>define_register_constraint</strong> <em>name regclass docstring</em></dt> |
| <dd><p>All three arguments are string constants. |
| <var>name</var> is the name of the constraint, as it will appear in |
| <code>match_operand</code> expressions. If <var>name</var> is a multi-letter |
| constraint its length shall be the same for all constraints starting |
| with the same letter. <var>regclass</var> can be either the |
| name of the corresponding register class (see <a href="Register-Classes.html#Register-Classes">Register Classes</a>), |
| or a C expression which evaluates to the appropriate register class. |
| If it is an expression, it must have no side effects, and it cannot |
| look at the operand. The usual use of expressions is to map some |
| register constraints to <code>NO_REGS</code> when the register class |
| is not available on a given subarchitecture. |
| </p> |
| <p><var>docstring</var> is a sentence documenting the meaning of the |
| constraint. Docstrings are explained further below. |
| </p></dd></dl> |
| |
| <p>Non-register constraints are more like predicates: the constraint |
| definition gives a Boolean expression which indicates whether the |
| constraint matches. |
| </p> |
| <dl> |
| <dt><a name="index-define_005fconstraint"></a>MD Expression: <strong>define_constraint</strong> <em>name docstring exp</em></dt> |
| <dd><p>The <var>name</var> and <var>docstring</var> arguments are the same as for |
| <code>define_register_constraint</code>, but note that the docstring comes |
| immediately after the name for these expressions. <var>exp</var> is an RTL |
| expression, obeying the same rules as the RTL expressions in predicate |
| definitions. See <a href="Defining-Predicates.html#Defining-Predicates">Defining Predicates</a>, for details. If it |
| evaluates true, the constraint matches; if it evaluates false, it |
| doesn’t. Constraint expressions should indicate which RTL codes they |
| might match, just like predicate expressions. |
| </p> |
| <p><code>match_test</code> C expressions have access to the |
| following variables: |
| </p> |
| <dl compact="compact"> |
| <dt><var>op</var></dt> |
| <dd><p>The RTL object defining the operand. |
| </p></dd> |
| <dt><var>mode</var></dt> |
| <dd><p>The machine mode of <var>op</var>. |
| </p></dd> |
| <dt><var>ival</var></dt> |
| <dd><p>‘<samp>INTVAL (<var>op</var>)</samp>’, if <var>op</var> is a <code>const_int</code>. |
| </p></dd> |
| <dt><var>hval</var></dt> |
| <dd><p>‘<samp>CONST_DOUBLE_HIGH (<var>op</var>)</samp>’, if <var>op</var> is an integer |
| <code>const_double</code>. |
| </p></dd> |
| <dt><var>lval</var></dt> |
| <dd><p>‘<samp>CONST_DOUBLE_LOW (<var>op</var>)</samp>’, if <var>op</var> is an integer |
| <code>const_double</code>. |
| </p></dd> |
| <dt><var>rval</var></dt> |
| <dd><p>‘<samp>CONST_DOUBLE_REAL_VALUE (<var>op</var>)</samp>’, if <var>op</var> is a floating-point |
| <code>const_double</code>. |
| </p></dd> |
| </dl> |
| |
| <p>The <var>*val</var> variables should only be used once another piece of the |
| expression has verified that <var>op</var> is the appropriate kind of RTL |
| object. |
| </p></dd></dl> |
| |
| <p>Most non-register constraints should be defined with |
| <code>define_constraint</code>. The remaining two definition expressions |
| are only appropriate for constraints that should be handled specially |
| by <code>reload</code> if they fail to match. |
| </p> |
| <dl> |
| <dt><a name="index-define_005fmemory_005fconstraint"></a>MD Expression: <strong>define_memory_constraint</strong> <em>name docstring exp</em></dt> |
| <dd><p>Use this expression for constraints that match a subset of all memory |
| operands: that is, <code>reload</code> can make them match by converting the |
| operand to the form ‘<samp>(mem (reg <var>X</var>))<!-- /@w --></samp>’, where <var>X</var> is a |
| base register (from the register class specified by |
| <code>BASE_REG_CLASS</code>, see <a href="Register-Classes.html#Register-Classes">Register Classes</a>). |
| </p> |
| <p>For example, on the S/390, some instructions do not accept arbitrary |
| memory references, but only those that do not make use of an index |
| register. The constraint letter ‘<samp>Q</samp>’ is defined to represent a |
| memory address of this type. If ‘<samp>Q</samp>’ is defined with |
| <code>define_memory_constraint</code>, a ‘<samp>Q</samp>’ constraint can handle any |
| memory operand, because <code>reload</code> knows it can simply copy the |
| memory address into a base register if required. This is analogous to |
| the way an ‘<samp>o</samp>’ constraint can handle any memory operand. |
| </p> |
| <p>The syntax and semantics are otherwise identical to |
| <code>define_constraint</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-define_005faddress_005fconstraint"></a>MD Expression: <strong>define_address_constraint</strong> <em>name docstring exp</em></dt> |
| <dd><p>Use this expression for constraints that match a subset of all address |
| operands: that is, <code>reload</code> can make the constraint match by |
| converting the operand to the form ‘<samp>(reg <var>X</var>)<!-- /@w --></samp>’, again |
| with <var>X</var> a base register. |
| </p> |
| <p>Constraints defined with <code>define_address_constraint</code> can only be |
| used with the <code>address_operand</code> predicate, or machine-specific |
| predicates that work the same way. They are treated analogously to |
| the generic ‘<samp>p</samp>’ constraint. |
| </p> |
| <p>The syntax and semantics are otherwise identical to |
| <code>define_constraint</code>. |
| </p></dd></dl> |
| |
| <p>For historical reasons, names beginning with the letters ‘<samp>G H</samp>’ |
| are reserved for constraints that match only <code>const_double</code>s, and |
| names beginning with the letters ‘<samp>I J K L M N O P</samp>’ are reserved |
| for constraints that match only <code>const_int</code>s. This may change in |
| the future. For the time being, constraints with these names must be |
| written in a stylized form, so that <code>genpreds</code> can tell you did |
| it correctly: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_constraint "[<var>GHIJKLMNOP</var>]…" |
| "<var>doc</var>…" |
| (and (match_code "const_int") ; <span class="roman"><code>const_double</code> for G/H</span> |
| <var>condition</var>…)) ; <span class="roman">usually a <code>match_test</code></span> |
| </pre></div> |
| |
| <p>It is fine to use names beginning with other letters for constraints |
| that match <code>const_double</code>s or <code>const_int</code>s. |
| </p> |
| <p>Each docstring in a constraint definition should be one or more complete |
| sentences, marked up in Texinfo format. <em>They are currently unused.</em> |
| In the future they will be copied into the GCC manual, in <a href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>, replacing the hand-maintained tables currently found in |
| that section. Also, in the future the compiler may use this to give |
| more helpful diagnostics when poor choice of <code>asm</code> constraints |
| causes a reload failure. |
| </p> |
| <p>If you put the pseudo-Texinfo directive ‘<samp>@internal</samp>’ at the |
| beginning of a docstring, then (in the future) it will appear only in |
| the internals manual’s version of the machine-specific constraint tables. |
| Use this for constraints that should not appear in <code>asm</code> statements. |
| </p> |
| <hr> |
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| Next: <a href="C-Constraint-Interface.html#C-Constraint-Interface" accesskey="n" rel="next">C Constraint Interface</a>, Previous: <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives" accesskey="p" rel="prev">Disable Insn Alternatives</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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