<div class="doc_title">Source Level Debugging with LLVM</div>
+<table class="layout" style="width:100%">
+ <tr class="layout">
+ <td class="left">
<ul>
-
-<img src="venusflytrap.jpg" width=247 height=369 align=right>
-
- <li><a href="#introduction">Introduction</a></li>
+ <li><a href="#introduction">Introduction</a>
<ol>
<li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
<li><a href="#debugopt">Debugging optimized code</a></li>
<li><a href="#future">Future work</a></li>
- </ol>
+ </ol></li>
<li><a href="#llvm-db">Using the <tt>llvm-db</tt> tool</a>
<ol>
<li><a href="#limitations">Limitations of <tt>llvm-db</tt></a></li>
<li><a href="#commands">Commands recognized by the debugger</a></li>
</ol></li>
- <li><a href="#architecture">Architecture of the LLVM debugger</a></li>
+ <li><a href="#architecture">Architecture of the LLVM debugger</a>
<ol>
+ <li><a href="#arch_debugger">The Debugger and InferiorProcess classes</a></li>
+ <li><a href="#arch_info">The RuntimeInfo, ProgramInfo, and SourceLanguage classes</a></li>
+ <li><a href="#arch_llvm-db">The <tt>llvm-db</tt> tool</a></li>
<li><a href="#arch_todo">Short-term TODO list</a></li>
- </ol>
+ </ol></li>
- <li><a href="#implementation">Debugging information implementation</a></li>
+ <li><a href="#format">Debugging information format</a>
<ol>
- <li><a href="#impl_common_anchors">Anchors for global objects</a></li>
- <li><a href="#impl_common_stoppoint">Representing stopping points in the source program</a></li>
- <li><a href="#impl_common_lifetime">Object lifetimes and scoping</a></li>
- <li><a href="#impl_common_descriptors">Object descriptor formats</a></li>
+ <li><a href="#format_common_anchors">Anchors for global objects</a></li>
+ <li><a href="#format_common_stoppoint">Representing stopping points in the source program</a></li>
+ <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>
+ <li><a href="#format_common_descriptors">Object descriptor formats</a>
<ul>
- <li><a href="#impl_common_source_files">Representation of source files</a></li>
- <li><a href="#impl_common_globals">Representation of global objects</a></li>
- <li><a href="#impl_common_localvars">Representation of local variables</a></li>
- </ul>
- <li><a href="#impl_common_intrinsics">Other intrinsic functions</a></li>
- </ol>
- <li><a href="#impl_ccxx">C/C++ front-end specific debug information</a></li>
+ <li><a href="#format_common_source_files">Representation of source files</a></li>
+ <li><a href="#format_common_program_objects">Representation of program objects</a></li>
+ <li><a href="#format_common_object_contexts">Program object contexts</a></li>
+ </ul></li>
+ <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a></li>
+ <li><a href="#format_common_tags">Values for debugger tags</a></li>
+ </ol></li>
+ <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
<ol>
- <li><a href="#impl_ccxx_descriptors">Object descriptor formats</a></li>
- </ol>
+ <li><a href="#ccxx_pse">Program Scope Entries</a>
+ <ul>
+ <li><a href="#ccxx_compilation_units">Compilation unit entries</a></li>
+ <li><a href="#ccxx_modules">Module, namespace, and importing entries</a></li>
+ </ul></li>
+ <li><a href="#ccxx_dataobjects">Data objects (program variables)</a></li>
+ </ol></li>
</ul>
+</td>
+<td class="right">
+<img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
+height="369">
+</td>
+</tr></table>
+
+<div class="doc_author">
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="introduction">Introduction</a></div>
+<div class="doc_section"><a name="introduction">Introduction</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document is the central repository for all information pertaining to
-debug information in LLVM. It describes how to use the <a
-href="CommandGuide/llvm-db.html"><tt>llvm-db</tt> tool</a>, which provides a
-powerful <a href="#llvm-db">source-level debugger</a> to users of LLVM-based
-compilers. When compiling a program in debug mode, the front-end in use adds
-LLVM debugging information to the program in the form of normal <a
-
href="LangRef.html">LLVM program objects</a> as well as a small set of LLVM <a
-href="#implementation">intrinsic functions</a>, which specify the mapping of the
-program in LLVM form to the program in the source language.
-</p>
+debug information in LLVM. It describes the <a href="#llvm-db">user
+interface</a> for the <tt>llvm-db</tt> tool, which provides a
+powerful <a href="#llvm-db">source-level debugger</a>
+to users of LLVM-based compilers. It then describes the <a
+href="#architecture">various components</a> that make up the debugger and the
+
libraries which future clients may use. Finally, it describes the <a
+href="#format">actual format that the LLVM debug information</a> takes,
+which is useful for those interested in creating front-ends or dealing directly
+with the information.</p>
</div>
<div class="doc_text">
-<p>
-The idea of the LLVM debugging information is to capture how the important
+<p>The idea of the LLVM debugging information is to capture how the important
pieces of the source-language's Abstract Syntax Tree map onto LLVM code.
Several design aspects have shaped the solution that appears here. The
important ones are:</p>
-<p><ul>
+<ul>
<li>Debugging information should have very little impact on the rest of the
compiler. No transformations, analyses, or code generators should need to be
modified because of debugging information.</li>
the debugging information should work with any language.</li>
<li>With code generator support, it should be possible to use an LLVM compiler
-to compile a program to native machine code with standard debugging formats.
+to compile a program to native machine code and standard debugging formats.
