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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM Link Time Optimization: Design and Implementation</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<h1>
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LLVM Link Time Optimization: Design and Implementation
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</h1>
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<ul>
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<li><a href="#desc">Description</a></li>
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<li><a href="#design">Design Philosophy</a>
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<ul>
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<li><a href="#example1">Example of link time optimization</a></li>
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<li><a href="#alternative_approaches">Alternative Approaches</a></li>
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</ul></li>
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<li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
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<ul>
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<li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
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<li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
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<li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
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<li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
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</ul></li>
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<li><a href="#lto">libLTO</a>
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<ul>
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<li><a href="#lto_module_t">lto_module_t</a></li>
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<li><a href="#lto_code_gen_t">lto_code_gen_t</a></li>
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</ul>
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</ul>
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<div class="doc_author">
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<p>Written by Devang Patel and Nick Kledzik</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="desc">Description</a>
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</h2>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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LLVM features powerful intermodular optimizations which can be used at link
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time. Link Time Optimization (LTO) is another name for intermodular optimization
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when performed during the link stage. This document describes the interface
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and design between the LTO optimizer and the linker.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="design">Design Philosophy</a>
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</h2>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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The LLVM Link Time Optimizer provides complete transparency, while doing
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intermodular optimization, in the compiler tool chain. Its main goal is to let
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the developer take advantage of intermodular optimizations without making any
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significant changes to the developer's makefiles or build system. This is
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achieved through tight integration with the linker. In this model, the linker
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treates LLVM bitcode files like native object files and allows mixing and
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matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
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object, to handle LLVM bitcode files. This tight integration between
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the linker and LLVM optimizer helps to do optimizations that are not possible
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in other models. The linker input allows the optimizer to avoid relying on
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conservative escape analysis.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="example1">Example of link time optimization</a>
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</h3>
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<div class="doc_text">
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<p>The following example illustrates the advantages of LTO's integrated
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approach and clean interface. This example requires a system linker which
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supports LTO through the interface described in this document. Here,
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llvm-gcc transparently invokes system linker. </p>
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<ul>
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<li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
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<li> Input source file <tt>main.c</tt> is compiled into native object code.
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</ul>
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<pre class="doc_code">
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--- a.h ---
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extern int foo1(void);
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extern void foo2(void);
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extern void foo4(void);
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--- a.c ---
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#include "a.h"
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static signed int i = 0;
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void foo2(void) {
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i = -1;
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}
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static int foo3() {
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foo4();
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return 10;
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}
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int foo1(void) {
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int data = 0;
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if (i < 0) { data = foo3(); }
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data = data + 42;
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return data;
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}
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--- main.c ---
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#include <stdio.h>
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#include "a.h"
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void foo4(void) {
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printf ("Hi\n");
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}
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int main() {
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return foo1();
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}
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--- command lines ---
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$ llvm-gcc --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
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$ llvm-gcc -c main.c -o main.o # <-- main.o is native object file
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$ llvm-gcc a.o main.o -o main # <-- standard link command without any modifications
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</pre>
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<p>In this example, the linker recognizes that <tt>foo2()</tt> is an
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externally visible symbol defined in LLVM bitcode file. The linker completes
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its usual symbol resolution
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pass and finds that <tt>foo2()</tt> is not used anywhere. This information
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is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
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<tt>foo2()</tt> is removed, the optimizer recognizes that condition
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<tt>i < 0</tt> is always false, which means <tt>foo3()</tt> is never
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used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
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enables linker to remove <tt>foo4()</tt>. This example illustrates the
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advantage of tight integration with the linker. Here, the optimizer can not
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remove <tt>foo3()</tt> without the linker's input.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="alternative_approaches">Alternative Approaches</a>
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</h3>
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<div class="doc_text">
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<dl>
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<dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
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<dd>In this model the link time optimizer is not able to take advantage of
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information collected during the linker's normal symbol resolution phase.
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In the above example, the optimizer can not remove <tt>foo2()</tt> without
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the linker's input because it is externally visible. This in turn prohibits
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the optimizer from removing <tt>foo3()</tt>.</dd>
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<dt><b>Use separate tool to collect symbol information from all object
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files.</b></dt>
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<dd>In this model, a new, separate, tool or library replicates the linker's
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capability to collect information for link time optimization. Not only is
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this code duplication difficult to justify, but it also has several other
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disadvantages. For example, the linking semantics and the features
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provided by the linker on various platform are not unique. This means,
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this new tool needs to support all such features and platforms in one
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super tool or a separate tool per platform is required. This increases
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maintenance cost for link time optimizer significantly, which is not
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necessary. This approach also requires staying synchronized with linker
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developements on various platforms, which is not the main focus of the link
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time optimizer. Finally, this approach increases end user's build time due
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to the duplication of work done by this separate tool and the linker itself.
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</dd>
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</dl>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="multiphase">Multi-phase communication between libLTO and linker</a>
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</h2>
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<div class="doc_text">
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<p>The linker collects information about symbol defininitions and uses in
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various link objects which is more accurate than any information collected
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by other tools during typical build cycles. The linker collects this
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information by looking at the definitions and uses of symbols in native .o
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files and using symbol visibility information. The linker also uses
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user-supplied information, such as a list of exported symbols. LLVM
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optimizer collects control flow information, data flow information and knows
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much more about program structure from the optimizer's point of view.
