[llvm-commits] CVS: llvm/docs/SourceLevelDebugging.html venusflytrap.jpg
Chris Lattner
lattner at cs.uiuc.edu
Sun Jan 4 23:07:01 PST 2004
Changes in directory llvm/docs:
SourceLevelDebugging.html added (r1.1)
venusflytrap.jpg added (r1.1)
---
Log message:
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.
---
Diffs of the changes: (+906 -0)
Index: llvm/docs/SourceLevelDebugging.html
diff -c /dev/null llvm/docs/SourceLevelDebugging.html:1.1
*** /dev/null Sun Jan 4 23:06:44 2004
--- llvm/docs/SourceLevelDebugging.html Sun Jan 4 23:06:33 2004
***************
*** 0 ****
--- 1,906 ----
+ <!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
+ <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 at nondot.org">Chris Lattner</a></address>
+ <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
+ <br>
+ Last modified: $Date: 2004/01/05 05:06:33 $
+ </div>
+
+ </body>
+ </html>
Index: llvm/docs/venusflytrap.jpg
More information about the llvm-commits
mailing list