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- <title>LLVM Link Time Optimization: design and implementation</title>
+ <title>LLVM Link Time Optimization: Design and Implementation</title>
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<div class="doc_title">
- LLVM Link Time Optimization: design and implementation
+ LLVM Link Time Optimization: Design and Implementation
</div>
<ul>
<div class="doc_text">
<p>
-LLVM features powerful intermodular optimization which can be used at link time.
-Link Time Optimization is another name of intermodular optimization when it
-is done during link stage. This document describes the interface between LLVM
-intermodular optimizer and the linker and its design.
-</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_text">
<p>
-The LLVM Link Time Optimizer seeks complete transparency, while doing intermodular
-optimization, in compiler tool chain. Its main goal is to let developer take
-advantage of intermodular optimizer without making any significant changes to
-their makefiles or build system. This is achieved through tight integration with
-linker. In this model, linker treates LLVM bytecode files like native objects
-file 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
-optimizer to avoid relying on conservative escape analysis.
+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>
<div class="doc_text">
-
-<p>Following example illustrates advantage of integrated approach that uses
-clean interface.
-<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>
-<code>
+ <p>The following example illustrates the advantages of LTO's integrated
+ approach and clean interface.</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>
+<pre>
--- a.h ---
-<br>extern int foo1(void);
-<br>extern void foo2(void);
-<br>extern void foo4(void);
-<br>--- a.c ---
-<br>#include "a.h"
-<br>
-<br>static signed int i = 0;
-<br>
-<br>void foo2(void) {
-<br> i = -1;
-<br>}
-<br>
-<br>static int foo3() {
-<br>foo4();
-<br>return 10;
-<br>}
-<br>
-<br>int foo1(void) {
-<br>int data = 0;
-<br>
-<br>if (i < 0) { data = foo3(); }
-<br>
-<br>data = data + 42;
-<br>return data;
-<br>}
-<br>
-<br>--- main.c ---
-<br>#include <stdio.h>
-<br>#include "a.h"
-<br>
-<br>void foo4(void) {
-<br> printf ("Hi\n");
-<br>}
-<br>
-<br>int main() {
-<br> return foo1();
-<br>}
-<br>
-<br>--- command lines ---
-<br> $ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
-<br> $ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
-<br> $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifications
-<br>
-</code>
-<p>
-In this example, the linker recognizes that <tt>foo2()</tt> is a externally visible
-symbol defined in LLVM byte code file. This information is collected using
-<a href="#readllvmobjectfile"> readLLVMObjectFile() </a>. Based on this
-information, linker completes its usual symbol resolution pass and finds that
-<tt>foo2()</tt> is not used anywhere. This information is used by LLVM optimizer
-and it removes <tt>foo2()</tt>. As soon as <tt>foo2()</tt> is removed, optimizer
-recognizes that condition <tt> i < 0 </tt> is always false, which means
-<tt>foo3()</tt> is never used. Hence, optimizer removes <tt>foo3()</tt> also.
-And this in turn, enables linker to remove <tt>foo4()</tt>.
-This example illustrates advantage of tight integration with linker. Here,
-optimizer can not remove <tt>foo3()</tt> without the linker's input.
-</p>
+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>
+ <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>
<div class="doc_text">
-<p>
-<ul>
-<li> Compiler driver invokes link time optimizer separately.
-<br><br>In this model link time optimizer is not able to take advantage of information
-collected during normal linker's symbol resolution phase. In above example,
-optimizer can not remove <tt>foo2()</tt> without linker's input because it is
-externally visible. And this in turn prohibits optimizer from removing <tt>foo3()</tt>.
-<br><br>
-<li> Use separate tool to collect symbol information from all object file.
-<br><br>In this model, this new separate tool or library replicates linker's
-capabilities to collect information for link time optimizer. Not only such code
-duplication is difficult to justify but it also has several other disadvantages.
-For example, the linking semantics and the features provided by 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 one new separate tool per
-platform is required. This increases maintance cost for link time optimizer
-significantly, which is not necessary. Plus, this approach requires staying
-synchronized with linker developements on various platforms, which is not the
-main focus of link time optimizer. Finally, this approach increases end user's build
-time due to duplicate work done by this separate tool and linker itself.
-</ul>
+ <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>
<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 cycle.
-The linker collects this information by looking at definitions and uses of
-symbols in native .o files and using symbol visibility information. The linker
-also uses user supplied information, such as list of exported symbol.
-LLVM optimizer collects control flow information, data flow information and
-knows much more about program structure from optimizer's point of view. Our
-goal is to take advantage of tight intergration between the linker and
-optimizer by sharing this information during various linking phases.
+ <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>
<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 dlopen()s the dynamic library. This is
-to allow changes to LLVM part without relinking the linker.
+ <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>
<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 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 linker collects list of live symbols.
-</p>
+ <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>
<!-- ======================================================================= -->
<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 .o
-files. This information is used by LLVM interprocedural optimizer. The
-linker uses <a href="#optimizemodules"> optimizeModules()</a> and requests
-optimized native object file of the LLVM portion of the program.
+ <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>
<div class="doc_text">
-<p>
-In this phase, the linker reads optimized native object file and updates internal
-global symbol table to reflect any changes. Linker also collects information
-about any change 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
-striping is enabled then linker refreshes live symbol information appropriately
-and performs dead code stripping.
-<br>
-After this phase, the linker continues linking as if it never saw LLVM bytecode
-files.
-</p>
+ <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>
<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
-internals. The intention is to keep the interface as stable as possible even
-when the LLVM optimizer continues to evolve.
-</p>
+ <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>
<div class="doc_text">
-<p>
-<tt>LLVMSymbol</tt> class is used to describe the externally visible functions
-and global variables, tdefined in LLVM bytecode files, to linker.
-This includes symbol visibility information. This information is used by linker
-to do symbol resolution. For example : function <tt>foo2()</tt> is defined inside
-a LLVM bytecode module and it is externally visible symbol.
-This helps linker connect use of <tt>foo2()</tt> in native object file with
-future definition of symbol <tt>foo2()</tt>. The linker will see actual definition
-of <tt>foo2()</tt> when it receives optimized native object file in <a href="#phase4">
-Symbol Resolution after optimization</a> phase. If the linker does not find any
-use 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>
+ <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>
<div class="doc_text">
-<p>
-<tt>readLLVMObjectFile()</tt> is used by the linker to read LLVM bytecode files
-and collect LLVMSymbol nformation. This routine also
-supplies list of externally defined symbols that are used by LLVM bytecode
-files. 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 list of external references used by bytecode file.<br>
-</p>
+ <p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
+ LLVM bytecode files and collect LLVMSymbol nformation. 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>
<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 LLVM intermodular optimizer and generates native object code
-as .o file at name and location provided by the linker.
-</p>
+ <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_text">
-<p><tt> ... incomplete ... </tt></p>
+<p><tt> ... To be completed ... </tt></p>
</div>
projects / oota-llvm.git / commitdiff