This allows compatibility with traditional machine-code level debuggers, like
GDB or DBX.</li>
-</ul></p>
+</ul>
-<p>
-The approach used by the LLVM implementation is to use a small set of <a
-href="#impl_common_intrinsics">intrinsic functions</a> to define a mapping
+<p>The approach used by the LLVM implementation is to use a small set of <a
+href="#format_common_intrinsics">intrinsic functions</a> to define a mapping
between LLVM program objects and the source-level objects. The description of
the source-level program is maintained in LLVM global variables in an <a
-href="#impl_ccxx">implementation-defined format</a> (the C/C++ front-end
+href="#ccxx_frontend">implementation-defined format</a> (the C/C++ front-end
currently uses working draft 7 of the <a
href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3 standard</a>).</p>
-<p>
-When a program is debugged, the debugger interacts with the user and turns the
-stored debug information into source-language specific information. As such,
-the debugger must be aware of the source-language, and is thus tied to a
+<p>When a program is debugged, the debugger interacts with the user and turns
+the stored debug information into source-language specific information. As
+such, the debugger must be aware of the source-language, and is thus tied to a
specific language of family of languages. The <a href="#llvm-db">LLVM
-debugger</a> is designed to be modular in its support for source-languages.
-</p>
+debugger</a> is designed to be modular in its support for source-languages.</p>
</div>
</div>
<div class="doc_text">
-<p>
-An extremely high priority of LLVM debugging information is to make it interact
-well with optimizations and analysis. In particular, the LLVM debug information
-provides the following guarantees:</p>
-<p><ul>
+<p>An extremely high priority of LLVM debugging information is to make it
+interact well with optimizations and analysis. In particular, the LLVM debug
+information provides the following guarantees:</p>
+
+<ul>
<li>LLVM debug information <b>always provides information to accurately read the
source-level state of the program</b>, regardless of which LLVM optimizations
automatically merged by the linker, and unused information is automatically
removed.</li>
-</ul></p>
+</ul>
-<p>
-Basically, the debug information allows you to compile a program with "<tt>-O0
--g</tt>" and get full debug information, allowing you to arbitrarily modify the
-program as it executes from the debugger. Compiling a program with "<tt>-O3
--g</tt>" gives you full debug information that is always available and accurate
-for reading (e.g., you get accurate stack traces despite tail call elimination
-and inlining), but you might lose the ability to modify the program and call
-functions where were optimized out of the program, or inlined away completely.
-</p>
+<p>Basically, the debug information allows you to compile a program with
+"<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily
+modify the program as it executes from the debugger. Compiling a program with
+"<tt>-O3 -g</tt>" gives you full debug information that is always available and
+accurate for reading (e.g., you get accurate stack traces despite tail call
+elimination and inlining), but you might lose the ability to modify the program
+and call functions where were optimized out of the program, or inlined away
+completely.</p>
</div>
-
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="future">Future work</a>
</div>
<div class="doc_text">
-<p>
-There are several important extensions that could be eventually added to the
+<p>There are several important extensions that could be eventually added to the
LLVM debugger. The most important extension would be to upgrade the LLVM code
generators to support debugging information. This would also allow, for
example, the X86 code generator to emit native objects that contain debugging
information consumable by traditional source-level debuggers like GDB or
DBX.</p>
-<p>
-Additionally, LLVM optimizations can be upgraded to incrementally update the
+<p>Additionally, LLVM optimizations can be upgraded to incrementally update the
debugging information, <a href="#commands">new commands</a> can be added to the
debugger, and thread support could be added to the debugger.</p>
-<p>
-The "SourceLanguage" modules provided by <tt>llvm-db</tt> could be substantially
-improved to provide good support for C++ language features like namespaces and
-scoping rules.</p>
+<p>The "SourceLanguage" modules provided by <tt>llvm-db</tt> could be
+substantially improved to provide good support for C++ language features like
+namespaces and scoping rules.</p>
-<p>
-After working with the debugger for a while, perhaps the nicest improvement
-would be to add some sort of line editor, such as GNU readline (but that is
+<p>After working with the debugger for a while, perhaps the nicest improvement
+would be to add some sort of line editor, such as GNU readline (but one that is
compatible with the LLVM license).</p>
-<p>
-For someone so inclined, it should be straight-forward to write different
+<p>For someone so inclined, it should be straight-forward to write different
front-ends for the LLVM debugger, as the LLVM debugging engine is cleanly
-seperated from the <tt>llvm-db</tt> front-end. A GUI debugger or IDE would be
-an interesting project.
-</p>
+separated from the <tt>llvm-db</tt> front-end. A new LLVM GUI debugger or IDE
+would be nice.</p>
</div>
-
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="llvm-db">Using the <tt>llvm-db</tt> tool</a>
<div class="doc_text">
-<p>
-The <tt>llvm-db</tt> tool provides a GDB-like interface for source-level
+<p>The <tt>llvm-db</tt> tool provides a GDB-like interface for source-level
debugging of programs. This tool provides many standard commands for inspecting
and modifying the program as it executes, loading new programs, single stepping,
-placing breakpoints, etc. This section describes how to use the debugger.
-</p>
+placing breakpoints, etc. This section describes how to use the debugger.</p>
<p><tt>llvm-db</tt> has been designed to be as similar to GDB in its user
interface as possible. This should make it extremely easy to learn
provides the subset of GDB commands that are applicable to LLVM debugging users.
If there is a command missing that make a reasonable amount of sense within the
<a href="#limitations">limitations of <tt>llvm-db</tt></a>, please report it as
-a bug or, better yet, submit a patch to add it. :)</p>
+a bug or, better yet, submit a patch to add it.</p>
</div>
<div class="doc_text">
-<p><tt>llvm-db</tt> is the first LLVM debugger, and as such was designed to be
-quick to prototype and build, and simple to extend. It is missing many many
-features, though they should be easy to add over time (patches welcomed!).
-Because the (currently only) debugger backend (implemented in
-"lib/Debugger/UnixLocalInferiorProcess.cpp") was designed to work without any
-cooperation from the code generators, it suffers from the following inherent
-limitations:</p>
+<p><tt>llvm-db</tt> is designed to be modular and easy to extend. This
+extensibility was key to getting the debugger up-and-running quickly, because we
+can start with simple-but-unsophisicated implementations of various components.
