| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- 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. --> |
| <!-- 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="Omega.html#Omega" rel="next" title="Omega"> |
| <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> |
| Next: <a href="Omega.html#Omega" accesskey="n" rel="next">Omega</a>, 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> |
| Next: <a href="Omega.html#Omega" accesskey="n" rel="next">Omega</a>, 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> |
