First version of this document. It is still missing some pretty big pieces, and
the debugging information formats will likely change, but it's a start, and I have to move on to other things in the short-term, so it might be a while before I get back to working on this. llvm-svn: 10683
This commit is contained in:
Chris Lattner
parent
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
|
||||
<head>
|
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<title>Source Level Debugging with LLVM</title>
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||||
<link rel="stylesheet" href="llvm.css" type="text/css">
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||||
</head>
|
||||
<body>
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||||
|
||||
<div class="doc_title">Source Level Debugging with LLVM</div>
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||||
|
||||
<ul>
|
||||
|
||||
<img src="venusflytrap.jpg" width=247 height=369 align=right>
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|
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<li><a href="#introduction">Introduction</a></li>
|
||||
<ol>
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||||
<li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
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||||
<li><a href="#debugopt">Debugging optimized code</a></li>
|
||||
<li><a href="#future">Future work</a></li>
|
||||
</ol>
|
||||
<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="#sample">A sample <tt>llvm-db</tt> session</a></li>
|
||||
<li><a href="#startup">Starting the debugger</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>
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||||
<ol>
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||||
<li><a href="#arch_todo">Short-term TODO list</a></li>
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||||
</ol>
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||||
|
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<li><a href="#implementation">Debugging information implementation</a></li>
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||||
<ol>
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<li><a href="#impl_common_anchors">Anchors for global objects</a></li>
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<li><a href="#impl_common_stoppoint">Representing stopping points in the source program</a></li>
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<li><a href="#impl_common_lifetime">Object lifetimes and scoping</a></li>
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<li><a href="#impl_common_descriptors">Object descriptor formats</a></li>
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<ul>
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<li><a href="#impl_common_source_files">Representation of source files</a></li>
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<li><a href="#impl_common_globals">Representation of global objects</a></li>
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<li><a href="#impl_common_localvars">Representation of local variables</a></li>
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</ul>
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<li><a href="#impl_common_intrinsics">Other intrinsic functions</a></li>
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</ol>
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<li><a href="#impl_ccxx">C/C++ front-end specific debug information</a></li>
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<ol>
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<li><a href="#impl_ccxx_descriptors">Object descriptor formats</a></li>
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||||
</ol>
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||||
</ul>
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||||
|
||||
<!-- *********************************************************************** -->
|
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<div class="doc_section"><a name="introduction">Introduction</a></div>
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<!-- *********************************************************************** -->
|
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|
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<div class="doc_text">
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|
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<p>This document is the central repository for all information pertaining to
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debug information in LLVM. It describes how to use the <a
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href="CommandGuide/llvm-db.html"><tt>llvm-db</tt> tool</a>, which provides a
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powerful <a href="#llvm-db">source-level debugger</a> to users of LLVM-based
|
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compilers. When compiling a program in debug mode, the front-end in use adds
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LLVM debugging information to the program in the form of normal <a
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href="LangRef.html">LLVM program objects</a> as well as a small set of LLVM <a
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href="#implementation">intrinsic functions</a>, which specify the mapping of the
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program in LLVM form to the program in the source language.
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</p>
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|
||||
</div>
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|
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<!-- ======================================================================= -->
|
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<div class="doc_subsection">
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<a name="phil">Philosophy behind LLVM debugging information</a>
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</div>
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|
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<div class="doc_text">
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<p>
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The idea of the LLVM debugging information is to capture how the important
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pieces of the source-language's Abstract Syntax Tree map onto LLVM code.
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Several design aspects have shaped the solution that appears here. The
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important ones are:</p>
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<p><ul>
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<li>Debugging information should have very little impact on the rest of the
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compiler. No transformations, analyses, or code generators should need to be
|
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modified because of debugging information.</li>
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<li>LLVM optimizations should interact in <a href="#debugopt">well-defined and
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easily described ways</a> with the debugging information.</li>
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|
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<li>Because LLVM is designed to support arbitrary programming languages,
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LLVM-to-LLVM tools should not need to know anything about the semantics of the
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source-level-language.</li>
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|
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<li>Source-level languages are often <b>widely</b> different from one another.
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LLVM should not put any restrictions of the flavor of the source-language, and
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the debugging information should work with any language.</li>
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|
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<li>With code generator support, it should be possible to use an LLVM compiler
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to compile a program to native machine code with standard debugging formats.
