Add SwiftShader dump from Feb 6 2013
diff --git a/src/LLVM/docs/LinkTimeOptimization.html b/src/LLVM/docs/LinkTimeOptimization.html
new file mode 100644
index 0000000..5cd9214
--- /dev/null
+++ b/src/LLVM/docs/LinkTimeOptimization.html
@@ -0,0 +1,400 @@
+<!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>
+
+<h1>
+ LLVM Link Time Optimization: Design and Implementation
+</h1>
+
+<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>
+
+<!-- *********************************************************************** -->
+<h2>
+<a name="desc">Description</a>
+</h2>
+<!-- *********************************************************************** -->
+
+<div>
+<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>
+
+<!-- *********************************************************************** -->
+<h2>
+<a name="design">Design Philosophy</a>
+</h2>
+<!-- *********************************************************************** -->
+
+<div>
+<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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="example1">Example of link time optimization</a>
+</h3>
+
+<div>
+ <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,
+ clang 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 ---
+$ clang -emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
+$ clang -c main.c -o main.o # <-- main.o is native object file
+$ clang a.o main.o -o main # <-- standard link command without any modifications
+</pre>
+
+<ul>
+ <li>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>.</li>
+ <li>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 also removes <tt>foo3()</tt>.</li>
+ <li>And this in turn, enables linker to remove <tt>foo4()</tt>.</li>
+</ul>
+
+<p>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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="alternative_approaches">Alternative Approaches</a>
+</h3>
+
+<div>
+ <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>
+
+<!-- *********************************************************************** -->
+<h2>
+ <a name="multiphase">Multi-phase communication between libLTO and linker</a>
+</h2>
+
+<div>
+ <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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
+</h3>
+
+<div>
+ <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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="phase2">Phase 2 : Symbol Resolution</a>
+</h3>
+
+<div>
+ <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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
+</h3>
+<div>
+ <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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
+</h3>
+
+<div>
+ <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>
+
+<!-- *********************************************************************** -->
+<h2>
+<a name="lto">libLTO</a>
+</h2>
+
+<div>
+ <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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="lto_module_t">lto_module_t</a>
+</h3>
+
+<div>
+
+<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>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="lto_code_gen_t">lto_code_gen_t</a>
+</h3>
+
+<div>
+
+<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>
+
+</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: 2011-09-18 08:51:05 -0400 (Sun, 18 Sep 2011) $
+</address>
+
+</body>
+</html>
+
