213 lines
11 KiB
HTML
213 lines
11 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: Dependency analysis</title>
|
|
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals: Dependency analysis">
|
|
<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Dependency analysis">
|
|
<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="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" rel="up" title="Loop Analysis and Representation">
|
|
<link href="Machine-Desc.html#Machine-Desc" rel="next" title="Machine Desc">
|
|
<link href="Number-of-iterations.html#Number-of-iterations" rel="prev" title="Number of iterations">
|
|
<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="Dependency-analysis"></a>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Number-of-iterations.html#Number-of-iterations" accesskey="p" rel="prev">Number of iterations</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</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="Data-Dependency-Analysis"></a>
|
|
<h3 class="section">15.8 Data Dependency Analysis</h3>
|
|
<a name="index-Data-Dependency-Analysis"></a>
|
|
|
|
<p>The code for the data dependence analysis can be found in
|
|
<samp>tree-data-ref.c</samp> and its interface and data structures are
|
|
described in <samp>tree-data-ref.h</samp>. The function that computes the
|
|
data dependences for all the array and pointer references for a given
|
|
loop is <code>compute_data_dependences_for_loop</code>. This function is
|
|
currently used by the linear loop transform and the vectorization
|
|
passes. Before calling this function, one has to allocate two vectors:
|
|
a first vector will contain the set of data references that are
|
|
contained in the analyzed loop body, and the second vector will contain
|
|
the dependence relations between the data references. Thus if the
|
|
vector of data references is of size <code>n</code>, the vector containing the
|
|
dependence relations will contain <code>n*n</code> elements. However if the
|
|
analyzed loop contains side effects, such as calls that potentially can
|
|
interfere with the data references in the current analyzed loop, the
|
|
analysis stops while scanning the loop body for data references, and
|
|
inserts a single <code>chrec_dont_know</code> in the dependence relation
|
|
array.
|
|
</p>
|
|
<p>The data references are discovered in a particular order during the
|
|
scanning of the loop body: the loop body is analyzed in execution order,
|
|
and the data references of each statement are pushed at the end of the
|
|
data reference array. Two data references syntactically occur in the
|
|
program in the same order as in the array of data references. This
|
|
syntactic order is important in some classical data dependence tests,
|
|
and mapping this order to the elements of this array avoids costly
|
|
queries to the loop body representation.
|
|
</p>
|
|
<p>Three types of data references are currently handled: ARRAY_REF,
|
|
INDIRECT_REF and COMPONENT_REF. The data structure for the data reference
|
|
is <code>data_reference</code>, where <code>data_reference_p</code> is a name of a
|
|
pointer to the data reference structure. The structure contains the
|
|
following elements:
|
|
</p>
|
|
<ul>
|
|
<li> <code>base_object_info</code>: Provides information about the base object
|
|
of the data reference and its access functions. These access functions
|
|
represent the evolution of the data reference in the loop relative to
|
|
its base, in keeping with the classical meaning of the data reference
|
|
access function for the support of arrays. For example, for a reference
|
|
<code>a.b[i][j]</code>, the base object is <code>a.b</code> and the access functions,
|
|
one for each array subscript, are:
|
|
<code>{i_init, + i_step}_1, {j_init, +, j_step}_2</code>.
|
|
|
|
</li><li> <code>first_location_in_loop</code>: Provides information about the first
|
|
location accessed by the data reference in the loop and about the access
|
|
function used to represent evolution relative to this location. This data
|
|
is used to support pointers, and is not used for arrays (for which we
|
|
have base objects). Pointer accesses are represented as a one-dimensional
|
|
access that starts from the first location accessed in the loop. For
|
|
example:
|
|
|
|
<div class="smallexample">
|
|
<pre class="smallexample"> for1 i
|
|
for2 j
|
|
*((int *)p + i + j) = a[i][j];
|
|
</pre></div>
|
|
|
|
<p>The access function of the pointer access is <code>{0, + 4B}_for2</code>
|
|
relative to <code>p + i</code>. The access functions of the array are
|
|
<code>{i_init, + i_step}_for1</code> and <code>{j_init, +, j_step}_for2</code>
|
|
relative to <code>a</code>.
|
|
</p>
|
|
<p>Usually, the object the pointer refers to is either unknown, or we can’t
|
|
prove that the access is confined to the boundaries of a certain object.
|
|
</p>
|
|
<p>Two data references can be compared only if at least one of these two
|
|
representations has all its fields filled for both data references.
|
|
</p>
|
|
<p>The current strategy for data dependence tests is as follows:
|
|
If both <code>a</code> and <code>b</code> are represented as arrays, compare
|
|
<code>a.base_object</code> and <code>b.base_object</code>;
|
|
if they are equal, apply dependence tests (use access functions based on
|
|
base_objects).
|
|
Else if both <code>a</code> and <code>b</code> are represented as pointers, compare
|
|
<code>a.first_location</code> and <code>b.first_location</code>;
|
|
if they are equal, apply dependence tests (use access functions based on
|
|
first location).
|
|
However, if <code>a</code> and <code>b</code> are represented differently, only try
|
|
to prove that the bases are definitely different.
|
|
</p>
|
|
</li><li> Aliasing information.
|
|
</li><li> Alignment information.
|
|
</li></ul>
|
|
|
|
<p>The structure describing the relation between two data references is
|
|
<code>data_dependence_relation</code> and the shorter name for a pointer to
|
|
such a structure is <code>ddr_p</code>. This structure contains:
|
|
</p>
|
|
<ul>
|
|
<li> a pointer to each data reference,
|
|
</li><li> a tree node <code>are_dependent</code> that is set to <code>chrec_known</code>
|
|
if the analysis has proved that there is no dependence between these two
|
|
data references, <code>chrec_dont_know</code> if the analysis was not able to
|
|
determine any useful result and potentially there could exist a
|
|
dependence between these data references, and <code>are_dependent</code> is
|
|
set to <code>NULL_TREE</code> if there exist a dependence relation between the
|
|
data references, and the description of this dependence relation is
|
|
given in the <code>subscripts</code>, <code>dir_vects</code>, and <code>dist_vects</code>
|
|
arrays,
|
|
</li><li> a boolean that determines whether the dependence relation can be
|
|
represented by a classical distance vector,
|
|
</li><li> an array <code>subscripts</code> that contains a description of each
|
|
subscript of the data references. Given two array accesses a
|
|
subscript is the tuple composed of the access functions for a given
|
|
dimension. For example, given <code>A[f1][f2][f3]</code> and
|
|
<code>B[g1][g2][g3]</code>, there are three subscripts: <code>(f1, g1), (f2,
|
|
g2), (f3, g3)</code>.
|
|
</li><li> two arrays <code>dir_vects</code> and <code>dist_vects</code> that contain
|
|
classical representations of the data dependences under the form of
|
|
direction and distance dependence vectors,
|
|
</li><li> an array of loops <code>loop_nest</code> that contains the loops to
|
|
which the distance and direction vectors refer to.
|
|
</li></ul>
|
|
|
|
<p>Several functions for pretty printing the information extracted by the
|
|
data dependence analysis are available: <code>dump_ddrs</code> prints with a
|
|
maximum verbosity the details of a data dependence relations array,
|
|
<code>dump_dist_dir_vectors</code> prints only the classical distance and
|
|
direction vectors for a data dependence relations array, and
|
|
<code>dump_data_references</code> prints the details of the data references
|
|
contained in a data reference array.
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Number-of-iterations.html#Number-of-iterations" accesskey="p" rel="prev">Number of iterations</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</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>
|