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<title>Source Level Debugging with LLVM</title>
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<body>
<div class="doc_title">Source Level Debugging with LLVM</div>
<ul>
<img src="venusflytrap.jpg" width=247 height=369 align=right>
<li><a href="#introduction">Introduction</a></li>
<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>
<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>
<ol>
<li><a href="#arch_todo">Short-term TODO list</a></li>
</ol>
<li><a href="#implementation">Debugging information implementation</a></li>
<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>
<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>
<ol>
<li><a href="#impl_ccxx_descriptors">Object descriptor formats</a></li>
</ol>
</ul>
<!-- *********************************************************************** -->
<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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="phil">Philosophy behind LLVM debugging information</a>
</div>
<div class="doc_text">
<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>
<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>
<li>LLVM optimizations should interact in <a href="#debugopt">well-defined and
easily described ways</a> with the debugging information.</li>
<li>Because LLVM is designed to support arbitrary programming languages,
LLVM-to-LLVM tools should not need to know anything about the semantics of the
source-level-language.</li>
<li>Source-level languages are often <b>widely</b> different from one another.
LLVM should not put any restrictions of the flavor of the source-language, and
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.
This allows compatibility with traditional machine-code level debuggers, like
GDB or DBX.</li>
</ul></p>
<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
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
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
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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="debugopt">Debugging optimized code</a>
</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>
<li>LLVM debug information <b>always provides information to accurately read the
source-level state of the program</b>, regardless of which LLVM optimizations
have been run, and without any modification to the optimizations themselves.
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
in the cases that would invalidate the debug info. This retains the LLVM
features making it easy to write new transformations.</li>
<li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
debugging information, allowing them to update the debugging information as they
perform aggressive optimizations. This means that, with effort, the LLVM
optimizers could optimize debug code just as well as non-debug code.</li>
<li>LLVM debug information does not prevent many important optimizations from
happening (for example inlining, basic block reordering/merging/cleanup, tail
duplication, etc), further reducing the amount of the compiler that eventually
is "aware" of debugging information.</li>
<li>LLVM debug information is automatically optimized along with the rest of the
program, using existing facilities. For example, duplicate information is
automatically merged by the linker, and unused information is automatically
removed.</li>
</ul></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
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
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>
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
compatible with the LLVM license).</p>
<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>
</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
&lt;program&gt;</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> &amp; <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: $Date$
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