+Because of this, it is currently missing many features, though they should be
+easy to add over time (patches welcomed!). The biggest inherent limitations of
+<tt>llvm-db</tt> are currently due to extremely simple <a
+href="#arch_debugger">debugger backend</a> (implemented in
+"lib/Debugger/UnixLocalInferiorProcess.cpp") which is designed to work without
+any cooperation from the code generators. Because it is so simple, it suffers
+from the following inherent limitations:</p>
-<p><ul>
+<ul>
<li>Running a program in <tt>llvm-db</tt> is a bit slower than running it with
-<tt>lli</tt>.</li>
+<tt>lli</tt> (i.e., in the JIT).</li>
<li>Inspection of the target hardware is not supported. This means that you
cannot, for example, print the contents of X86 registers.</li>
<li>Attaching to existing processes and core files is not currently
supported.</li>
-</ul></p>
+</ul>
-<p>That said, it is still quite useful, and all of these limitations can be
-eliminated by integrating support for the debugger into the code generators.
-See the <a href="#future">future work</a> section for ideas of how to extend
-the LLVM debugger despite these limitations.</p>
+<p>That said, the debugger is still quite useful, and all of these limitations
+can be eliminated by integrating support for the debugger into the code
+generators, and writing a new <a href="#arch_debugger">InferiorProcess</a>
+subclass to use it. See the <a href="#future">future work</a> section for ideas
+of how to extend the LLVM debugger despite these limitations.</p>
</div>
<div class="doc_text">
-<p>
-TODO
-</p>
+<p>TODO: this is obviously lame, when more is implemented, this can be much
+better.</p>
+
+<pre>
+$ <b>llvm-db funccall</b>
+llvm-db: The LLVM source-level debugger
+Loading program... successfully loaded 'funccall.bc'!
+(llvm-db) <b>create</b>
+Starting program: funccall.bc
+main at funccall.c:9:2
+9 -> q = 0;
+(llvm-db) <b>list main</b>
+4 void foo() {
+5 int t = q;
+6 q = t + 1;
+7 }
+8 int main() {
+9 -> q = 0;
+10 foo();
+11 q = q - 1;
+12
+13 return q;
+(llvm-db) <b>list</b>
+14 }
+(llvm-db) <b>step</b>
+10 -> foo();
+(llvm-db) <b>s</b>
+foo at funccall.c:5:2
+5 -> int t = q;
+(llvm-db) <b>bt</b>
+#0 -> 0x85ffba0 in foo at funccall.c:5:2
+#1 0x85ffd98 in main at funccall.c:10:2
+(llvm-db) <b>finish</b>
+main at funccall.c:11:2
+11 -> q = q - 1;
+(llvm-db) <b>s</b>
+13 -> return q;
+(llvm-db) <b>s</b>
+The program stopped with exit code 0
+(llvm-db) <b>quit</b>
+$
+</pre>
</div>
<p>When run with no options, just <tt>llvm-db</tt>, the debugger starts up
without a program loaded at all. You must use the <a
href="#c_file"><tt>file</tt> command</a> to load a program, and the <a
-href="c_set_args"><tt>set args</tt></a> or <a href="#c_run"><tt>run</tt></a>
+href="
#c_set_args"><tt>set args</tt></a> or <a href="#c_run"><tt>run</tt></a>
commands to specify the arguments for the program.</p>
<p>If you start the debugger with one argument, as <tt>llvm-db
<program></tt>, the debugger will start up and load in the specified
program. You can then optionally specify arguments to the program with the <a
-href="c_set_args"><tt>set args</tt></a> or <a href="#c_run"><tt>run</tt></a>
+href="
#c_set_args"><tt>set args</tt></a> or <a href="#c_run"><tt>run</tt></a>
commands.</p>
<p>The third way to start the program is with the <tt>--args</tt> option. This
<li>set listsize</li>
<li>show language</li>
<li>set language</li>
+<li>show args</li>
+<li>set args [args]</li>
</ul>
<h2>TODO:</h2>
<li>info catch</li>
<li>... many others</li>
</ul>
-</p>
+
</div>
<!-- *********************************************************************** -->
<!-- *********************************************************************** -->
<div class="doc_text">
+<p>The LLVM debugger is built out of three distinct layers of software. These
+layers provide clients with different interface options depending on what pieces
+of they want to implement themselves, and it also promotes code modularity and
+good design. The three layers are the <a href="#arch_debugger">Debugger
+interface</a>, the <a href="#arch_info">"info" interfaces</a>, and the <a
+href="#arch_llvm-db"><tt>llvm-db</tt> tool</a> itself.</p>
+</div>
-<p><pre>
-lib/Debugger
- - UnixLocalInferiorProcess.cpp
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="arch_debugger">The Debugger and InferiorProcess classes</a>
+</div>
-tools/llvm-db
- - SourceLanguage interfaces
- - ProgramInfo/RuntimeInfo
- - Commands
+<div class="doc_text">
+<p>The Debugger class (defined in the <tt>include/llvm/Debugger/</tt> directory)
+is a low-level class which is used to maintain information about the loaded
+program, as well as start and stop the program running as necessary. This class
+does not provide any high-level analysis or control over the program, only
+exposing simple interfaces like <tt>load/unloadProgram</tt>,
+<tt>create/killProgram</tt>, <tt>step/next/finish/contProgram</tt>, and
+low-level methods for installing breakpoints.</p>
-</pre></p>
+<p>
+The Debugger class is itself a wrapper around the lowest-level InferiorProcess
+class. This class is used to represent an instance of the program running under
+debugger control. The InferiorProcess class can be implemented in different
+ways for different targets and execution scenarios (e.g., remote debugging).
+The InferiorProcess class exposes a small and simple collection of interfaces
+which are useful for inspecting the current state of the program (such as
+collecting stack trace information, reading the memory image of the process,
+etc). The interfaces in this class are designed to be as low-level and simple
+as possible, to make it easy to create new instances of the class.
