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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gccint/Profile-information.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
Change-Id: I7303388733328cd98ab9aa3c30236db67f2e9e9c
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diff --git a/share/doc/gccint/Profile-information.html b/share/doc/gccint/Profile-information.html new file mode 100644 index 0000000..b5f4178 --- /dev/null +++ b/share/doc/gccint/Profile-information.html @@ -0,0 +1,186 @@ +<!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: Profile information</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Profile information"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Profile information"> +<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="Control-Flow.html#Control-Flow" rel="up" title="Control Flow"> +<link href="Maintaining-the-CFG.html#Maintaining-the-CFG" rel="next" title="Maintaining the CFG"> +<link href="Edges.html#Edges" rel="previous" title="Edges"> +<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="Profile-information"></a> +<div class="header"> +<p> +Next: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="n" rel="next">Maintaining the CFG</a>, Previous: <a href="Edges.html#Edges" accesskey="p" rel="previous">Edges</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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="Profile-information-1"></a> +<h3 class="section">15.3 Profile information</h3> + +<a name="index-profile-representation"></a> +<p>In many cases a compiler must make a choice whether to trade speed in +one part of code for speed in another, or to trade code size for code +speed. In such cases it is useful to know information about how often +some given block will be executed. That is the purpose for +maintaining profile within the flow graph. +GCC can handle profile information obtained through <em>profile +feedback</em>, but it can also estimate branch probabilities based on +statics and heuristics. +</p> +<a name="index-profile-feedback"></a> +<p>The feedback based profile is produced by compiling the program with +instrumentation, executing it on a train run and reading the numbers +of executions of basic blocks and edges back to the compiler while +re-compiling the program to produce the final executable. This method +provides very accurate information about where a program spends most +of its time on the train run. Whether it matches the average run of +course depends on the choice of train data set, but several studies +have shown that the behavior of a program usually changes just +marginally over different data sets. +</p> +<a name="index-Static-profile-estimation"></a> +<a name="index-branch-prediction"></a> +<a name="index-predict_002edef"></a> +<p>When profile feedback is not available, the compiler may be asked to +attempt to predict the behavior of each branch in the program using a +set of heuristics (see <samp>predict.def</samp> for details) and compute +estimated frequencies of each basic block by propagating the +probabilities over the graph. +</p> +<a name="index-frequency_002c-count_002c-BB_005fFREQ_005fBASE"></a> +<p>Each <code>basic_block</code> contains two integer fields to represent +profile information: <code>frequency</code> and <code>count</code>. The +<code>frequency</code> is an estimation how often is basic block executed +within a function. It is represented as an integer scaled in the +range from 0 to <code>BB_FREQ_BASE</code>. The most frequently executed +basic block in function is initially set to <code>BB_FREQ_BASE</code> and +the rest of frequencies are scaled accordingly. During optimization, +the frequency of the most frequent basic block can both decrease (for +instance by loop unrolling) or grow (for instance by cross-jumping +optimization), so scaling sometimes has to be performed multiple +times. +</p> +<a name="index-gcov_005ftype"></a> +<p>The <code>count</code> contains hard-counted numbers of execution measured +during training runs and is nonzero only when profile feedback is +available. This value is represented as the host’s widest integer +(typically a 64 bit integer) of the special type <code>gcov_type</code>. +</p> +<p>Most optimization passes can use only the frequency information of a +basic block, but a few passes may want to know hard execution counts. +The frequencies should always match the counts after scaling, however +during updating of the profile information numerical error may +accumulate into quite large errors. +</p> +<a name="index-REG_005fBR_005fPROB_005fBASE_002c-EDGE_005fFREQUENCY"></a> +<p>Each edge also contains a branch probability field: an integer in the +range from 0 to <code>REG_BR_PROB_BASE</code>. It represents probability of +passing control from the end of the <code>src</code> basic block to the +<code>dest</code> basic block, i.e. the probability that control will flow +along this edge. The <code>EDGE_FREQUENCY</code> macro is available to +compute how frequently a given edge is taken. There is a <code>count</code> +field for each edge as well, representing same information as for a +basic block. +</p> +<p>The basic block frequencies are not represented in the instruction +stream, but in the RTL representation the edge frequencies are +represented for conditional jumps (via the <code>REG_BR_PROB</code> +macro) since they are used when instructions are output to the +assembly file and the flow graph is no longer maintained. +</p> +<a name="index-reverse-probability"></a> +<p>The probability that control flow arrives via a given edge to its +destination basic block is called <em>reverse probability</em> and is not +directly represented, but it may be easily computed from frequencies +of basic blocks. +</p> +<a name="index-redirect_005fedge_005fand_005fbranch"></a> +<p>Updating profile information is a delicate task that can unfortunately +not be easily integrated with the CFG manipulation API. Many of the +functions and hooks to modify the CFG, such as +<code>redirect_edge_and_branch</code>, do not have enough information to +easily update the profile, so updating it is in the majority of cases +left up to the caller. It is difficult to uncover bugs in the profile +updating code, because they manifest themselves only by producing +worse code, and checking profile consistency is not possible because +of numeric error accumulation. Hence special attention needs to be +given to this issue in each pass that modifies the CFG. +</p> +<a name="index-REG_005fBR_005fPROB_005fBASE_002c-BB_005fFREQ_005fBASE_002c-count"></a> +<p>It is important to point out that <code>REG_BR_PROB_BASE</code> and +<code>BB_FREQ_BASE</code> are both set low enough to be possible to compute +second power of any frequency or probability in the flow graph, it is +not possible to even square the <code>count</code> field, as modern CPUs are +fast enough to execute $2^32$ operations quickly. +</p> + +<hr> +<div class="header"> +<p> +Next: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="n" rel="next">Maintaining the CFG</a>, Previous: <a href="Edges.html#Edges" accesskey="p" rel="previous">Edges</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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> |