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diff --git a/docs/LinkTimeOptimization.html b/docs/LinkTimeOptimization.html index 3033474..289da23 100644 --- a/docs/LinkTimeOptimization.html +++ b/docs/LinkTimeOptimization.html @@ -6,9 +6,9 @@ <link rel="stylesheet" href="llvm.css" type="text/css"> </head> -<div class="doc_title"> +<h1> LLVM Link Time Optimization: Design and Implementation -</div> +</h1> <ul> <li><a href="#desc">Description</a></li> @@ -36,12 +36,12 @@ </div> <!-- *********************************************************************** --> -<div class="doc_section"> +<h2> <a name="desc">Description</a> -</div> +</h2> <!-- *********************************************************************** --> -<div class="doc_text"> +<div> <p> LLVM features powerful intermodular optimizations which can be used at link time. Link Time Optimization (LTO) is another name for intermodular optimization @@ -50,12 +50,12 @@ and design between the LTO optimizer and the linker.</p> </div> <!-- *********************************************************************** --> -<div class="doc_section"> +<h2> <a name="design">Design Philosophy</a> -</div> +</h2> <!-- *********************************************************************** --> -<div class="doc_text"> +<div> <p> The LLVM Link Time Optimizer provides complete transparency, while doing intermodular optimization, in the compiler tool chain. Its main goal is to let @@ -69,14 +69,13 @@ the linker and LLVM optimizer helps to do optimizations that are not possible in other models. The linker input allows the optimizer to avoid relying on conservative escape analysis. </p> -</div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="example1">Example of link time optimization</a> -</div> +</h3> -<div class="doc_text"> +<div> <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, @@ -145,11 +144,11 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific </div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="alternative_approaches">Alternative Approaches</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -175,12 +174,14 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific </dl> </div> +</div> + <!-- *********************************************************************** --> -<div class="doc_section"> +<h2> <a name="multiphase">Multi-phase communication between libLTO and linker</a> -</div> +</h2> -<div class="doc_text"> +<div> <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 @@ -192,14 +193,13 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific Our goal is to take advantage of tight integration between the linker and the optimizer by sharing this information during various linking phases. </p> -</div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -219,11 +219,11 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific </div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="phase2">Phase 2 : Symbol Resolution</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -233,10 +233,10 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific </div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="phase3">Phase 3 : Optimize Bitcode Files</a> -</div> -<div class="doc_text"> +</h3> +<div> <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 @@ -248,11 +248,11 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific </div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="phase4">Phase 4 : Symbol Resolution after optimization</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -264,12 +264,14 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific bitcode files.</p> </div> +</div> + <!-- *********************************************************************** --> -<div class="doc_section"> +<h2> <a name="lto">libLTO</a> -</div> +</h2> -<div class="doc_text"> +<div> <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 @@ -278,14 +280,13 @@ $ llvm-gcc a.o main.o -o main # <-- standard link command without any modific 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> -</div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="lto_module_t">lto_module_t</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -325,11 +326,11 @@ lto_module_get_symbol_attribute(lto_module_t, unsigned int) </div> <!-- ======================================================================= --> -<div class="doc_subsection"> +<h3> <a name="lto_code_gen_t">lto_code_gen_t</a> -</div> +</h3> -<div class="doc_text"> +<div> <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 @@ -371,6 +372,8 @@ of the native object files.</p> </div> +</div> + <!-- *********************************************************************** --> <hr> @@ -381,8 +384,8 @@ of the native object files.</p> src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> Devang Patel and Nick Kledzik<br> - <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br> - Last modified: $Date: 2010-09-29 22:09:55 +0200 (Wed, 29 Sep 2010) $ + <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> + Last modified: $Date: 2011-04-23 02:30:22 +0200 (Sat, 23 Apr 2011) $ </address> </body> |
