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. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10683 91177308-0d34-0410-b5e6-96231b3b80d8
author: Chris Lattner <sabre@nondot.org> 2004-01-05 05:06:33 +0000
committer: Chris Lattner <sabre@nondot.org> 2004-01-05 05:06:33 +0000
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+<html>
+<head>
+  <title>Source Level Debugging with LLVM</title>
+  <link rel="stylesheet" href="llvm.css" type="text/css">
+</head>
+<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: $Date$
+</div>
+
+</body>
+</html>
author	Chris Lattner <sabre@nondot.org>	2004-01-05 05:06:33 +0000
committer	Chris Lattner <sabre@nondot.org>	2004-01-05 05:06:33 +0000
commit	bdfb339b8d1d0480c42bfbcf76b96c1f7fcdec75 (patch)
tree	85a316d998ca120b1bb172f5fb20157a61f24e07 /docs/SourceLevelDebugging.html
parent	271bd2d7f1c8dca9906a9b9b9d3081b9040e2f48 (diff)