+</p>
+<p>
+The Debugger class exposes the currently active instance of InferiorProcess
+through the <tt>Debugger::getRunningProcess</tt> method, which returns a
+<tt>const</tt> reference to the class. This means that clients of the Debugger
+class can only <b>inspect</b> the running instance of the program directly. To
+change the executing process in some way, they must use the interces exposed by
+the Debugger class.
+</p>
</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="arch_info">The RuntimeInfo, ProgramInfo, and SourceLanguage classes</a>
+</div>
+
+<div class="doc_text">
+<p>
+The next-highest level of debugger abstraction is provided through the
+ProgramInfo, RuntimeInfo, SourceLanguage and related classes (also defined in
+the <tt>include/llvm/Debugger/</tt> directory). These classes efficiently
+decode the debugging information and low-level interfaces exposed by
+InferiorProcess into a higher-level representation, suitable for analysis by the
+debugger.
+</p>
+
+<p>
+The ProgramInfo class exposes a variety of different kinds of information about
+the program objects in the source-level-language. The SourceFileInfo class
+represents a source-file in the program (e.g. a .cpp or .h file). The
+SourceFileInfo class captures information such as which SourceLanguage was used
+to compile the file, where the debugger can get access to the actual file text
+(which is lazily loaded on demand), etc. The SourceFunctionInfo class
+represents a... <b>FIXME: finish</b>. The ProgramInfo class provides interfaces
+to lazily find and decode the information needed to create the Source*Info
+classes requested by the debugger.
+</p>
+
+<p>
+The RuntimeInfo class exposes information about the currently executed program,
+by decoding information from the InferiorProcess and ProgramInfo classes. It
+provides a StackFrame class which provides an easy-to-use interface for
+inspecting the current and suspended stack frames in the program.
+</p>
+
+<p>
+The SourceLanguage class is an abstract interface used by the debugger to
+perform all source-language-specific tasks. For example, this interface is used
+by the ProgramInfo class to decode language-specific types and functions and by
+the debugger front-end (such as <a href="#arch_llvm-db"><tt>llvm-db</tt></a> to
+evaluate source-langauge expressions typed into the debugger. This class uses
+the RuntimeInfo & ProgramInfo classes to get information about the current
+execution context and the loaded program, respectively.
+</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="arch_llvm-db">The <tt>llvm-db</tt> tool</a>
+</div>
+
+<div class="doc_text">
+<p>
+The <tt>llvm-db</tt> is designed to be a debugger providing an interface as <a
+href="#llvm-db">similar to GDB</a> as reasonable, but no more so than that.
+Because the <a href="#arch_debugger">Debugger</a> and <a
+href="#arch_info">info</a> classes implement all of the heavy lifting and
+analysis, <tt>llvm-db</tt> (which lives in <tt>llvm/tools/llvm-db</tt>) consists
+mainly of of code to interact with the user and parse commands. The CLIDebugger
+constructor registers all of the builtin commands for the debugger, and each
+command is implemented as a CLIDebugger::[name]Command method.
+</p>
+</div>
+
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="arch_todo">Short-term TODO list</a>
reloads.
</p>
-<p>
-<b>run (with args)</b> & <b>set args</b>: These need to be implemented.
-Currently run doesn't support setting arguments as part of the command. The
-only tricky thing is handling quotes right and stuff.</p>
-
<p>
<b>UnixLocalInferiorProcess.cpp speedup</b>: There is no reason for the debugged
process to code gen the globals corresponding to debug information. The
would also allow us to eliminate the mapping back and forth between physical
addresses that must be done.</p>
+<p>
+<b>Process deaths</b>: The InferiorProcessDead exception should be extended to
+know "how" a process died, i.e., it was killed by a signal. This is easy to
+collect in the UnixLocalInferiorProcess, we just need to represent it.</p>
+
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
- <a name="implementation">Debugging information implementation</a>
+ <a name="format">Debugging information format</a>
</div>
<!-- *********************************************************************** -->
become dead and be removed by the optimizer.</p>
<p>The debugger is designed to be agnostic about the contents of most of the
-debugging information. It uses a source-language-specific module to decode the
-information that represents variables, types, functions, namespaces, etc: this
-allows for arbitrary source-language semantics and type-systems to be used, as
-long as there is a module written for the debugger to interpret the information.
-</p>
+debugging information. It uses a <a href="#arch_info">source-language-specific
+module</a> to decode the information that represents variables, types,
+functions, namespaces, etc: this allows for arbitrary source-language semantics
+and type-systems to be used, as long as there is a module written for the
+debugger to interpret the information.</p>
-<p>
-To provide basic functionality, the LLVM debugger does have to make some
+<p>To provide basic functionality, the LLVM debugger does have to make some
assumptions about the source-level language being debugged, though it keeps
these to a minimum. The only common features that the LLVM debugger assumes
-exist are <a href="#impl_common_source_files">source files</a>, <a
-href="#impl_common_globals">global objects</a> (aka methods, messages, global
-variables, etc), and <a href="#impl_common_localvars">local variables</a>.
-These abstract objects are used by the debugger to form stack traces, show
-information about local variables, etc.
+exist are <a href="#format_common_source_files">source files</a>, and <a
+href="#format_program_objects">program objects</a>. These abstract objects are
+used by the debugger to form stack traces, show information about local
+variables, etc.</p>
<p>This section of the documentation first describes the representation aspects
-<a href="#impl_common">common to any source-language</a>. The next section
-describes the data layout conventions used by the <a href="#impl_ccxx">C and C++
-front-ends</a>.</p>
+common to any source-language. The <a href="#ccxx_frontend">next section</a>
+describes the data layout conventions used by the C and C++ front-ends.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_common_anchors">Anchors for global objects</a>
+ <a name="format_common_anchors">Anchors for global objects</a>
</div>
<div class="doc_text">
-<p>
-One important aspect of the LLVM debug representation is that it allows the LLVM
-debugger to efficiently index all of the global objects without having the scan
-the program. To do this, all of the global objects use "anchor" globals of type
-"<tt>{}</tt>", with designated names. These anchor objects obviously do not
-contain any content or meaning by themselves, but all of the global objects of a
-particular type (e.g., source file descriptors) contain a pointer to the anchor.
