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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>LLVM Link Time Optimization: Design and Implementation</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<div class="doc_title">
LLVM Link Time Optimization: Design and Implementation
</div>
<ul>
<li><a href="#desc">Description</a></li>
<li><a href="#design">Design Philosophy</a>
<ul>
<li><a href="#example1">Example of link time optimization</a></li>
<li><a href="#alternative_approaches">Alternative Approaches</a></li>
</ul></li>
<li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
<ul>
<li><a href="#phase1">Phase 1 : Read LLVM Bytecode Files</a></li>
<li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
<li><a href="#phase3">Phase 3 : Optimize Bytecode Files</a></li>
<li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
</ul></li>
<li><a href="#lto">LLVMlto</a>
<ul>
<li><a href="#llvmsymbol">LLVMSymbol</a></li>
<li><a href="#readllvmobjectfile">readLLVMObjectFile()</a></li>
<li><a href="#optimizemodules">optimizeModules()</a></li>
<li><a href="#gettargettriple">getTargetTriple()</a></li>
<li><a href="#removemodule">removeModule()</a></li>
<li><a href="#getalignment">getAlignment()</a></li>
</ul></li>
<li><a href="#debug">Debugging Information</a></li>
</ul>
<div class="doc_author">
<p>Written by Devang Patel</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="desc">Description</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
LLVM features powerful intermodular optimizations which can be used at link
time. Link Time Optimization is another name for intermodular optimization
when performed during the link stage. This document describes the interface
and design between the LLVM intermodular optimizer and the linker.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="design">Design Philosophy</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
The LLVM Link Time Optimizer provides complete transparency, while doing
intermodular optimization, in the compiler tool chain. Its main goal is to let
the developer take advantage of intermodular optimizations without making any
significant changes to the developer's makefiles or build system. This is
achieved through tight integration with the linker. In this model, the linker
treates LLVM bytecode files like native object files and allows mixing and
matching among them. The linker uses <a href="#lto">LLVMlto</a>, a dynamically
loaded library, to handle LLVM bytecode files. This tight integration between
the linker and LLVM optimizer helps to do optimizations that are not possible
in other models. The linker input allows the optimizer to avoid relying on
conservative escape analysis.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="example1">Example of link time optimization</a>
</div>
<div class="doc_text">
<p>The following example illustrates the advantages of LTO's integrated
approach and clean interface. This example requires a system linker which
supports LTO through the interface described in this document. Here,
llvm-gcc4 transparently invokes system linker. </p>
<ul>
<li> Input source file <tt>a.c</tt> is compiled into LLVM byte code form.
<li> Input source file <tt>main.c</tt> is compiled into native object code.
</ul>
<div class="doc_code"><pre>
--- a.h ---
extern int foo1(void);
extern void foo2(void);
extern void foo4(void);
--- a.c ---
#include "a.h"
static signed int i = 0;
void foo2(void) {
i = -1;
}
static int foo3() {
foo4();
return 10;
}
int foo1(void) {
int data = 0;
if (i < 0) { data = foo3(); }
data = data + 42;
return data;
}
--- main.c ---
#include <stdio.h>
#include "a.h"
void foo4(void) {
printf ("Hi\n");
}
int main() {
return foo1();
}
--- command lines ---
$ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
$ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
$ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifications
</pre></div>
<p>In this example, the linker recognizes that <tt>foo2()</tt> is an
externally visible symbol defined in LLVM byte code file. This information
is collected using <a href="#readllvmobjectfile"> readLLVMObjectFile()</a>.
Based on this information, the linker completes its usual symbol resolution
pass and finds that <tt>foo2()</tt> is not used anywhere. This information
is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
<tt>foo2()</tt> is removed, the optimizer recognizes that condition
<tt>i < 0</tt> is always false, which means <tt>foo3()</tt> is never
used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
enables linker to remove <tt>foo4()</tt>. This example illustrates the
advantage of tight integration with the linker. Here, the optimizer can not
remove <tt>foo3()</tt> without the linker's input.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="alternative_approaches">Alternative Approaches</a>
</div>
<div class="doc_text">
<dl>
<dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
<dd>In this model the link time optimizer is not able to take advantage of
information collected during the linker's normal symbol resolution phase.
In the above example, the optimizer can not remove <tt>foo2()</tt> without
the linker's input because it is externally visible. This in turn prohibits
the optimizer from removing <tt>foo3()</tt>.</dd>
<dt><b>Use separate tool to collect symbol information from all object
files.</b></dt>
<dd>In this model, a new, separate, tool or library replicates the linker's
capability to collect information for link time optimization. Not only is
this code duplication difficult to justify, but it also has several other
disadvantages. For example, the linking semantics and the features
provided by the linker on various platform are not unique. This means,
this new tool needs to support all such features and platforms in one
super tool or a separate tool per platform is required. This increases
maintance cost for link time optimizer significantly, which is not
necessary. This approach also requires staying synchronized with linker
developements on various platforms, which is not the main focus of the link
time optimizer. Finally, this approach increases end user's build time due
to the duplication of work done by this separate tool and the linker itself.
</dd>
</dl>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="multiphase">Multi-phase communication between LLVM and linker</a>
</div>
<div class="doc_text">
<p>The linker collects information about symbol defininitions and uses in
various link objects which is more accurate than any information collected
by other tools during typical build cycles. The linker collects this
information by looking at the definitions and uses of symbols in native .o
files and using symbol visibility information. The linker also uses
user-supplied information, such as a list of exported symbols. LLVM
optimizer collects control flow information, data flow information and knows
much more about program structure from the optimizer's point of view.
