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author | Chris Lattner <sabre@nondot.org> | 2004-01-05 05:06:33 +0000 |
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committer | Chris Lattner <sabre@nondot.org> | 2004-01-05 05:06:33 +0000 |
commit | bdfb339b8d1d0480c42bfbcf76b96c1f7fcdec75 (patch) | |
tree | 85a316d998ca120b1bb172f5fb20157a61f24e07 | |
parent | 271bd2d7f1c8dca9906a9b9b9d3081b9040e2f48 (diff) | |
download | external_llvm-bdfb339b8d1d0480c42bfbcf76b96c1f7fcdec75.zip external_llvm-bdfb339b8d1d0480c42bfbcf76b96c1f7fcdec75.tar.gz external_llvm-bdfb339b8d1d0480c42bfbcf76b96c1f7fcdec75.tar.bz2 |
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
-rw-r--r-- | docs/SourceLevelDebugging.html | 906 | ||||
-rw-r--r-- | docs/venusflytrap.jpg | bin | 0 -> 56606 bytes |
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diff --git a/docs/SourceLevelDebugging.html b/docs/SourceLevelDebugging.html new file mode 100644 index 0000000..ab9af99 --- /dev/null +++ b/docs/SourceLevelDebugging.html @@ -0,0 +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@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> diff --git a/docs/venusflytrap.jpg b/docs/venusflytrap.jpg Binary files differnew file mode 100644 index 0000000..59340ef --- /dev/null +++ b/docs/venusflytrap.jpg |