-This pointer allows the debugger to use def-use chains to find all global
-objects of that type.
-</p>
-
-<p>
-So far, the following names are recognized as anchors by the LLVM debugger:
-</p>
-
-<p><pre>
- %<a href="#impl_common_source_files">llvm.dbg.translation_units</a> = linkonce global {} {}
- %<a href="#impl_common_globals">llvm.dbg.globals</a> = linkonce global {} {}
-</pre></p>
-
-<p>
-Using anchors in this way (where the source file descriptor points to the
+<p>One important aspect of the LLVM debug representation is that it allows the
+LLVM debugger to efficiently index all of the global objects without having the
+scan the program. To do this, all of the global objects use "anchor" globals of
+type "<tt>{}</tt>", with designated names. These anchor objects obviously do
+not contain any content or meaning by themselves, but all of the global objects
+of a particular type (e.g., source file descriptors) contain a pointer to the
+anchor. This pointer allows the debugger to use def-use chains to find all
+global objects of that type.</p>
+
+<p>So far, the following names are recognized as anchors by the LLVM
+debugger:</p>
+
+<pre>
+ %<a href="#format_common_source_files">llvm.dbg.translation_units</a> = linkonce global {} {}
+ %<a href="#format_program_objects">llvm.dbg.globals</a> = linkonce global {} {}
+</pre>
+
+<p>Using anchors in this way (where the source file descriptor points to the
anchors, as opposed to having a list of source file descriptors) allows for the
standard dead global elimination and merging passes to automatically remove
unused debugging information. If the globals were kept track of through lists,
there would always be an object pointing to the descriptors, thus would never be
-deleted.
-</p>
+deleted.</p>
</div>
-
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_common_stoppoint">
+ <a name="format_common_stoppoint">
Representing stopping points in the source program
</a>
</div>
at every point in the program where the debugger should be able to inspect the
program (these correspond to places the debugger stops when you "<tt>step</tt>"
through it). The front-end can choose to place these as fine-grained as it
-would like (for example, before every subexpression was evaluated), but it is
-recommended to only put them after every source statement.</p>
+would like (for example, before every subexpression evaluated), but it is
+recommended to only put them after every source statement that includes
+executable code.</p>
-<p>
-Using calls to this intrinsic function to demark legal points for the debugger
-to inspect the program automatically disables any optimizations that could
-potentially confuse debugging information. To non-debug-information-aware
+<p>Using calls to this intrinsic function to demark legal points for the
+debugger to inspect the program automatically disables any optimizations that
+could potentially confuse debugging information. To non-debug-information-aware
transformations, these calls simply look like calls to an external function,
which they must assume to do anything (including reading or writing to any part
of reachable memory). On the other hand, it does not impact many optimizations,
such as code motion of non-trapping instructions, nor does it impact
-optimization of subexpressions, or any other code between the stop points.</p>
+optimization of subexpressions, code duplication transformations, or basic-block
+reordering transformations.</p>
-<p>
-An important aspect of the calls to the <tt>%llvm.dbg.stoppoint</tt> intrinsic
-is that the function-local debugging information is woven together with use-def
-chains. This makes it easy for the debugger to, for example, locate the 'next'
-stop point. For a concrete example of stop points, see <a
-href="#impl_common_lifetime">the next section</a>.</p>
+<p>An important aspect of the calls to the <tt>%llvm.dbg.stoppoint</tt>
+intrinsic is that the function-local debugging information is woven together
+with use-def chains. This makes it easy for the debugger to, for example,
+locate the 'next' stop point. For a concrete example of stop points, see the
+example in <a href="#format_common_lifetime">the next section</a>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_common_lifetime">Object lifetimes and scoping</a>
+ <a name="format_common_lifetime">Object lifetimes and scoping</a>
</div>
<div class="doc_text">
-<p>
-In many languages, the local variables in functions can have their lifetime or
-scope limited to a subset of a function. In the C family of languages, for
+<p>In many languages, the local variables in functions can have their lifetime
+or scope limited to a subset of a function. In the C family of languages, for
example, variables are only live (readable and writable) within the source block
that they are defined in. In functional languages, values are only readable
after they have been defined. Though this is a very obvious concept, it is also
non-trivial to model in LLVM, because it has no notion of scoping in this sense,
-and does not want to be tied to a language's scoping rules.
-</p>
+and does not want to be tied to a language's scoping rules.</p>
-<p>
-In order to handle this, the LLVM debug format uses the notion of "regions" of a
-function, delineated by calls to intrinsic functions. These intrinsic functions
-define new regions of the program and indicate when the region lifetime expires.
-Consider the following C fragment, for example:
-</p>
+<p>In order to handle this, the LLVM debug format uses the notion of "regions"
+of a function, delineated by calls to intrinsic functions. These intrinsic
+functions define new regions of the program and indicate when the region
+lifetime expires. Consider the following C fragment, for example:</p>
-<p><pre>
+<pre>
1. void foo() {
2. int X = ...;
3. int Y = ...;
7. }
8. ...
9. }
-</pre></p>
+</pre>
-<p>
-Compiled to LLVM, this function would be represented like this (FIXME: CHECK AND
-UPDATE THIS):
-</p>
+<p>Compiled to LLVM, this function would be represented like this (FIXME: CHECK
+AND UPDATE THIS):</p>
-<p><pre>
+<pre>
void %foo() {
%X = alloca int
%Y = alloca int
%Z = alloca int
- <a name="#icl_ex_D1">%D1</a> = call {}* %llvm.dbg.func.start(<a href="#impl_common_globals">%lldb.global</a>* %d.foo)
- %D2 = call {}* <a href="#impl_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D1, uint 2, uint 2, <a href="#impl_common_source_files">%lldb.compile_unit</a>* %file)
+ <a name="#icl_ex_D1">%D1</a> = call {}* %llvm.dbg.func.start(<a href="#format_program_objects">%lldb.global</a>* %d.foo)
+ %D2 = call {}* <a href="#format_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D1, uint 2, uint 2, <a href="#format_common_source_files">%lldb.compile_unit</a>* %file)
%D3 = call {}* %llvm.dbg.DEFINEVARIABLE({}* %D2, ...)
