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LLVM Link Time Optimization: Design and Implementation
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<ul>
<li><a href="#desc">Description</a></li>
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<li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
<ul>
- <li><a href="#phase1">Phase 1 : Read LLVM Bytecode Files</a></li>
+ <li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
<li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
<li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
<li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
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<a name="desc">Description</a>
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<p>
LLVM features powerful intermodular optimizations which can be used at link
time. Link Time Optimization (LTO) is another name for intermodular optimization
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<a name="design">Design Philosophy</a>
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<p>
The LLVM Link Time Optimizer provides complete transparency, while doing
intermodular optimization, in the compiler tool chain. Its main goal is to let
in other models. The linker input allows the optimizer to avoid relying on
conservative escape analysis.
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<a name="example1">Example of link time optimization</a>
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<p>The following example illustrates the advantages of LTO's integrated
approach and clean interface. This example requires a system linker which
supports LTO through the interface described in this document. Here,
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<a name="alternative_approaches">Alternative Approaches</a>
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<dl>
<dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
<dd>In this model the link time optimizer is not able to take advantage of
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<a name="multiphase">Multi-phase communication between libLTO and linker</a>
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<p>The linker collects information about symbol defininitions and uses in
various link objects which is more accurate than any information collected
by other tools during typical build cycles. The linker collects this
Our goal is to take advantage of tight integration between the linker and
the optimizer by sharing this information during various linking phases.
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<a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
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<p>The linker first reads all object files in natural order and collects
symbol information. This includes native object files as well as LLVM bitcode
files. To minimize the cost to the linker in the case that all .o files
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<a name="phase2">Phase 2 : Symbol Resolution</a>
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<p>In this stage, the linker resolves symbols using global symbol table.
It may report undefined symbol errors, read archive members, replace
weak symbols, etc. The linker is able to do this seamlessly even though it
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<a name="phase3">Phase 3 : Optimize Bitcode Files</a>
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<p>After symbol resolution, the linker tells the LTO shared object which
symbols are needed by native object files. In the example above, the linker
reports that only <tt>foo1()</tt> is used by native object files using
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<a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
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<p>In this phase, the linker reads optimized a native object file and
updates the internal global symbol table to reflect any changes. The linker
also collects information about any changes in use of external symbols by
bitcode files.</p>
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<a name="lto">libLTO</a>
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<p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
interface to use the LLVM interprocedural optimizer without exposing details
be possible for a completely different compilation technology to provide
a different libLTO that works with their object files and the standard
linker tool.</p>
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<a name="lto_module_t">lto_module_t</a>
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<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
The following functions allow the linker to check if a file (on disk
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<a name="lto_code_gen_t">lto_code_gen_t</a>
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<p>Once the linker has loaded each non-native object files into an
<tt>lto_module_t</tt>, it can request libLTO to process them all and
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Devang Patel and Nick Kledzik<br>
- <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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