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This allows compatibility with traditional machine-code level debuggers, like
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GDB or DBX.</li>
|
||||
|
||||
</ul></p>
|
||||
|
||||
<p>
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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
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between LLVM program objects and the source-level objects. The description of
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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
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currently uses working draft 7 of the <a
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||||
href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3 standard</a>).</p>
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|
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<p>
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When a program is debugged, the debugger interacts with the user and turns the
|
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stored debug information into source-language specific information. As such,
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the debugger must be aware of the source-language, and is thus tied to a
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specific language of family of languages. The <a href="#llvm-db">LLVM
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debugger</a> is designed to be modular in its support for source-languages.
|
||||
</p>
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||||
|
||||
</div>
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|
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|
||||
<!-- ======================================================================= -->
|
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<div class="doc_subsection">
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<a name="debugopt">Debugging optimized code</a>
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</div>
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|
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<div class="doc_text">
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<p>
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An extremely high priority of LLVM debugging information is to make it interact
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well with optimizations and analysis. In particular, the LLVM debug information
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provides the following guarantees:</p>
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|
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<p><ul>
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|
||||
<li>LLVM debug information <b>always provides information to accurately read the
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source-level state of the program</b>, regardless of which LLVM optimizations
|
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have been run, and without any modification to the optimizations themselves.
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||||
However, some optimizations may impact the ability to modify the current state
|
||||
of the program with a debugger, such as setting program variables, or calling
|
||||
function that have been deleted.</li>
|
||||
|
||||
<li>LLVM optimizations gracefully interact with debugging information. If they
|
||||
are not aware of debug information, they are automatically disabled as necessary
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in the cases that would invalidate the debug info. This retains the LLVM
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features making it easy to write new transformations.</li>
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|
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<li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
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debugging information, allowing them to update the debugging information as they
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perform aggressive optimizations. This means that, with effort, the LLVM
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optimizers could optimize debug code just as well as non-debug code.</li>
|
||||
|
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<li>LLVM debug information does not prevent many important optimizations from
|
||||
happening (for example inlining, basic block reordering/merging/cleanup, tail
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duplication, etc), further reducing the amount of the compiler that eventually
|
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is "aware" of debugging information.</li>
|
||||
|
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<li>LLVM debug information is automatically optimized along with the rest of the
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program, using existing facilities. For example, duplicate information is
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automatically merged by the linker, and unused information is automatically
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removed.</li>
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|
||||
</ul></p>
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|
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<p>
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Basically, the debug information allows you to compile a program with "<tt>-O0
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-g</tt>" and get full debug information, allowing you to arbitrarily modify the
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program as it executes from the debugger. Compiling a program with "<tt>-O3
|
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-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
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functions where were optimized out of the program, or inlined away completely.
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||||
</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
|
||||
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
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||||
debugging information, <a href="#commands">new commands</a> can be added to the
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||||
debugger, and thread support could be added to the debugger.</p>
|
||||
|
||||
<p>
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||||
The "SourceLanguage" modules provided by <tt>llvm-db</tt> could be substantially
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||||
improved to provide good support for C++ language features like namespaces and
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||||
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
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compatible with the LLVM license).</p>
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||||
|
||||
<p>
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||||
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>
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||||
|
||||
</div>
|
||||
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
<div class="doc_section">
|
||||
<a name="llvm-db">Using the <tt>llvm-db</tt> tool</a>
|
||||
</div>
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<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>
|
||||
|
||||
<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
|
||||
<tt>llvm-db</tt> if you already know <tt>GDB</tt>. In general, <tt>llvm-db</tt>
|
||||
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>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="limitations">Limitations of <tt>llvm-db</tt></a>
|
||||
</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><ul>
|
||||
|
||||
<li>Running a program in <tt>llvm-db</tt> is a bit slower than running it with
|
||||
<tt>lli</tt>.</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>Inspection of LLVM code is not supported. This means that you cannot print
|
||||
the contents of arbitrary LLVM values, or use commands such as <tt>stepi</tt>.
