You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
124 lines
6.0 KiB
HTML
124 lines
6.0 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Delay Slots - 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="Insn-Attributes.html#Insn-Attributes" title="Insn Attributes">
|
||
|
|
<link rel="prev" href="Mnemonic-Attribute.html#Mnemonic-Attribute" title="Mnemonic Attribute">
|
||
|
|
<link rel="next" href="Processor-pipeline-description.html#Processor-pipeline-description" title="Processor pipeline description">
|
||
|
|
<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="Delay-Slots"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Processor-pipeline-description.html#Processor-pipeline-description">Processor pipeline description</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h4 class="subsection">16.19.8 Delay Slot Scheduling</h4>
|
||
|
|
|
||
|
|
<p><a name="index-delay-slots_002c-defining-3778"></a>
|
||
|
|
The insn attribute mechanism can be used to specify the requirements for
|
||
|
|
delay slots, if any, on a target machine. An instruction is said to
|
||
|
|
require a <dfn>delay slot</dfn> if some instructions that are physically
|
||
|
|
after the instruction are executed as if they were located before it.
|
||
|
|
Classic examples are branch and call instructions, which often execute
|
||
|
|
the following instruction before the branch or call is performed.
|
||
|
|
|
||
|
|
<p>On some machines, conditional branch instructions can optionally
|
||
|
|
<dfn>annul</dfn> instructions in the delay slot. This means that the
|
||
|
|
instruction will not be executed for certain branch outcomes. Both
|
||
|
|
instructions that annul if the branch is true and instructions that
|
||
|
|
annul if the branch is false are supported.
|
||
|
|
|
||
|
|
<p>Delay slot scheduling differs from instruction scheduling in that
|
||
|
|
determining whether an instruction needs a delay slot is dependent only
|
||
|
|
on the type of instruction being generated, not on data flow between the
|
||
|
|
instructions. See the next section for a discussion of data-dependent
|
||
|
|
instruction scheduling.
|
||
|
|
|
||
|
|
<p><a name="index-define_005fdelay-3779"></a>The requirement of an insn needing one or more delay slots is indicated
|
||
|
|
via the <code>define_delay</code> expression. It has the following form:
|
||
|
|
|
||
|
|
<pre class="smallexample"> (define_delay <var>test</var>
|
||
|
|
[<var>delay-1</var> <var>annul-true-1</var> <var>annul-false-1</var>
|
||
|
|
<var>delay-2</var> <var>annul-true-2</var> <var>annul-false-2</var>
|
||
|
|
...])
|
||
|
|
</pre>
|
||
|
|
<p><var>test</var> is an attribute test that indicates whether this
|
||
|
|
<code>define_delay</code> applies to a particular insn. If so, the number of
|
||
|
|
required delay slots is determined by the length of the vector specified
|
||
|
|
as the second argument. An insn placed in delay slot <var>n</var> must
|
||
|
|
satisfy attribute test <var>delay-n</var>. <var>annul-true-n</var> is an
|
||
|
|
attribute test that specifies which insns may be annulled if the branch
|
||
|
|
is true. Similarly, <var>annul-false-n</var> specifies which insns in the
|
||
|
|
delay slot may be annulled if the branch is false. If annulling is not
|
||
|
|
supported for that delay slot, <code>(nil)</code> should be coded.
|
||
|
|
|
||
|
|
<p>For example, in the common case where branch and call insns require
|
||
|
|
a single delay slot, which may contain any insn other than a branch or
|
||
|
|
call, the following would be placed in the <samp><span class="file">md</span></samp> file:
|
||
|
|
|
||
|
|
<pre class="smallexample"> (define_delay (eq_attr "type" "branch,call")
|
||
|
|
[(eq_attr "type" "!branch,call") (nil) (nil)])
|
||
|
|
</pre>
|
||
|
|
<p>Multiple <code>define_delay</code> expressions may be specified. In this
|
||
|
|
case, each such expression specifies different delay slot requirements
|
||
|
|
and there must be no insn for which tests in two <code>define_delay</code>
|
||
|
|
expressions are both true.
|
||
|
|
|
||
|
|
<p>For example, if we have a machine that requires one delay slot for branches
|
||
|
|
but two for calls, no delay slot can contain a branch or call insn,
|
||
|
|
and any valid insn in the delay slot for the branch can be annulled if the
|
||
|
|
branch is true, we might represent this as follows:
|
||
|
|
|
||
|
|
<pre class="smallexample"> (define_delay (eq_attr "type" "branch")
|
||
|
|
[(eq_attr "type" "!branch,call")
|
||
|
|
(eq_attr "type" "!branch,call")
|
||
|
|
(nil)])
|
||
|
|
|
||
|
|
(define_delay (eq_attr "type" "call")
|
||
|
|
[(eq_attr "type" "!branch,call") (nil) (nil)
|
||
|
|
(eq_attr "type" "!branch,call") (nil) (nil)])
|
||
|
|
</pre>
|
||
|
|
<!-- the above is *still* too long. -mew 4feb93 -->
|
||
|
|
</body></html>
|
||
|
|
|