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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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| <a name="Whole-program-assumptions_002c-linker-plugin-and-symbol-visibilities"></a> |
| <h3 class="section">24.4 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>Currently, the linker plugin works only in combination |
| with the Gold linker, but a GNU ld implementation is under |
| development. |
| </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> |
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