|
||||
This also means that you cannot debug code without debug information.</li>
|
||||
|
||||
<li>Portions of the debugger run in the same address space as the program being
|
||||
debugged. This means that memory corruption by the program could trample on
|
||||
portions of the debugger.</li>
|
||||
|
||||
<li>Attaching to existing processes and core files is not currently
|
||||
supported.</li>
|
||||
|
||||
</ul></p>
|
||||
|
||||
<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>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="sample">A sample <tt>llvm-db</tt> session</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>
|
||||
TODO
|
||||
</p>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="startup">Starting the debugger</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>There are three ways to start up the <tt>llvm-db</tt> debugger:</p>
|
||||
|
||||
<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>
|
||||
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>
|
||||
commands.</p>
|
||||
|
||||
<p>The third way to start the program is with the <tt>--args</tt> option. This
|
||||
option allows you to specify the program to load and the arguments to start out
|
||||
with. <!-- No options to <tt>llvm-db</tt> may be specified after the
|
||||
<tt>-args</tt> option. --> Example use: <tt>llvm-db --args ls /home</tt></p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="commands">Commands recognized by the debugger</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>FIXME: this needs work obviously. See the <a
|
||||
href="http://sources.redhat.com/gdb/documentation/">GDB documentation</a> for
|
||||
information about what these do, or try '<tt>help [command]</tt>' within
|
||||
<tt>llvm-db</tt> to get information.</p>
|
||||
|
||||
<p>
|
||||
<h2>General usage:</h2>
|
||||
<ul>
|
||||
<li>help [command]</li>
|
||||
<li>quit</li>
|
||||
<li><a name="c_file">file</a> [program]</li>
|
||||
</ul>
|
||||
|
||||
<h2>Program inspection and interaction:</h2>
|
||||
<ul>
|
||||
<li>create (start the program, stopping it ASAP in <tt>main</tt>)</li>
|
||||
<li>kill</li>
|
||||
<li>run [args]</li>
|
||||
<li>step [num]</li>
|
||||
<li>next [num]</li>
|
||||
<li>cont</li>
|
||||
<li>finish</li>
|
||||
|
||||
<li>list [start[, end]]</li>
|
||||
<li>info source</li>
|
||||
<li>info sources</li>
|
||||
<li>info functions</li>
|
||||
</ul>
|
||||
|
||||
<h2>Call stack inspection:</h2>
|
||||
<ul>
|
||||
<li>backtrace</li>
|
||||
<li>up [n]</li>
|
||||
<li>down [n]</li>
|
||||
<li>frame [n]</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<h2>Debugger inspection and interaction:</h2>
|
||||
<ul>
|
||||
<li>info target</li>
|
||||
<li>show prompt</li>
|
||||
<li>set prompt</li>
|
||||
<li>show listsize</li>
|
||||
<li>set listsize</li>
|
||||
<li>show language</li>
|
||||
<li>set language</li>
|
||||
</ul>
|
||||
|
||||
<h2>TODO:</h2>
|
||||
<ul>
|
||||
<li>info frame</li>
|
||||
<li>break</li>
|
||||
<li>print</li>
|
||||
<li>ptype</li>
|
||||
|
||||
<li>info types</li>
|
||||
<li>info variables</li>
|
||||
<li>info program</li>
|
||||
|
||||
<li>info args</li>
|
||||
<li>info locals</li>
|
||||
<li>info catch</li>
|
||||
<li>... many others</li>
|
||||
</ul>
|
||||
</p>
|
||||
</div>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
<div class="doc_section">
|
||||
<a name="architecture">Architecture of the LLVM debugger</a>
|
||||
</div>
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p><pre>
|
||||
lib/Debugger
|
||||
- UnixLocalInferiorProcess.cpp
|
||||
|
||||
tools/llvm-db
|
||||
- SourceLanguage interfaces
|
||||
- ProgramInfo/RuntimeInfo
|
||||
- Commands
|
||||
|
||||
</pre></p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="arch_todo">Short-term TODO list</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>
|
||||
FIXME: this section will eventually go away. These are notes to myself of
|
||||
things that should be implemented, but haven't yet.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
<b>Breakpoints:</b> Support is already implemented in the 'InferiorProcess'
|
||||
class, though it hasn't been tested yet. To finish breakpoint support, we need
|
||||
to implement breakCommand (which should reuse the linespec parser from the list
|
||||
command), and handle the fact that 'break foo' or 'break file.c:53' may insert
|
||||
multiple breakpoints. Also, if you say 'break file.c:53' and there is no
|
||||
stoppoint on line 53, the breakpoint should go on the next available line. My
|
||||
idea was to have the Debugger class provide a "Breakpoint" class which
|
||||
encapsulated this messiness, giving the debugger front-end a simple interface.
|
||||
The debugger front-end would have to map the really complex semantics of
|
||||
temporary breakpoints and 'conditional' breakpoints onto this intermediate
|
||||
level. Also, breakpoints should survive as much as possible across program
|
||||
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
|
||||
IntrinsicLowering object could instead change descriptors into constant expr
|
||||
casts of the constant address of the LLVM objects for the descriptors. This
|
||||
would also allow us to eliminate the mapping back and forth between physical
|
||||
addresses that must be done.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
<div class="doc_section">
|
||||
<a name="implementation">Debugging information implementation</a>
|
||||
</div>
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>LLVM debugging information has been carefully designed to make it possible
|
||||
for the optimizer to optimize the program and debugging information without
|
||||
necessarily having to know anything about debugging information. In particular,
|
||||
the global constant merging pass automatically eliminates duplicated debugging
|
||||
information (often caused by header files), the global dead code elimination
|
||||
pass automatically deletes debugging information for a function if it decides to
|
||||
delete the function, and the linker eliminates debug information when it merges
|
||||
<tt>linkonce</tt> functions.</p>
|
||||
|
||||
<p>To do this, most of the debugging information (descriptors for types,
|
||||
variables, functions, source files, etc) is inserted by the language front-end
|
||||
in the form of LLVM global variables. These LLVM global variables are no
|
||||
different from any other global variables, except that they have a web of LLVM
|
||||
intrinsic functions that point to them. If the last references to a particular
|
||||
piece of debugging information are deleted (for example, by the
|
||||
<tt>-globaldce</tt> pass), the extraneous debug information will automatically
|
||||
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>
|
||||
|
||||
<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.