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Our goal is to take advantage of tight integration between the linker and
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the optimizer by sharing this information during various linking phases.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
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</h3>
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<div class="doc_text">
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<p>The linker first reads all object files in natural order and collects
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symbol information. This includes native object files as well as LLVM bitcode
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files. To minimize the cost to the linker in the case that all .o files
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are native object files, the linker only calls <tt>lto_module_create()</tt>
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when a supplied object file is found to not be a native object file. If
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<tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
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the linker
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then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
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<tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
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referenced.
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This information is added to the linker's global symbol table.
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</p>
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<p>The lto* functions are all implemented in a shared object libLTO. This
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allows the LLVM LTO code to be updated independently of the linker tool.
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On platforms that support it, the shared object is lazily loaded.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="phase2">Phase 2 : Symbol Resolution</a>
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</h3>
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<div class="doc_text">
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<p>In this stage, the linker resolves symbols using global symbol table.
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It may report undefined symbol errors, read archive members, replace
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weak symbols, etc. The linker is able to do this seamlessly even though it
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does not know the exact content of input LLVM bitcode files. If dead code
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stripping is enabled then the linker collects the list of live symbols.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="phase3">Phase 3 : Optimize Bitcode Files</a>
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</h3>
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<div class="doc_text">
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<p>After symbol resolution, the linker tells the LTO shared object which
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symbols are needed by native object files. In the example above, the linker
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reports that only <tt>foo1()</tt> is used by native object files using
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<tt>lto_codegen_add_must_preserve_symbol()</tt>. Next the linker invokes
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the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
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which returns a native object file creating by merging the LLVM bitcode files
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and applying various optimization passes.
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</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
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</h3>
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<div class="doc_text">
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<p>In this phase, the linker reads optimized a native object file and
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updates the internal global symbol table to reflect any changes. The linker
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also collects information about any changes in use of external symbols by
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LLVM bitcode files. In the example above, the linker notes that
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<tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
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the linker refreshes the live symbol information appropriately and performs
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dead code stripping.</p>
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<p>After this phase, the linker continues linking as if it never saw LLVM
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bitcode files.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="lto">libLTO</a>
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</h2>
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<div class="doc_text">
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<p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
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is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
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interface to use the LLVM interprocedural optimizer without exposing details
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of LLVM's internals. The intention is to keep the interface as stable as
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possible even when the LLVM optimizer continues to evolve. It should even
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be possible for a completely different compilation technology to provide
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a different libLTO that works with their object files and the standard
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linker tool.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="lto_module_t">lto_module_t</a>
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</h3>
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<div class="doc_text">
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<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
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The following functions allow the linker to check if a file (on disk
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or in a memory buffer) is a file which libLTO can process:</p>
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<pre class="doc_code">
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lto_module_is_object_file(const char*)
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lto_module_is_object_file_for_target(const char*, const char*)
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lto_module_is_object_file_in_memory(const void*, size_t)
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lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
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</pre>
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<p>If the object file can be processed by libLTO, the linker creates a
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<tt>lto_module_t</tt> by using one of</p>
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<pre class="doc_code">
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lto_module_create(const char*)
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lto_module_create_from_memory(const void*, size_t)
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</pre>
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<p>and when done, the handle is released via</p>
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<pre class="doc_code">
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lto_module_dispose(lto_module_t)
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</pre>
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<p>The linker can introspect the non-native object file by getting the number of
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symbols and getting the name and attributes of each symbol via:</p>
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<pre class="doc_code">
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lto_module_get_num_symbols(lto_module_t)
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lto_module_get_symbol_name(lto_module_t, unsigned int)
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lto_module_get_symbol_attribute(lto_module_t, unsigned int)
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</pre>
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<p>The attributes of a symbol include the alignment, visibility, and kind.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="lto_code_gen_t">lto_code_gen_t</a>
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</h3>
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<div class="doc_text">
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<p>Once the linker has loaded each non-native object files into an
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<tt>lto_module_t</tt>, it can request libLTO to process them all and
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generate a native object file. This is done in a couple of steps.
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First, a code generator is created with:</p>
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<pre class="doc_code">lto_codegen_create()</pre>
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<p>Then, each non-native object file is added to the code generator with:</p>
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<pre class="doc_code">
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lto_codegen_add_module(lto_code_gen_t, lto_module_t)
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</pre>
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<p>The linker then has the option of setting some codegen options. Whether or
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not to generate DWARF debug info is set with:</p>
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<pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
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<p>Which kind of position independence is set with:</p>
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<pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
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<p>And each symbol that is referenced by a native object file or otherwise must
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not be optimized away is set with:</p>
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<pre class="doc_code">
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lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
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</pre>
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<p>After all these settings are done, the linker requests that a native object
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file be created from the modules with the settings using:</p>
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<pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
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<p>which returns a pointer to a buffer containing the generated native
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object file. The linker then parses that and links it with the rest
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of the native object files.</p>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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Devang Patel and Nick Kledzik<br>
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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