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docs/LinkTimeOptimization.html
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docs/LinkTimeOptimization.html
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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 implentation
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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></li>
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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></li>
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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>
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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</a></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 optimization which can be used at link time.
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Link Time Optimization is another name of intermodular optimization when it
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is done during link stage. This document describes the interface between LLVM
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intermodular optimizer and the linker and its design.
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</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 seeks complete transparency, while doing intermodular
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optimization, in compiler tool chain. Its main goal is to let developer take
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advantage of intermodular optimizer without making any significant changes to
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their makefiles or build system. This is achieved through tight integration with
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linker. In this model, linker treates LLVM bytecode files like native objects
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file and allows mixing and matching among them. The linker uses
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<a href="#lto">LLVMlto</a>, a dynamically loaded library, to handle LLVM bytecode
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files. This tight integration between the linker and LLVM optimizer helps to do
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optimizations that are not possible in other models. The linker input allows
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optimizer to avoid relying on conservative escape analysis.
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</p>
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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>Following example illustrates advantage of integrated approach that uses
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clean interface.
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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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<br>
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<code>
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<br>--- a.h ---
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<br>extern int foo1(void);
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<br>extern void foo2(void);
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<br>extern void foo4(void);
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<br>--- a.c ---
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<br>#include "a.h"
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<br>
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<br>static signed int i = 0;
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<br>
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<br>void foo2(void) {
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<br> i = -1;
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<br>}
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<br>
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<br>static int foo3() {
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<br>foo4();
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<br>return 10;
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<br>}
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<br>
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<br>int foo1(void) {
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<br>int data = 0;
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<br>
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<br>if (i < 0) { data = foo3(); }
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<br>
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<br>data = data + 42;
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<br>return data;
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<br>}
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<br>
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<br>--- main.c ---
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<br>#include <stdio.h>
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<br>#include "a.h"
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<br>
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<br>void foo4(void) {
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<br> printf ("Hi\n");
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<br>}
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<br>
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<br>int main() {
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<br> return foo1();
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<br>}
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<br>
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<br>--- command lines ---
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<br> $ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
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<br> $ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
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<br> $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifications
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<br>
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</code>
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</p>
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<p>
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In this example, the linker recognizes that <tt>foo2()</tt> is a externally visible
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symbol defined in LLVM byte code file. This information is collected using
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<a href=#lreadllvmbytecodefile> readLLVMByteCodeFile() </a>. Based on this
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information, linker completes its usual symbol resolution pass and finds that
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<tt>foo2()</tt> is not used anywhere. This information is used by LLVM optimizer
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and it removes <tt>foo2()</tt>. As soon as <tt>foo2()</tt> is removed, optimizer
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recognizes that condition <tt> i < 0 </tt> is always false, which means
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<tt>foo3()</tt> is never used. Hence, optimizer removes <tt>foo3()</tt> also.
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And this in turn, enables linker to remove <tt>foo4()</tt>.
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This example illustrates advantage of tight integration with linker. Here,
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optimizer can not 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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<p>
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<li> Compiler driver invokes link time optimizer separately.
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<br><br>In this model link time optimizer is not able to take advantage of information
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collected during normal linker's symbol resolution phase. In above example,
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optimizer can not remove <tt>foo2()</tt> without linker's input because it is
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externally visible. And this in turn prohibits optimizer from removing <tt>foo3()</tt>.
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<br><br>
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<li> Use separate tool to collect symbol information from all object file.
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<br><br>In this model, this new separate tool or library replicates linker's
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capabilities to collect information for link time optimizer. Not only such code
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duplication is difficult to justify but it also has several other disadvantages.
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For example, the linking semantics and the features provided by linker on
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various platform are not unique. This means, this new tool needs to support all
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such features and platforms in one super tool or one new separate tool per
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platform is required. This increases maintance cost for link time optimizer
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significantly, which is not necessary. Plus, this approach requires staying
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synchronized with linker developements on various platforms, which is not the
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main focus of link time optimizer. Finally, this approach increases end user's build
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time due to duplicate work done by this separate tool and linker itself.
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</p>
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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>
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The linker collects information about symbol defininitions and uses in various
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link objects which is more accurate than any information collected by other tools
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during typical build cycle.
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The linker collects this information by looking at definitions and uses of
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symbols in native .o files and using symbol visibility information. The linker
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also uses user supplied information, such as list of exported symbol.