|
||||
|
||||
<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>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_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
|
||||
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>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_common_stoppoint">
|
||||
Representing stopping points in the source program
|
||||
</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
|
||||
<p>LLVM debugger "stop points" are a key part of the debugging representation
|
||||
that allows the LLVM to maintain simple semantics for <a
|
||||
href="#debugopt">debugging optimized code</a>. The basic idea is that the
|
||||
front-end inserts calls to the <tt>%llvm.dbg.stoppoint</tt> intrinsic function
|
||||
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>
|
||||
|
||||
<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>
|
||||
|
||||
<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>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_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
|
||||
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>
|
||||
|
||||
<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>
|
||||
1. void foo() {
|
||||
2. int X = ...;
|
||||
3. int Y = ...;
|
||||
4. {
|
||||
5. int Z = ...;
|
||||
6. ...
|
||||
7. }
|
||||
8. ...
|
||||
9. }
|
||||
</pre></p>
|
||||
|
||||
<p>
|
||||
Compiled to LLVM, this function would be represented like this (FIXME: CHECK AND
|
||||
UPDATE THIS):
|
||||
</p>
|
||||
|
||||
<p><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)
|
||||
|
||||
%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)
|
||||
|
||||
%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)
|
||||
|
||||
<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)
|
||||
|
||||
<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)
|
||||
|
||||
<i>;; Code for line 8.</i>
|
||||
<a name="#icl_ex_D1">%D12</a> = call {}* %llvm.region.end({}* %D11)
|
||||
ret void
|
||||
}
|
||||
</pre></p>
|
||||
|
||||
<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>
|
||||
|
||||
<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
|
||||
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
|
||||
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>
|
||||
|
||||
<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>
|
||||
|
||||
<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>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_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
|
||||
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.
|
||||
</p>
|
||||
</div>
|
||||
|
||||
|
||||
<!----------------------------------------------------------------------------->
|
||||
<div class="doc_subsubsection">
|
||||
<a name="impl_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>
|
||||
|
||||
<p><pre>
|
||||
%lldb.compile_unit = type {
|
||||
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>
|
||||
}
|
||||
</pre></p>
|
||||
|
||||
<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
|
||||
descriptor:
|
||||
</p>
|
||||
|
||||
<p><pre>
|
||||
%arraytest_source_file = internal constant %lldb.compile_unit {
|
||||
ushort 0, ; Version #0
|
||||
ushort 1, ; DW_LANG_C89
|
||||
sbyte* getelementptr ([12 x sbyte]* %.str_1, long 0, long 0), ; filename
|
||||
sbyte* getelementptr ([12 x sbyte]* %.str_2, long 0, long 0), ; working dir
|
||||
sbyte* getelementptr ([12 x sbyte]* %.str_3, long 0, long 0), ; producer
|
||||
{}* %llvm.dbg.translation_units ; Anchor
|
||||
}
|
||||
%.str_1 = internal constant [12 x sbyte] c"arraytest.c\00"
|
||||
%.str_2 = internal constant [12 x sbyte] c"/home/sabre\00"
|
||||
%.str_3 = internal constant [12 x sbyte] c"llvmgcc 3.4\00"
|
||||
</pre></p>
|
||||
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
<!----------------------------------------------------------------------------->
|
||||
<div class="doc_subsubsection">
|
||||
<a name="impl_common_globals">Representation of global 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:
|
||||
</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></p>
|
||||
|
||||
<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>
|
||||
|
||||
|
||||
|
||||
<!----------------------------------------------------------------------------->
|
||||
<div class="doc_subsubsection">
|
||||
<a name="impl_common_localvars">Representation of local variables</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_common_intrinsics">Other intrinsic functions</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
|
||||
</p>
|
||||
</div>
|
||||
|
||||
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
<div class="doc_section">
|
||||
<a name="impl_ccxx">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
|
||||
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>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
<div class="doc_subsection">
|
||||
<a name="impl_ccxx_descriptors">Object Descriptor Formats</a>
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
|
||||
</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>
|
||||
Last modified: $Date$
|
||||
</div>
|
||||
|
||||
</body>
|
||||
</html>
|
||||
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Reference in New Issue