192 lines
8.1 KiB
HTML
192 lines
8.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2016 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. -->
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>GNU Compiler Collection (GCC) Internals: Disable Insn Alternatives</title>
|
|
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals: Disable Insn Alternatives">
|
|
<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Disable Insn Alternatives">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Constraints.html#Constraints" rel="up" title="Constraints">
|
|
<link href="Define-Constraints.html#Define-Constraints" rel="next" title="Define Constraints">
|
|
<link href="Machine-Constraints.html#Machine-Constraints" rel="prev" title="Machine Constraints">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.indentedblock {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style:oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nocodebreak {white-space:nowrap}
|
|
span.nolinebreak {white-space:nowrap}
|
|
span.roman {font-family:serif; font-weight:normal}
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
|
<a name="Disable-Insn-Alternatives"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Define-Constraints.html#Define-Constraints" accesskey="n" rel="next">Define Constraints</a>, Previous: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="p" rel="prev">Machine 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>
|
|
</div>
|
|
<hr>
|
|
<a name="Disable-insn-alternatives-using-the-enabled-attribute"></a>
|
|
<h4 class="subsection">16.8.6 Disable insn alternatives using the <code>enabled</code> attribute</h4>
|
|
<a name="index-enabled"></a>
|
|
|
|
<p>There are three insn attributes that may be used to selectively disable
|
|
instruction alternatives:
|
|
</p>
|
|
<dl compact="compact">
|
|
<dt><code>enabled</code></dt>
|
|
<dd><p>Says whether an alternative is available on the current subtarget.
|
|
</p>
|
|
</dd>
|
|
<dt><code>preferred_for_size</code></dt>
|
|
<dd><p>Says whether an enabled alternative should be used in code that is
|
|
optimized for size.
|
|
</p>
|
|
</dd>
|
|
<dt><code>preferred_for_speed</code></dt>
|
|
<dd><p>Says whether an enabled alternative should be used in code that is
|
|
optimized for speed.
|
|
</p></dd>
|
|
</dl>
|
|
|
|
<p>All these attributes should use <code>(const_int 1)</code> to allow an alternative
|
|
or <code>(const_int 0)</code> to disallow it. The attributes must be a static
|
|
property of the subtarget; they cannot for example depend on the
|
|
current operands, on the current optimization level, on the location
|
|
of the insn within the body of a loop, on whether register allocation
|
|
has finished, or on the current compiler pass.
|
|
</p>
|
|
<p>The <code>enabled</code> attribute is a correctness property. It tells GCC to act
|
|
as though the disabled alternatives were never defined in the first place.
|
|
This is useful when adding new instructions to an existing pattern in
|
|
cases where the new instructions are only available for certain cpu
|
|
architecture levels (typically mapped to the <code>-march=</code> command-line
|
|
option).
|
|
</p>
|
|
<p>In contrast, the <code>preferred_for_size</code> and <code>preferred_for_speed</code>
|
|
attributes are strong optimization hints rather than correctness properties.
|
|
<code>preferred_for_size</code> tells GCC which alternatives to consider when
|
|
adding or modifying an instruction that GCC wants to optimize for size.
|
|
<code>preferred_for_speed</code> does the same thing for speed. Note that things
|
|
like code motion can lead to cases where code optimized for size uses
|
|
alternatives that are not preferred for size, and similarly for speed.
|
|
</p>
|
|
<p>Although <code>define_insn</code>s can in principle specify the <code>enabled</code>
|
|
attribute directly, it is often clearer to have subsiduary attributes
|
|
for each architectural feature of interest. The <code>define_insn</code>s
|
|
can then use these subsiduary attributes to say which alternatives
|
|
require which features. The example below does this for <code>cpu_facility</code>.
|
|
</p>
|
|
<p>E.g. the following two patterns could easily be merged using the <code>enabled</code>
|
|
attribute:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">
|
|
(define_insn "*movdi_old"
|
|
[(set (match_operand:DI 0 "register_operand" "=d")
|
|
(match_operand:DI 1 "register_operand" " d"))]
|
|
"!TARGET_NEW"
|
|
"lgr %0,%1")
|
|
|
|
(define_insn "*movdi_new"
|
|
[(set (match_operand:DI 0 "register_operand" "=d,f,d")
|
|
(match_operand:DI 1 "register_operand" " d,d,f"))]
|
|
"TARGET_NEW"
|
|
"@
|
|
lgr %0,%1
|
|
ldgr %0,%1
|
|
lgdr %0,%1")
|
|
|
|
</pre></div>
|
|
|
|
<p>to:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">
|
|
(define_insn "*movdi_combined"
|
|
[(set (match_operand:DI 0 "register_operand" "=d,f,d")
|
|
(match_operand:DI 1 "register_operand" " d,d,f"))]
|
|
""
|
|
"@
|
|
lgr %0,%1
|
|
ldgr %0,%1
|
|
lgdr %0,%1"
|
|
[(set_attr "cpu_facility" "*,new,new")])
|
|
|
|
</pre></div>
|
|
|
|
<p>with the <code>enabled</code> attribute defined like this:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">
|
|
(define_attr "cpu_facility" "standard,new" (const_string "standard"))
|
|
|
|
(define_attr "enabled" ""
|
|
(cond [(eq_attr "cpu_facility" "standard") (const_int 1)
|
|
(and (eq_attr "cpu_facility" "new")
|
|
(ne (symbol_ref "TARGET_NEW") (const_int 0)))
|
|
(const_int 1)]
|
|
(const_int 0)))
|
|
|
|
</pre></div>
|
|
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Define-Constraints.html#Define-Constraints" accesskey="n" rel="next">Define Constraints</a>, Previous: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="p" rel="prev">Machine 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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|