mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-12-11 21:45:16 +00:00
391 lines
15 KiB
HTML
391 lines
15 KiB
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 Bitcode 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>
|
|
<pre class="doc_code">
|
|
--- 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>
|
|
<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
|
|
maintenance 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 integration 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 example 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:</p>
|
|
|
|
<pre class="doc_code">
|
|
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>
|
|
|
|
<p>If the object file can be processed by libLTO, the linker creates a
|
|
<tt>lto_module_t</tt> by using one of</p>
|
|
|
|
<pre class="doc_code">
|
|
lto_module_create(const char*)
|
|
lto_module_create_from_memory(const void*, size_t)
|
|
</pre>
|
|
|
|
<p>and when done, the handle is released via</p>
|
|
|
|
<pre class="doc_code">
|
|
lto_module_dispose(lto_module_t)
|
|
</pre>
|
|
|
|
<p>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:</p>
|
|
|
|
<pre class="doc_code">
|
|
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>
|
|
|
|
<p>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:</p>
|
|
|
|
<pre class="doc_code">lto_codegen_create()</pre>
|
|
|
|
<p>Then, each non-native object file is added to the code generator with:</p>
|
|
|
|
<pre class="doc_code">
|
|
lto_codegen_add_module(lto_code_gen_t, lto_module_t)
|
|
</pre>
|
|
|
|
<p>The linker then has the option of setting some codegen options. Whether or
|
|
not to generate DWARF debug info is set with:</p>
|
|
|
|
<pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
|
|
|
|
<p>Which kind of position independence is set with:</p>
|
|
|
|
<pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
|
|
|
|
<p>And each symbol that is referenced by a native object file or otherwise must
|
|
not be optimized away is set with:</p>
|
|
|
|
<pre class="doc_code">
|
|
lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
|
|
</pre>
|
|
|
|
<p>After all these settings are done, the linker requests that a native object
|
|
file be created from the modules with the settings using:</p>
|
|
|
|
<pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
|
|
|
|
<p>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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401-blue" 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>
|
|
|