<i>;; Evaluate expression on line 2, assigning to X.</i>
- %D4 = call {}* <a href="#impl_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D3, uint 3, uint 2, <a href="#impl_common_source_files">%lldb.compile_unit</a>* %file)
+ %D4 = call {}* <a href="#format_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D3, uint 3, uint 2, <a href="#format_common_source_files">%lldb.compile_unit</a>* %file)
%D5 = call {}* %llvm.dbg.DEFINEVARIABLE({}* %D4, ...)
<i>;; Evaluate expression on line 3, assigning to Y.</i>
- %D6 = call {}* <a href="#impl_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D5, uint 5, uint 4, <a href="#impl_common_source_files">%lldb.compile_unit</a>* %file)
+ %D6 = call {}* <a href="#format_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D5, uint 5, uint 4, <a href="#format_common_source_files">%lldb.compile_unit</a>* %file)
<a name="#icl_ex_D1">%D7</a> = call {}* %llvm.region.start({}* %D6)
%D8 = call {}* %llvm.dbg.DEFINEVARIABLE({}* %D7, ...)
<i>;; Evaluate expression on line 5, assigning to Z.</i>
- %D9 = call {}* <a href="#impl_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D8, uint 6, uint 4, <a href="#impl_common_source_files">%lldb.compile_unit</a>* %file)
+ %D9 = call {}* <a href="#format_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D8, uint 6, uint 4, <a href="#format_common_source_files">%lldb.compile_unit</a>* %file)
<i>;; Code for line 6.</i>
%D10 = call {}* %llvm.region.end({}* %D9)
- %D11 = call {}* <a href="#impl_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D10, uint 8, uint 2, <a href="#impl_common_source_files">%lldb.compile_unit</a>* %file)
+ %D11 = call {}* <a href="#format_common_stoppoint">%llvm.dbg.stoppoint</a>({}* %D10, uint 8, uint 2, <a href="#format_common_source_files">%lldb.compile_unit</a>* %file)
<i>;; Code for line 8.</i>
<a name="#icl_ex_D1">%D12</a> = call {}* %llvm.region.end({}* %D11)
ret void
}
-</pre></p>
+</pre>
-<p>
-This example illustrates a few important details about the LLVM debugging
+<p>This example illustrates a few important details about the LLVM debugging
information. In particular, it shows how the various intrinsics used are woven
together with def-use and use-def chains, similar to how <a
-href="#impl_common_anchors">anchors</a> are used with globals. This allows the
-debugger to analyze the relationship between statements, variable definitions,
-and the code used to implement the function.</p>
+href="#format_common_anchors">anchors</a> are used with globals. This allows
+the debugger to analyze the relationship between statements, variable
+definitions, and the code used to implement the function.</p>
-<p>
-In this example, two explicit regions are defined, one with the <a
+<p>In this example, two explicit regions are defined, one with the <a
href="#icl_ex_D1">definition of the <tt>%D1</tt> variable</a> and one with the
<a href="#icl_ex_D7">definition of <tt>%D7</tt></a>. In the case of
<tt>%D1</tt>, the debug information indicates that the function whose <a
-href="#impl_common_globals">descriptor</a> is specified as an argument to the
+href="#format_program_objects">descriptor</a> is specified as an argument to the
intrinsic. This defines a new stack frame whose lifetime ends when the region
is ended by <a href="#icl_ex_D12">the <tt>%D12</tt> call</a>.</p>
-<p>
-Representing the boundaries of functions with regions allows normal LLVM
-interprocedural optimizations to change the boundaries of functions without
-having to worry about breaking mapping information between LLVM and source-level
-functions. In particular, the inlining optimization requires no modification to
-support inlining with debugging information: there is no correlation drawn
-between LLVM functions and their source-level counterparts.</p>
-
-<p>
-Once the function has been defined, the <a
-href="#impl_common_stoppoint">stopping point</a> corresponding to line #2 of the
-function is encountered. At this point in the function, <b>no</b> local
+<p>Using regions to represent the boundaries of source-level functions allow
+LLVM interprocedural optimizations to arbitrarily modify LLVM functions without
+having to worry about breaking mapping information between the LLVM code and the
+and source-level program. In particular, the inliner requires no modification
+to support inlining with debugging information: there is no explicit correlation
+drawn between LLVM functions and their source-level counterparts (note however,
+that if the inliner inlines all instances of a non-strong-linkage function into
+its caller that it will not be possible for the user to manually invoke the
+inlined function from the debugger).</p>
+
+<p>Once the function has been defined, the <a
+href="#format_common_stoppoint">stopping point</a> corresponding to line #2 of
+the function is encountered. At this point in the function, <b>no</b> local
variables are live. As lines 2 and 3 of the example are executed, their
variable definitions are automatically introduced into the program, without the
need to specify a new region. These variables do not require new regions to be
introduced because they go out of scope at the same point in the program: line
-9.
-</p>
+9.</p>
-<p>
-In contrast, the <tt>Z</tt> variable goes out of scope at a different time, on
-line 7. For this reason, it is defined within <a href="#icl_ex_D7">the
+<p>In contrast, the <tt>Z</tt> variable goes out of scope at a different time,
+on line 7. For this reason, it is defined within <a href="#icl_ex_D7">the
<tt>%D7</tt> region</a>, which kills the availability of <tt>Z</tt> before the
-code for line 8 is executed. Through the use of LLVM debugger regions,
-arbitrary source-language scoping rules can be supported, as long as they can
-only be nested (ie, one scope cannot partially overlap with a part of another
-scope).