Our goal is to take advantage of tight intergration between the linker and
the optimizer by sharing this information during various linking phases.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="phase1">Phase 1 : Read LLVM Bytecode Files</a>
</div>
<div class="doc_text">
<p>The linker first reads all object files in natural order and collects
symbol information. This includes native object files as well as LLVM byte
code files. In this phase, the linker uses
<a href="#readllvmobjectfile"> readLLVMObjectFile() </a> to collect symbol
information from each LLVM bytecode files and updates its internal global
symbol table accordingly. The intent of this interface is to avoid overhead
in the non LLVM case, where all input object files are native object files,
by putting this code in the error path of the linker. When the linker sees
the first llvm .o file, it <tt>dlopen()</tt>s the dynamic library. This is
to allow changes to the LLVM LTO code without relinking the linker.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="phase2">Phase 2 : Symbol Resolution</a>
</div>
<div class="doc_text">
<p>In this stage, the linker resolves symbols using global symbol table
information to report undefined symbol errors, read archive members, resolve
weak symbols, etc. The linker is able to do this seamlessly even though it
does not know the exact content of input LLVM bytecode files because it uses
symbol information provided by
<a href="#readllvmobjectfile">readLLVMObjectFile()</a>. If dead code
stripping is enabled then the linker collects the list of live symbols.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="phase3">Phase 3 : Optimize Bytecode Files</a>
</div>
<div class="doc_text">
<p>After symbol resolution, the linker updates symbol information supplied
by LLVM bytecode files appropriately. For example, whether certain LLVM
bytecode supplied symbols are used or not. In the example above, the linker
reports that <tt>foo2()</tt> is not used anywhere in the program, including
native <tt>.o</tt> files. This information is used by the LLVM interprocedural
optimizer. The linker uses <a href="#optimizemodules">optimizeModules()</a>
and requests an optimized native object file of the LLVM portion of the
program.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
</div>
<div class="doc_text">
<p>In this phase, the linker reads optimized a native object file and
updates the internal global symbol table to reflect any changes. The linker
also collects information about any changes in use of external symbols by
LLVM bytecode files. In the examle above, the linker notes that
<tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
the linker refreshes the live symbol information appropriately and performs
dead code stripping.</p>
<p>After this phase, the linker continues linking as if it never saw LLVM
bytecode files.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="lto">LLVMlto</a>
</div>
<div class="doc_text">
<p><tt>LLVMlto</tt> is a dynamic library that is part of the LLVM tools, and
is intended for use by a linker. <tt>LLVMlto</tt> provides an abstract C++
interface to use the LLVM interprocedural optimizer without exposing details
of LLVM's internals. The intention is to keep the interface as stable as
possible even when the LLVM optimizer continues to evolve.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="llvmsymbol">LLVMSymbol</a>
</div>
<div class="doc_text">
<p>The <tt>LLVMSymbol</tt> class is used to describe the externally visible
functions and global variables, defined in LLVM bytecode files, to the linker.
This includes symbol visibility information. This information is used by
the linker to do symbol resolution. For example: function <tt>foo2()</tt> is
defined inside an LLVM bytecode module and it is an externally visible symbol.
This helps the linker connect the use of <tt>foo2()</tt> in native object
files with a future definition of the symbol <tt>foo2()</tt>. The linker
will see the actual definition of <tt>foo2()</tt> when it receives the
optimized native object file in
<a href="#phase4">Symbol Resolution after optimization</a> phase. If the
linker does not find any uses of <tt>foo2()</tt>, it updates LLVMSymbol
visibility information to notify LLVM intermodular optimizer that it is dead.
The LLVM intermodular optimizer takes advantage of such information to
generate better code.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="readllvmobjectfile">readLLVMObjectFile()</a>
</div>
<div class="doc_text">
<p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
LLVM bytecode files and collect LLVMSymbol information. This routine also
supplies a list of externally defined symbols that are used by LLVM bytecode
files. The linker uses this symbol information to do symbol resolution.
Internally, <a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in
memory. This function also provides a list of external references used by
bytecode files.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="optimizemodules">optimizeModules()</a>
</div>
<div class="doc_text">
<p>The linker invokes <tt>optimizeModules</tt> to optimize already read
LLVM bytecode files by applying LLVM intermodular optimization techniques.
This function runs the LLVM intermodular optimizer and generates native
object code as <tt>.o</tt> files at the name and location provided by the
linker.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="gettargettriple">getTargetTriple()</a>
</div>
<div class="doc_text">
<p>The linker may use <tt>getTargetTriple()</tt> to query target architecture
while validating LLVM bytecode file.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="removemodule">removeModule()</a>
</div>
<div class="doc_text">
<p>Internally, <a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in
memory. The linker may use <tt>removeModule()</tt> method to remove desired
modules from memory. </p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="getalignment">getAlignment()</a>
</div>
<div class="doc_text">
<p>The linker may use <a href="#llvmsymbol">LLVMSymbol</a> method
<tt>getAlignment()</tt> to query symbol alignment information.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="debug">Debugging Information</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p><tt> ... To be completed ... </tt></p>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
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Devang Patel<br>
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
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|
