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.
187 lines
9.1 KiB
HTML
187 lines
9.1 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Looping Patterns (GNU Compiler Collection (GCC) Internals)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Looping Patterns (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="keywords" content="Looping Patterns (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<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="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
|
||
|
|
<link href="Insn-Canonicalizations.html#Insn-Canonicalizations" rel="next" title="Insn Canonicalizations">
|
||
|
|
<link href="Jump-Patterns.html#Jump-Patterns" rel="prev" title="Jump Patterns">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
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.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Looping-Patterns"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Insn-Canonicalizations.html#Insn-Canonicalizations" accesskey="n" rel="next">Insn Canonicalizations</a>, Previous: <a href="Jump-Patterns.html#Jump-Patterns" accesskey="p" rel="prev">Jump Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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="Defining-Looping-Instruction-Patterns"></a>
|
||
|
|
<h3 class="section">17.13 Defining Looping Instruction Patterns</h3>
|
||
|
|
<a name="index-looping-instruction-patterns"></a>
|
||
|
|
<a name="index-defining-looping-instruction-patterns"></a>
|
||
|
|
|
||
|
|
<p>Some machines have special jump instructions that can be utilized to
|
||
|
|
make loops more efficient. A common example is the 68000 ‘<samp>dbra</samp>’
|
||
|
|
instruction which performs a decrement of a register and a branch if the
|
||
|
|
result was greater than zero. Other machines, in particular digital
|
||
|
|
signal processors (DSPs), have special block repeat instructions to
|
||
|
|
provide low-overhead loop support. For example, the TI TMS320C3x/C4x
|
||
|
|
DSPs have a block repeat instruction that loads special registers to
|
||
|
|
mark the top and end of a loop and to count the number of loop
|
||
|
|
iterations. This avoids the need for fetching and executing a
|
||
|
|
‘<samp>dbra</samp>’-like instruction and avoids pipeline stalls associated with
|
||
|
|
the jump.
|
||
|
|
</p>
|
||
|
|
<p>GCC has three special named patterns to support low overhead looping.
|
||
|
|
They are ‘<samp>decrement_and_branch_until_zero</samp>’, ‘<samp>doloop_begin</samp>’,
|
||
|
|
and ‘<samp>doloop_end</samp>’. The first pattern,
|
||
|
|
‘<samp>decrement_and_branch_until_zero</samp>’, is not emitted during RTL
|
||
|
|
generation but may be emitted during the instruction combination phase.
|
||
|
|
This requires the assistance of the loop optimizer, using information
|
||
|
|
collected during strength reduction, to reverse a loop to count down to
|
||
|
|
zero. Some targets also require the loop optimizer to add a
|
||
|
|
<code>REG_NONNEG</code> note to indicate that the iteration count is always
|
||
|
|
positive. This is needed if the target performs a signed loop
|
||
|
|
termination test. For example, the 68000 uses a pattern similar to the
|
||
|
|
following for its <code>dbra</code> instruction:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(define_insn "decrement_and_branch_until_zero"
|
||
|
|
[(set (pc)
|
||
|
|
(if_then_else
|
||
|
|
(ge (plus:SI (match_operand:SI 0 "general_operand" "+d*am")
|
||
|
|
(const_int -1))
|
||
|
|
(const_int 0))
|
||
|
|
(label_ref (match_operand 1 "" ""))
|
||
|
|
(pc)))
|
||
|
|
(set (match_dup 0)
|
||
|
|
(plus:SI (match_dup 0)
|
||
|
|
(const_int -1)))]
|
||
|
|
"find_reg_note (insn, REG_NONNEG, 0)"
|
||
|
|
"…")
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>Note that since the insn is both a jump insn and has an output, it must
|
||
|
|
deal with its own reloads, hence the ‘m’ constraints. Also note that
|
||
|
|
since this insn is generated by the instruction combination phase
|
||
|
|
combining two sequential insns together into an implicit parallel insn,
|
||
|
|
the iteration counter needs to be biased by the same amount as the
|
||
|
|
decrement operation, in this case -1. Note that the following similar
|
||
|
|
pattern will not be matched by the combiner.
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(define_insn "decrement_and_branch_until_zero"
|
||
|
|
[(set (pc)
|
||
|
|
(if_then_else
|
||
|
|
(ge (match_operand:SI 0 "general_operand" "+d*am")
|
||
|
|
(const_int 1))
|
||
|
|
(label_ref (match_operand 1 "" ""))
|
||
|
|
(pc)))
|
||
|
|
(set (match_dup 0)
|
||
|
|
(plus:SI (match_dup 0)
|
||
|
|
(const_int -1)))]
|
||
|
|
"find_reg_note (insn, REG_NONNEG, 0)"
|
||
|
|
"…")
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>The other two special looping patterns, ‘<samp>doloop_begin</samp>’ and
|
||
|
|
‘<samp>doloop_end</samp>’, are emitted by the loop optimizer for certain
|
||
|
|
well-behaved loops with a finite number of loop iterations using
|
||
|
|
information collected during strength reduction.
|
||
|
|
</p>
|
||
|
|
<p>The ‘<samp>doloop_end</samp>’ pattern describes the actual looping instruction
|
||
|
|
(or the implicit looping operation) and the ‘<samp>doloop_begin</samp>’ pattern
|
||
|
|
is an optional companion pattern that can be used for initialization
|
||
|
|
needed for some low-overhead looping instructions.
|
||
|
|
</p>
|
||
|
|
<p>Note that some machines require the actual looping instruction to be
|
||
|
|
emitted at the top of the loop (e.g., the TMS320C3x/C4x DSPs). Emitting
|
||
|
|
the true RTL for a looping instruction at the top of the loop can cause
|
||
|
|
problems with flow analysis. So instead, a dummy <code>doloop</code> insn is
|
||
|
|
emitted at the end of the loop. The machine dependent reorg pass checks
|
||
|
|
for the presence of this <code>doloop</code> insn and then searches back to
|
||
|
|
the top of the loop, where it inserts the true looping insn (provided
|
||
|
|
there are no instructions in the loop which would cause problems). Any
|
||
|
|
additional labels can be emitted at this point. In addition, if the
|
||
|
|
desired special iteration counter register was not allocated, this
|
||
|
|
machine dependent reorg pass could emit a traditional compare and jump
|
||
|
|
instruction pair.
|
||
|
|
</p>
|
||
|
|
<p>The essential difference between the
|
||
|
|
‘<samp>decrement_and_branch_until_zero</samp>’ and the ‘<samp>doloop_end</samp>’
|
||
|
|
patterns is that the loop optimizer allocates an additional pseudo
|
||
|
|
register for the latter as an iteration counter. This pseudo register
|
||
|
|
cannot be used within the loop (i.e., general induction variables cannot
|
||
|
|
be derived from it), however, in many cases the loop induction variable
|
||
|
|
may become redundant and removed by the flow pass.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Insn-Canonicalizations.html#Insn-Canonicalizations" accesskey="n" rel="next">Insn Canonicalizations</a>, Previous: <a href="Jump-Patterns.html#Jump-Patterns" accesskey="p" rel="prev">Jump Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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>
|