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.
186 lines
9.2 KiB
HTML
186 lines
9.2 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>Profile information (GNU Compiler Collection (GCC) Internals)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Profile information (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="keywords" content="Profile information (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="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="prev" title="Edges">
|
||
|
|
<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="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="prev">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="prev">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>
|