-</p>
+code for line 8 is executed. In this way, regions can support arbitrary
+source-language scoping rules, as long as they can only be nested (ie, one scope
+cannot partially overlap with a part of another scope).</p>
-<p>
-It is worth noting that this scoping mechanism is used to control scoping of all
-declarations, not just variable declarations. For example, the scope of a C++
-using declaration is controlled with this, and the <tt>llvm-db</tt> C++ support
-routines could use this to change how name lookup is performed (though this is
-not yet implemented).
-</p>
+<p>It is worth noting that this scoping mechanism is used to control scoping of
+all declarations, not just variable declarations. For example, the scope of a
+C++ using declaration is controlled with this, and the <tt>llvm-db</tt> C++
+support routines could use this to change how name lookup is performed (though
+this is not implemented yet).</p>
</div>
-
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_common_descriptors">Object descriptor formats</a>
+ <a name="format_common_descriptors">Object descriptor formats</a>
</div>
<div class="doc_text">
-<p>
-The LLVM debugger expects the descriptors for global objects to start in a
+<p>The LLVM debugger expects the descriptors for program objects to start in a
canonical format, but the descriptors can include additional information
-appended at the end. All LLVM debugging information is versioned, allowing
-backwards compatibility in the case that the core structures need to change in
-some way. The lowest-level descriptor are those describing <a
-href="#impl_common_source_files">the files containing the program source
-code</a>, all other descriptors refer to them.
+appended at the end that is source-language specific. All LLVM debugging
+information is versioned, allowing backwards compatibility in the case that the
+core structures need to change in some way. Also, all debugging information
+objects start with a <a href="#format_common_tags">tag</a> to indicate what type
+of object it is. The source-language is allows to define its own objects, by
+using unreserved tag numbers.</p>
+
+<p>The lowest-level descriptor are those describing <a
+href="#format_common_source_files">the files containing the program source
+code</a>, as most other descriptors (sometimes indirectly) refer to them.
</p>
</div>
-<!----------------------------------------------------------------------------->
+<!-- ------------------------------------------------------------------------ ->
<div class="doc_subsubsection">
- <a name="impl_common_source_files">Representation of source files</a>
+ <a name="format_common_source_files">Representation of source files</a>
</div>
<div class="doc_text">
<p>
-Source file descriptors were roughly patterned after the Dwarf "compile_unit"
-object. The descriptor currently is defined to have the following LLVM
-type:</p>
+Source file descriptors are patterned after the Dwarf "compile_unit" object.
+The descriptor currently is defined to have at least the following LLVM
+type entries:</p>
-<p><pre>
+<pre>
%lldb.compile_unit = type {
+ uint, <i>;; Tag: <a href="#tag_compile_unit">LLVM_COMPILE_UNIT</a></i>
ushort, <i>;; LLVM debug version number</i>
ushort, <i>;; Dwarf language identifier</i>
sbyte*, <i>;; Filename</i>
sbyte*, <i>;; Working directory when compiled</i>
- sbyte*, <i>;; Producer of the debug information</i>
- {}* <i>;; Anchor for llvm.dbg.translation_units</i>
+ sbyte* <i>;; Producer of the debug information</i>
}
-</pre></p>
+</pre>
<p>
These descriptors contain the version number for the debug info, a source
language ID for the file (we use the Dwarf 3.0 ID numbers, such as
<tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>, <tt>DW_LANG_Cobol74</tt>,
etc), three strings describing the filename, working directory of the compiler,
-and an identifier string for the compiler that produced it, and the <a
-href="#impl_common_anchors">anchor</a> for the descriptor. Here is an example
+and an identifier string for the compiler that produced it. Note that actual
+compile_unit declarations must also include an <a
+href="#format_common_anchors">anchor</a> to <tt>llvm.dbg.translation_units</tt>,
+but it is not specified where the anchor is to be located. Here is an example
descriptor:
</p>
<p><pre>
%arraytest_source_file = internal constant %lldb.compile_unit {
+ <a href="#tag_compile_unit">uint 17</a>, ; Tag value
ushort 0, ; Version #0
ushort 1, ; DW_LANG_C89
sbyte* getelementptr ([12 x sbyte]* %.str_1, long 0, long 0), ; filename
%.str_3 = internal constant [12 x sbyte] c"llvmgcc 3.4\00"
</pre></p>
+<p>
+Note that the LLVM constant merging pass should eliminate duplicate copies of
+the strings that get emitted to each translation unit, such as the producer.
+</p>
</div>
-<!----------------------------------------------------------------------------->
+<!-- ----------------------------------------------------------------------- -->
<div class="doc_subsubsection">
- <a name="impl_common_globals">Representation of global objects</a>
+ <a name="format_program_objects">Representation of program objects</a>
</div>
<div class="doc_text">
<p>
-The LLVM debugger needs to know what the source-language global objects, in
-order to build stack traces and other related activities. Because
-source-languages have widly varying forms of global objects, the LLVM debugger
-only expects the following fields in the descriptor for each global:
+The LLVM debugger needs to know about some source-language program objects, in
+order to build stack traces, print information about local variables, and other
+related activities. The LLVM debugger differentiates between three different
+types of program objects: subprograms (functions, messages, methods, etc),
+variables (locals and globals), and others. Because source-languages have
+widely varying forms of these objects, the LLVM debugger expects only a few
+fields in the descriptor for each object:
</p>
-<p><pre>
-%lldb.global = type {
- <a href="#impl_common_source_files">%lldb.compile_unit</a>*, <i>;; The translation unit containing the global</i>
- sbyte*, <i>;; The global object 'name'</i>
- [type]*, <i>;; Source-language type descriptor for global</i>
- {}* <i>;; The anchor for llvm.dbg.globals</i>
+<pre>
+%lldb.object = type {
+ uint, <i>;; <a href="#format_common_tag">A tag</a></i>
+ <i>any</i>*, <i>;; The <a href="#format_common_object_contexts">context</a> for the object</i>
+ sbyte* <i>;; The object 'name'</i>
}
-</pre></p>
+</pre>
+
+<p>The first field contains a tag for the descriptor. The second field contains
+either a pointer to the descriptor for the containing <a
+href="#format_common_source_files">source file</a>, or it contains a pointer to
+another program object whose context pointer eventually reaches a source file.
