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diff --git a/docs/LinkTimeOptimization.html b/docs/LinkTimeOptimization.html new file mode 100644 index 000000000000..c9d1e190eab0 --- /dev/null +++ b/docs/LinkTimeOptimization.html @@ -0,0 +1,390 @@ +<!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> +<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 + 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:</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: 2008-12-16 04:07:49 +0100 (Tue, 16 Dec 2008) $ +</address> + +</body> +</html> + |