docs/LinkTimeOptimization.html - fp2-dev/platform/external/llvm - Gitiles

 <!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 Bitcode Files</a></li>
     <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
   </ul></li>
   <li><a href="#lto">libLTO</a>
   <ul>
     <li><a href="#lto_module_t">lto_module_t</a></li>
     <li><a href="#lto_code_gen_t">lto_code_gen_t</a></li>
   </ul>
 </ul>

 <div class="doc_author">
 <p>Written by Devang Patel and Nick Kledzik</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 (LTO) is another name for intermodular optimization
 when performed during the link stage. This document describes the interface
 and design between the LTO 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 bitcode files like native object files and allows mixing and
 matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
 object, to handle LLVM bitcode 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-gcc transparently invokes system linker. </p>
   <ul>
     <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode 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 &lt; 0) { data = foo3(); }

 data = data + 42;
 return data;
 }

 --- main.c ---
 #include &lt;stdio.h&gt;
 #include "a.h"

 void foo4(void) {
  printf ("Hi\n");
 }

 int main() {
  return foo1();
 }

 --- command lines ---
 $ llvm-gcc --emit-llvm -c a.c -o a.o  # &lt;-- a.o is LLVM bitcode file
 $ llvm-gcc -c main.c -o main.o # &lt;-- main.o is native object file
 $ llvm-gcc a.o main.o -o main # &lt;-- 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 bitcode file. 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 &lt; 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 libLTO 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 Bitcode 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 bitcode
   files.  To minimize the cost to the linker in the case that all .o files
   are native object files, the linker only calls <tt>lto_module_create()</tt>
   when a supplied object file is found to not be a native object file.  If
   <tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
   the linker
   then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
   <tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
   referenced.
   This information is added to the linker's global symbol table.
 </p>
   <p>The lto* functions are all implemented in a shared object libLTO.  This
   allows the LLVM LTO code to be updated independently of the linker tool.
   On platforms that support it, the shared object is lazily loaded.
 </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.
   It may report undefined symbol errors, read archive members, replace
   weak symbols, etc.  The linker is able to do this seamlessly even though it
   does not know the exact content of input LLVM bitcode files.  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 Bitcode Files</a>
 </div>
 <div class="doc_text">
   <p>After symbol resolution, the linker tells the LTO shared object which
   symbols are needed by native object files.  In the example above, the linker
   reports that only <tt>foo1()</tt> is used by native object files using
   <tt>lto_codegen_add_must_preserve_symbol()</tt>.  Next the linker invokes
   the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
   which returns a native object file creating by merging the LLVM bitcode files
   and applying various optimization passes.
 </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 bitcode 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
   bitcode files.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
 <a name="lto">libLTO</a>
 </div>

 <div class="doc_text">
   <p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
   is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
   interface to use the LLVM interprocedural optimizer without exposing details
   of LLVM's internals. The intention is to keep the interface as stable as
   possible even when the LLVM optimizer continues to evolve. It should even
   be possible for a completely different compilation technology to provide
   a different libLTO that works with their object files and the standard
   linker tool.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="lto_module_t">lto_module_t</a>
 </div>

 <div class="doc_text">
    <p>A non-native object file is handled via an <tt>lto_module_t</tt>.
    The following functions allow the linker to check if a file (on disk
    or in a memory buffer) is a file which libLTO can process: <pre>
    lto_module_is_object_file(const char*)
    lto_module_is_object_file_for_target(const char*, const char*)
    lto_module_is_object_file_in_memory(const void*, size_t)
    lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)</pre>
   If the object file can be processed by libLTO, the linker creates a
   <tt>lto_module_t</tt> by using one of <pre>
    lto_module_create(const char*)
    lto_module_create_from_memory(const void*, size_t)</pre>
   and when done, the handle is released via<pre>
    lto_module_dispose(lto_module_t)</pre>
   The linker can introspect the non-native object file by getting the number
   of symbols and getting the name and attributes of each symbol via: <pre>
    lto_module_get_num_symbols(lto_module_t)
    lto_module_get_symbol_name(lto_module_t, unsigned int)
    lto_module_get_symbol_attribute(lto_module_t, unsigned int)</pre>
   The attributes of a symbol include the alignment, visibility, and kind.
 </p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="lto_code_gen_t">lto_code_gen_t</a>
 </div>

 <div class="doc_text">
   <p>Once the linker has loaded each non-native object files into an
   <tt>lto_module_t</tt>, it can request libLTO to process them all and
   generate a native object file.  This is done in a couple of steps.
   First a code generator is created with:<pre>
     lto_codegen_create() </pre>
   then each non-native object file is added to the code generator with:<pre>
     lto_codegen_add_module(lto_code_gen_t, lto_module_t)</pre>
   The linker then has the option of setting some codegen options.  Whether
   or not to generate DWARF debug info is set with: <pre>
     lto_codegen_set_debug_model(lto_code_gen_t) </pre>
   Which kind of position independence is set with: <pre>
     lto_codegen_set_pic_model(lto_code_gen_t) </pre>
   And each symbol that is referenced by a native object file or otherwise
   must not be optimized away is set with: <pre>
     lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)</pre>
   After all these settings are done, the linker requests that a native
   object file be created from the modules with the settings using:
     lto_codegen_compile(lto_code_gen_t, size*)</pre>
   which returns a pointer to a buffer containing the generated native
   object file.  The linker then parses that and links it with the rest
   of the native object files.
 </div>