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LLVM optimizer collects control flow information, data flow information and
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knows much more about program structure from optimizer's point of view. Our
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goal is to take advantage of tight intergration between the linker and
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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>
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The linker first reads all object files in natural order and collects symbol
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information. This includes native object files as well as LLVM byte code files.
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In this phase, the linker uses <a href=#lreadllvmbytecodefile> readLLVMByteCodeFile() </a>
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to collect symbol information from each LLVM bytecode files and updates its
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internal global symbol table accordingly. The intent of this interface is to
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avoid overhead in the non LLVM case, where all input object files are native
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object files, by putting this code in the error path of the linker. When the
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linker sees the first llvm .o file, it dlopen()s the dynamic library. This is
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to allow changes to LLVM part 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>
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In this stage, the linker resolves symbols using global symbol table information
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to report undefined symbol errors, read archive members, resolve weak
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symbols etc... The linker is able to do this seamlessly even though it does not
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know exact content of input LLVM bytecode files because it uses symbol information
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provided by <a href=#lreadllvmbytecodefile> readLLVMByteCodeFile() </a>.
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If dead code stripping is enabled then linker collects 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>
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After symbol resolution, the linker updates symbol information supplied by LLVM
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bytecode files appropriately. For example, whether certain LLVM bytecode
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supplied symbols are used or not. In the example above, the linker reports
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that <tt>foo2()</tt> is not used anywhere in the program, including native .o
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files. This information is used by LLVM interprocedural optimizer. The
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linker uses <a href="#optimizemodules"> optimizeModules()</a> and requests
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optimized native object file of the LLVM portion of the 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>
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In this phase, the linker reads optimized native object file and updates internal
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global symbol table to reflect any changes. Linker also collects information
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about any change in use of external symbols by LLVM bytecode files. In the examle
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above, the linker notes that <tt>foo4()</tt> is not used any more. If dead code
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striping is enabled then linker refreshes live symbol information appropriately
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and performs dead code stripping.
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<br>
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After this phase, the linker continues linking as if it never saw LLVM bytecode
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files.
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</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>
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<tt>LLVMlto</tt> is a dynamic library that is part of the LLVM tools, and is
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intended for use by a linker. <tt>LLVMlto</tt> provides an abstract C++ interface
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to use the LLVM interprocedural optimizer without exposing details of LLVM
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internals. The intention is to keep the interface as stable as possible even
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when the LLVM optimizer continues to evolve.
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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="llvmsymbol">LLVMSymbol</a>
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</div>
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<div class="doc_text">
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<p>
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<tt>LLVMSymbol</tt> class is used to describe the externally visible functions
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and global variables, tdefined in LLVM bytecode files, to linker.
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This includes symbol visibility information. This information is used by linker
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to do symbol resolution. For example : function <tt>foo2()</tt> is defined inside
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a LLVM bytecode module and it is externally visible symbol.
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This helps linker connect use of <tt>foo2()</tt> in native object file with
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future definition of symbol <tt>foo2()</tt>. The linker will see actual definition
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of <tt>foo2()</tt> when it receives optimized native object file in <a href="#phase4">
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Symbol Resolution after optimization</a> phase. If the linker does not find any
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use of <tt>foo2()</tt>, it updates LLVMSymbol visibility information to notify
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LLVM intermodular optimizer that it is dead. The LLVM intermodular optimizer
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takes advantage of such information to generate better code.
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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="readllvmobjectfile">readLLVMObjectFile()</a>
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</div>
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<div class="doc_text">
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<p>
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<tt>readLLVMObjectFile()</tt> is used by the linker to read LLVM bytecode files
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and collect LLVMSymbol nformation. This routine also
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supplies list of externally defined symbols that are used by LLVM bytecode
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files. Linker uses this symbol information to do symbol resolution. Internally,
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<a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in memory. This
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function also provides list of external references used by bytecode file.<br>
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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="optimizemodules">optimizeModules()</a>
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</div>
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<div class="doc_text">
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<p>
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The linker invokes <tt>optimizeModules</tt> to optimize already read LLVM
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bytecode files by applying LLVM intermodular optimization techniques. This
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function runs LLVM intermodular optimizer and generates native object code
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as .o file at name and location provided by the linker.
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</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> ... incomplete ... </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</a><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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