+Through this <a href="#format_common_object_contexts">context</a> pointer, the
+LLVM debugger can establish the debug version number of the object.</p>
+
+<p>The third field contains a string that the debugger can use to identify the
+object if it does not contain explicit support for the source-language in use
+(ie, the 'unknown' source language handler uses this string). This should be
+some sort of unmangled string that corresponds to the object, but it is a
+quality of implementation issue what exactly it contains (it is legal, though
+not useful, for all of these strings to be null).</p>
+
+<p>Note again that descriptors can be extended to include
+source-language-specific information in addition to the fields required by the
+LLVM debugger. See the <a href="#ccxx_descriptors">section on the C/C++
+front-end</a> for more information. Also remember that global objects
+(functions, selectors, global variables, etc) must contain an <a
+href="#format_common_anchors">anchor</a> to the <tt>llvm.dbg.globals</tt>
+variable.</p>
+</div>
-<p>
-The first field contains a pointer to the translation unit the function is
-defined in. This pointer allows the debugger to find out which version of debug
-information the function corresponds to. The second field contains a string
-that the debugger can use to identify the subprogram if it does not contain
-explicit support for the source-language in use. This should be some sort of
-unmangled string that corresponds to the function somehow.
-</p>
-<p>
-Note again that descriptors can be extended to include source-language-specific
-information in addition to the fields required by the LLVM debugger. See the <a
-href="#impl_ccxx_descriptors">section on the C/C++ front-end</a> for more
-information.
-</p>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="format_common_object_contexts">Program object contexts</a>
</div>
+<div class="doc_text">
+<pre>
+Allow source-language specific contexts, use to identify namespaces etc
+Must end up in a source file descriptor.
+Debugger core ignores all unknown context objects.
+</pre>
+</div>
-
-<!----------------------------------------------------------------------------->
-<div class="doc_subsubsection">
- <a name="impl_common_localvars">Representation of local variables</a>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="format_common_intrinsics">Debugger intrinsic functions</a>
</div>
<div class="doc_text">
-<p>
-</p>
+<pre>
+Define each intrinsics, as an extension of the language reference manual.
+
+llvm.dbg.stoppoint
+llvm.dbg.region.start
+llvm.dbg.region.end
+llvm.dbg.function.start
+llvm.dbg.declare
+</pre>
</div>
-
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_common_intrinsics">Other intrinsic functions</a>
+ <a name="format_common_tags">Values for debugger tags</a>
</div>
<div class="doc_text">
-<p>
-</p>
+<p>Happen to be the same value as the similarly named Dwarf-3 tags, this may
+change in the future.</p>
+
+<pre>
+ <a name="tag_compile_unit">LLVM_COMPILE_UNIT</a> : 17
+ <a name="tag_subprogram">LLVM_SUBPROGRAM</a> : 46
+ <a name="tag_variable">LLVM_VARIABLE</a> : 52
+<!-- <a name="tag_formal_parameter">LLVM_FORMAL_PARAMETER : 5-->
+</pre>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
- <a name="impl_ccxx">C/C++ front-end specific debug information</a>
+ <a name="ccxx_frontend">C/C++ front-end specific debug information</a>
</div>
+<!-- *********************************************************************** -->
<div class="doc_text">
-<p>
-The C and C++ front-ends represent information about the program in a format
+<p>The C and C++ front-ends represent information about the program in a format
that is effectively identical to <a
href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3.0</a> in terms of
information content. This allows code generators to trivially support native
debuggers by generating standard dwarf information, and contains enough
-information for non-dwarf targets to translate it other as needed.</p>
-
-<p>
-TODO: document extensions to standard debugging objects, document how we
-represent source types, etc.
-</p>
+information for non-dwarf targets to translate it as needed.</p>
+
+<p>The basic debug information required by the debugger is (intentionally)
+designed to be as minimal as possible. This basic information is so minimal
+that it is unlikely that <b>any</b> source-language could be adequately
+described by it. Because of this, the debugger format was designed for
+extension to support source-language-specific information. The extended
+descriptors are read and interpreted by the <a
+href="#arch_info">language-specific</a> modules in the debugger if there is
+support available, otherwise it is ignored.</p>
+
+<p>This section describes the extensions used to represent C and C++ programs.
+Other languages could pattern themselves after this (which itself is tuned to
+representing programs in the same way that Dwarf 3 does), or they could choose
+to provide completely different extensions if they don't fit into the Dwarf
+model. As support for debugging information gets added to the various LLVM
+source-language front-ends, the information used should be documented here.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="impl_ccxx_descriptors">Object Descriptor Formats</a>
+ <a name="ccxx_pse">Program Scope Entries</a>
</div>
<div class="doc_text">
-<p>
+<p>TODO</p>
+</div>
+
+<!-- -------------------------------------------------------------------------->
+<div class="doc_subsubsection">
+ <a name="ccxx_compilation_units">Compilation unit entries</a>
+</div>
+<div class="doc_text">
+<p>
+Translation units do not add any information over the standard <a
+href="#format_common_source_files">source file representation</a> already
+expected by the debugger. As such, it uses descriptors of the type specified,
+with a trailing <a href="#format_common_anchors">anchor</a>.
</p>
</div>
+<!-- -------------------------------------------------------------------------->
+<div class="doc_subsubsection">
+ <a name="ccxx_modules">Module, namespace, and importing entries</a>
+</div>
+
+<div class="doc_text">
+<p>TODO</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="ccxx_dataobjects">Data objects (program variables)</a>
+</div>
+
+<div class="doc_text">
+<p>TODO</p>
+</div>
<!-- *********************************************************************** -->
+
<hr>
-<div class="doc_footer">
- <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
- <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
- <br>
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+ <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+ <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
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projects / oota-llvm.git / blobdiff