172 lines
8.4 KiB
HTML
172 lines
8.4 KiB
HTML
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: WHOPR</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: WHOPR">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: WHOPR">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="LTO.html#LTO" rel="up" title="LTO">
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<link href="Internal-flags.html#Internal-flags" rel="next" title="Internal flags">
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<link href="IPA.html#IPA" rel="prev" title="IPA">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="WHOPR"></a>
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<div class="header">
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<p>
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Next: <a href="Internal-flags.html#Internal-flags" accesskey="n" rel="next">Internal flags</a>, Previous: <a href="IPA.html#IPA" accesskey="p" rel="prev">IPA</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</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>
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</div>
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<hr>
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<a name="Whole-program-assumptions_002c-linker-plugin-and-symbol-visibilities"></a>
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<h3 class="section">24.4 Whole program assumptions, linker plugin and symbol visibilities</h3>
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<p>Link-time optimization gives relatively minor benefits when used
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alone. The problem is that propagation of inter-procedural
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information does not work well across functions and variables
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that are called or referenced by other compilation units (such as
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from a dynamically linked library). We say that such functions
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and variables are <em>externally visible</em>.
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</p>
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<p>To make the situation even more difficult, many applications
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organize themselves as a set of shared libraries, and the default
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ELF visibility rules allow one to overwrite any externally
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visible symbol with a different symbol at runtime. This
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basically disables any optimizations across such functions and
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variables, because the compiler cannot be sure that the function
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body it is seeing is the same function body that will be used at
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runtime. Any function or variable not declared <code>static</code> in
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the sources degrades the quality of inter-procedural
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optimization.
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</p>
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<p>To avoid this problem the compiler must assume that it sees the
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whole program when doing link-time optimization. Strictly
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speaking, the whole program is rarely visible even at link-time.
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Standard system libraries are usually linked dynamically or not
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provided with the link-time information. In GCC, the whole
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program option (<samp>-fwhole-program</samp>) asserts that every
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function and variable defined in the current compilation
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unit is static, except for function <code>main</code> (note: at
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link time, the current unit is the union of all objects compiled
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with LTO). Since some functions and variables need to
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be referenced externally, for example by another DSO or from an
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assembler file, GCC also provides the function and variable
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attribute <code>externally_visible</code> which can be used to disable
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the effect of <samp>-fwhole-program</samp> on a specific symbol.
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</p>
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<p>The whole program mode assumptions are slightly more complex in
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C++, where inline functions in headers are put into <em>COMDAT</em>
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sections. COMDAT function and variables can be defined by
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multiple object files and their bodies are unified at link-time
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and dynamic link-time. COMDAT functions are changed to local only
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when their address is not taken and thus un-sharing them with a
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library is not harmful. COMDAT variables always remain externally
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visible, however for readonly variables it is assumed that their
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initializers cannot be overwritten by a different value.
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</p>
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<p>GCC provides the function and variable attribute
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<code>visibility</code> that can be used to specify the visibility of
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externally visible symbols (or alternatively an
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<samp>-fdefault-visibility</samp> command line option). ELF defines
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the <code>default</code>, <code>protected</code>, <code>hidden</code> and
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<code>internal</code> visibilities.
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</p>
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<p>The most commonly used is visibility is <code>hidden</code>. It
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specifies that the symbol cannot be referenced from outside of
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the current shared library. Unfortunately, this information
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cannot be used directly by the link-time optimization in the
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compiler since the whole shared library also might contain
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non-LTO objects and those are not visible to the compiler.
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</p>
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<p>GCC solves this problem using linker plugins. A <em>linker
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plugin</em> is an interface to the linker that allows an external
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program to claim the ownership of a given object file. The linker
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then performs the linking procedure by querying the plugin about
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the symbol table of the claimed objects and once the linking
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decisions are complete, the plugin is allowed to provide the
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final object file before the actual linking is made. The linker
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plugin obtains the symbol resolution information which specifies
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which symbols provided by the claimed objects are bound from the
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rest of a binary being linked.
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</p>
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<p>GCC is designed to be independent of the rest of the toolchain
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and aims to support linkers without plugin support. For this
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reason it does not use the linker plugin by default. Instead,
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the object files are examined by <code>collect2</code> before being
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passed to the linker and objects found to have LTO sections are
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passed to <code>lto1</code> first. This mode does not work for
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library archives. The decision on what object files from the
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archive are needed depends on the actual linking and thus GCC
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would have to implement the linker itself. The resolution
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information is missing too and thus GCC needs to make an educated
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guess based on <samp>-fwhole-program</samp>. Without the linker
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plugin GCC also assumes that symbols are declared <code>hidden</code>
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and not referred by non-LTO code by default.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Internal-flags.html#Internal-flags" accesskey="n" rel="next">Internal flags</a>, Previous: <a href="IPA.html#IPA" accesskey="p" rel="prev">IPA</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</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>
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</div>
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</body>
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</html>
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