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diff --git a/share/doc/gccint/Dependency-analysis.html b/share/doc/gccint/Dependency-analysis.html new file mode 100644 index 0000000..1e9fb9d --- /dev/null +++ b/share/doc/gccint/Dependency-analysis.html @@ -0,0 +1,212 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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 5.1, 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="previous" 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="previous">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">16.8 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.cc</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 cannot +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="previous">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> |