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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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<div class="doc_title">
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LLVM Link Time Optimization: Design and Implementation
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</div>
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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 Bytecode 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 Bytecode 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">LLVMlto</a>
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<ul>
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<li><a href="#llvmsymbol">LLVMSymbol</a></li>
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<li><a href="#readllvmobjectfile">readLLVMObjectFile()</a></li>
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<li><a href="#optimizemodules">optimizeModules()</a></li>
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</ul></li>
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<li><a href="#debug">Debugging Information</a></li>
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</ul>
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<div class="doc_author">
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<p>Written by Devang Patel</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="desc">Description</a>
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</div>
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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 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 LLVM intermodular optimizer and the linker.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="design">Design Philosophy</a>
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</div>
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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 bytecode files like native object files and allows mixing and
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matching among them. The linker uses <a href="#lto">LLVMlto</a>, a dynamically
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loaded library, to handle LLVM bytecode 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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<div class="doc_subsection">
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<a name="example1">Example of link time optimization</a>
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</div>
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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.</p>
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<ul>
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<li> Input source file <tt>a.c</tt> is compiled into LLVM byte code 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>
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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-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
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$ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
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$ llvm-gcc4 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 byte code file. This information
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is collected using <a href="#readllvmobjectfile"> readLLVMObjectFile()</a>.
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Based on this information, the linker completes 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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<div class="doc_subsection">
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<a name="alternative_approaches">Alternative Approaches</a>
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</div>
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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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maintance 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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<div class="doc_section">
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<a name="multiphase">Multi-phase communication between LLVM and linker</a>
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</div>
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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 intergration 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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<div class="doc_subsection">
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<a name="phase1">Phase 1 : Read LLVM Bytecode Files</a>
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</div>
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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 byte
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code files. In this phase, the linker uses
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<a href="#readllvmobjectfile"> readLLVMObjectFile() </a> to collect symbol
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information from each LLVM bytecode files and updates its internal global
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symbol table accordingly. The intent of this interface is to avoid overhead
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in the non LLVM case, where all input object files are native object files,
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by putting this code in the error path of the linker. When the linker sees
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the first llvm .o file, it <tt>dlopen()</tt>s the dynamic library. This is
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to allow changes to the LLVM LTO code without relinking the linker.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="phase2">Phase 2 : Symbol Resolution</a>
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</div>
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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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information to report undefined symbol errors, read archive members, resolve
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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 bytecode files because it uses
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symbol information provided by
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<a href="#readllvmobjectfile">readLLVMObjectFile()</a>. 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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<div class="doc_subsection">
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<a name="phase3">Phase 3 : Optimize Bytecode Files</a>
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</div>
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<div class="doc_text">
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<p>After symbol resolution, the linker updates symbol information supplied
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by LLVM bytecode files appropriately. For example, whether certain LLVM
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bytecode supplied symbols are used or not. In the example above, the linker
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reports that <tt>foo2()</tt> is not used anywhere in the program, including
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native <tt>.o</tt> files. This information is used by the LLVM interprocedural
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optimizer. The linker uses <a href="#optimizemodules">optimizeModules()</a>
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and requests an optimized native object file of the LLVM portion of the
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program.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
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</div>
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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 bytecode files. In the examle 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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bytecode files.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="lto">LLVMlto</a>
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</div>
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<div class="doc_text">
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<p><tt>LLVMlto</tt> is a dynamic library that is part of the LLVM tools, and
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is intended for use by a linker. <tt>LLVMlto</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.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="llvmsymbol">LLVMSymbol</a>
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</div>
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<div class="doc_text">
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<p>The <tt>LLVMSymbol</tt> class is used to describe the externally visible
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functions and global variables, defined in LLVM bytecode files, to the linker.
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This includes symbol visibility information. This information is used by
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the linker to do symbol resolution. For example: function <tt>foo2()</tt> is
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defined inside an LLVM bytecode module and it is an externally visible symbol.
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This helps the linker connect the use of <tt>foo2()</tt> in native object
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files with a future definition of the symbol <tt>foo2()</tt>. The linker
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will see the actual definition of <tt>foo2()</tt> when it receives the
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optimized native object file in
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<a href="#phase4">Symbol Resolution after optimization</a> phase. If the
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linker does not find any uses of <tt>foo2()</tt>, it updates LLVMSymbol
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visibility information to notify LLVM intermodular optimizer that it is dead.
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The LLVM intermodular optimizer takes advantage of such information to
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generate better code.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="readllvmobjectfile">readLLVMObjectFile()</a>
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</div>
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<div class="doc_text">
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<p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
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LLVM bytecode files and collect LLVMSymbol information. This routine also
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supplies a list of externally defined symbols that are used by LLVM bytecode
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files. The linker uses this symbol information to do symbol resolution.
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Internally, <a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in
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memory. This function also provides a list of external references used by
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bytecode files.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="optimizemodules">optimizeModules()</a>
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</div>
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<div class="doc_text">
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<p>The linker invokes <tt>optimizeModules</tt> to optimize already read
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LLVM bytecode files by applying LLVM intermodular optimization techniques.
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This function runs the LLVM intermodular optimizer and generates native
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object code as <tt>.o</tt> files at the name and location provided by the
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linker.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="debug">Debugging Information</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p><tt> ... To be completed ... </tt></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<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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</body>
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</html>
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