1 files changed, 171 insertions, 0 deletions
diff --git a/share/doc/gccint/WHOPR.html b/share/doc/gccint/WHOPR.html
new file mode 100644
index 0000000..773de6a
--- /dev/null
+++ b/share/doc/gccint/WHOPR.html
@@ -0,0 +1,171 @@
+<!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: WHOPR</title>
+
+<meta name="description" content="GNU Compiler Collection (GCC) Internals: WHOPR">
+<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: WHOPR">
+<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="LTO.html#LTO" rel="up" title="LTO">
+<link href="Internal-flags.html#Internal-flags" rel="next" title="Internal flags">
+<link href="IPA.html#IPA" rel="previous" title="IPA">
+<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="WHOPR"></a>
+<div class="header">
+<p>
+Next: <a href="Internal-flags.html#Internal-flags" accesskey="n" rel="next">Internal flags</a>, Previous: <a href="IPA.html#IPA" accesskey="p" rel="previous">IPA</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</a> &nbsp; [<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="Whole-program-assumptions_002c-linker-plugin-and-symbol-visibilities"></a>
+<h3 class="section">25.4 Whole program assumptions, linker plugin and symbol visibilities</h3>
+
+<p>Link-time optimization gives relatively minor benefits when used
+alone.  The problem is that propagation of inter-procedural
+information does not work well across functions and variables
+that are called or referenced by other compilation units (such as
+from a dynamically linked library).  We say that such functions
+and variables are <em>externally visible</em>.
+</p>
+<p>To make the situation even more difficult, many applications
+organize themselves as a set of shared libraries, and the default
+ELF visibility rules allow one to overwrite any externally
+visible symbol with a different symbol at runtime.  This
+basically disables any optimizations across such functions and
+variables, because the compiler cannot be sure that the function
+body it is seeing is the same function body that will be used at
+runtime.  Any function or variable not declared <code>static</code> in
+the sources degrades the quality of inter-procedural
+optimization.
+</p>
+<p>To avoid this problem the compiler must assume that it sees the
+whole program when doing link-time optimization.  Strictly
+speaking, the whole program is rarely visible even at link-time.
+Standard system libraries are usually linked dynamically or not
+provided with the link-time information.  In GCC, the whole
+program option (<samp>-fwhole-program</samp>) asserts that every
+function and variable defined in the current compilation
+unit is static, except for function <code>main</code> (note: at
+link time, the current unit is the union of all objects compiled
+with LTO).  Since some functions and variables need to
+be referenced externally, for example by another DSO or from an
+assembler file, GCC also provides the function and variable
+attribute <code>externally_visible</code> which can be used to disable
+the effect of <samp>-fwhole-program</samp> on a specific symbol.
+</p>
+<p>The whole program mode assumptions are slightly more complex in
+C++, where inline functions in headers are put into <em>COMDAT</em>
+sections.  COMDAT function and variables can be defined by
+multiple object files and their bodies are unified at link-time
+and dynamic link-time.  COMDAT functions are changed to local only
+when their address is not taken and thus un-sharing them with a
+library is not harmful.  COMDAT variables always remain externally
+visible, however for readonly variables it is assumed that their
+initializers cannot be overwritten by a different value.
+</p>
+<p>GCC provides the function and variable attribute
+<code>visibility</code> that can be used to specify the visibility of
+externally visible symbols (or alternatively an
+<samp>-fdefault-visibility</samp> command line option).  ELF defines
+the <code>default</code>, <code>protected</code>, <code>hidden</code> and
+<code>internal</code> visibilities.
+</p>
+<p>The most commonly used is visibility is <code>hidden</code>.  It
+specifies that the symbol cannot be referenced from outside of
+the current shared library.  Unfortunately, this information
+cannot be used directly by the link-time optimization in the
+compiler since the whole shared library also might contain
+non-LTO objects and those are not visible to the compiler.
+</p>
+<p>GCC solves this problem using linker plugins.  A <em>linker
+plugin</em> is an interface to the linker that allows an external
+program to claim the ownership of a given object file.  The linker
+then performs the linking procedure by querying the plugin about
+the symbol table of the claimed objects and once the linking
+decisions are complete, the plugin is allowed to provide the
+final object file before the actual linking is made.  The linker
+plugin obtains the symbol resolution information which specifies
+which symbols provided by the claimed objects are bound from the
+rest of a binary being linked.
+</p>
+<p>GCC is designed to be independent of the rest of the toolchain
+and aims to support linkers without plugin support.  For this
+reason it does not use the linker plugin by default.  Instead,
+the object files are examined by <code>collect2</code> before being
+passed to the linker and objects found to have LTO sections are
+passed to <code>lto1</code> first.  This mode does not work for
+library archives.  The decision on what object files from the
+archive are needed depends on the actual linking and thus GCC
+would have to implement the linker itself.  The resolution
+information is missing too and thus GCC needs to make an educated
+guess based on <samp>-fwhole-program</samp>.  Without the linker
+plugin GCC also assumes that symbols are declared <code>hidden</code>
+and not referred by non-LTO code by default.
+</p>
+<hr>
+<div class="header">
+<p>
+Next: <a href="Internal-flags.html#Internal-flags" accesskey="n" rel="next">Internal flags</a>, Previous: <a href="IPA.html#IPA" accesskey="p" rel="previous">IPA</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</a> &nbsp; [<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>