 <!-- *********************************************************************** -->

 <hr>
 <address>
   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
   src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
   <a href="http://validator.w3.org/check/referer"><img
   src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>

   Devang Patel and Nick Kledzik<br>
   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
   Last modified: $Date$
 </address>

 </body>
 </html>
	<!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 Bitcode Files</a></li>
	<li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
	</ul></li>
	<li><a href="#lto">libLTO</a>
	<ul>
	<li><a href="#lto_module_t">lto_module_t</a></li>
	<li><a href="#lto_code_gen_t">lto_code_gen_t</a></li>
	</ul>
	</ul>

	<div class="doc_author">
	<p>Written by Devang Patel and Nick Kledzik</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 (LTO) is another name for intermodular optimization
	when performed during the link stage. This document describes the interface
	and design between the LTO 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 bitcode files like native object files and allows mixing and
	matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
	object, to handle LLVM bitcode 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-gcc transparently invokes system linker. </p>
	<ul>
	<li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode 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-gcc --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
	$ llvm-gcc -c main.c -o main.o # <-- main.o is native object file
	$ llvm-gcc 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 bitcode file. 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 libLTO 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 Bitcode 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 bitcode
	files. To minimize the cost to the linker in the case that all .o files
	are native object files, the linker only calls <tt>lto_module_create()</tt>
	when a supplied object file is found to not be a native object file. If
	<tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
	the linker
	then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
	<tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
	referenced.
	This information is added to the linker's global symbol table.
	</p>
	<p>The lto* functions are all implemented in a shared object libLTO. This
	allows the LLVM LTO code to be updated independently of the linker tool.
	On platforms that support it, the shared object is lazily loaded.
	</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.
	It may report undefined symbol errors, read archive members, replace
	weak symbols, etc. The linker is able to do this seamlessly even though it
	does not know the exact content of input LLVM bitcode files. 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 Bitcode Files</a>
	</div>
	<div class="doc_text">
	<p>After symbol resolution, the linker tells the LTO shared object which
	symbols are needed by native object files. In the example above, the linker
	reports that only <tt>foo1()</tt> is used by native object files using
	<tt>lto_codegen_add_must_preserve_symbol()</tt>. Next the linker invokes
	the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
	which returns a native object file creating by merging the LLVM bitcode files
	and applying various optimization passes.
	</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 bitcode 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
	bitcode files.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="lto">libLTO</a>
	</div>

	<div class="doc_text">
	<p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
	is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
	interface to use the LLVM interprocedural optimizer without exposing details
	of LLVM's internals. The intention is to keep the interface as stable as
	possible even when the LLVM optimizer continues to evolve. It should even
	be possible for a completely different compilation technology to provide
	a different libLTO that works with their object files and the standard
	linker tool.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="lto_module_t">lto_module_t</a>
	</div>

	<div class="doc_text">
	<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
	The following functions allow the linker to check if a file (on disk
	or in a memory buffer) is a file which libLTO can process: <pre>
	lto_module_is_object_file(const char*)
	lto_module_is_object_file_for_target(const char, const char)
	lto_module_is_object_file_in_memory(const void*, size_t)
	lto_module_is_object_file_in_memory_for_target(const void, size_t, const char)</pre>
	If the object file can be processed by libLTO, the linker creates a
	<tt>lto_module_t</tt> by using one of <pre>
	lto_module_create(const char*)
	lto_module_create_from_memory(const void*, size_t)</pre>
	and when done, the handle is released via<pre>
	lto_module_dispose(lto_module_t)</pre>
	The linker can introspect the non-native object file by getting the number
	of symbols and getting the name and attributes of each symbol via: <pre>
	lto_module_get_num_symbols(lto_module_t)
	lto_module_get_symbol_name(lto_module_t, unsigned int)
	lto_module_get_symbol_attribute(lto_module_t, unsigned int)</pre>
	The attributes of a symbol include the alignment, visibility, and kind.
	</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="lto_code_gen_t">lto_code_gen_t</a>
	</div>

	<div class="doc_text">
	<p>Once the linker has loaded each non-native object files into an
	<tt>lto_module_t</tt>, it can request libLTO to process them all and
	generate a native object file. This is done in a couple of steps.
	First a code generator is created with:<pre>
	lto_codegen_create() </pre>
	then each non-native object file is added to the code generator with:<pre>
	lto_codegen_add_module(lto_code_gen_t, lto_module_t)</pre>
	The linker then has the option of setting some codegen options. Whether
	or not to generate DWARF debug info is set with: <pre>
	lto_codegen_set_debug_model(lto_code_gen_t) </pre>
	Which kind of position independence is set with: <pre>
	lto_codegen_set_pic_model(lto_code_gen_t) </pre>
	And each symbol that is referenced by a native object file or otherwise
	must not be optimized away is set with: <pre>
	lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)</pre>
	After all these settings are done, the linker requests that a native
	object file be created from the modules with the settings using:
	lto_codegen_compile(lto_code_gen_t, size*)</pre>
	which returns a pointer to a buffer containing the generated native
	object file. The linker then parses that and links it with the rest
	of the native object files.
	</div>

	<!-- *********************************************************************** -->

	<hr>
	<address>
	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
	src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
	<a href="http://validator.w3.org/check/referer"><img
	src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>

	Devang Patel and Nick Kledzik<br>
	<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
	Last modified: $Date$
	</address>

	</body>
	</html>