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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<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>
<table class="layout" style="width:100%">
<tr class="layout">
<td class="left">
<ul>
<li><a href="#introduction">Introduction</a>
<ol>
<li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
<li><a href="#consumers">Debug information consumers</a></li>
<li><a href="#debugopt">Debugging optimized code</a></li>
</ol></li>
<li><a href="#format">Debugging information format</a>
<ol>
<li><a href="#debug_info_descriptors">Debug information descriptors</a>
<ul>
<li><a href="#format_compile_units">Compile unit descriptors</a></li>
<li><a href="#format_global_variables">Global variable descriptors</a></li>
<li><a href="#format_subprograms">Subprogram descriptors</a></li>
<li><a href="#format_blocks">Block descriptors</a></li>
<li><a href="#format_basic_type">Basic type descriptors</a></li>
<li><a href="#format_derived_type">Derived type descriptors</a></li>
<li><a href="#format_composite_type">Composite type descriptors</a></li>
<li><a href="#format_subrange">Subrange descriptors</a></li>
<li><a href="#format_enumeration">Enumerator descriptors</a></li>
<li><a href="#format_variables">Local variables</a></li>
</ul></li>
<li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>
<ul>
<li><a href="#format_common_stoppoint">llvm.dbg.stoppoint</a></li>
<li><a href="#format_common_func_start">llvm.dbg.func.start</a></li>
<li><a href="#format_common_region_start">llvm.dbg.region.start</a></li>
<li><a href="#format_common_region_end">llvm.dbg.region.end</a></li>
<li><a href="#format_common_declare">llvm.dbg.declare</a></li>
</ul></li>
<li><a href="#format_common_stoppoints">Representing stopping points in the
source program</a></li>
</ol></li>
<li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
<ol>
<li><a href="#ccxx_compile_units">C/C++ source file information</a></li>
<li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>
<li><a href="#ccxx_subprogram">C/C++ function information</a></li>
<li><a href="#ccxx_basic_types">C/C++ basic types</a></li>
<li><a href="#ccxx_derived_types">C/C++ derived types</a></li>
<li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>
<li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>
</ol></li>
</ul>
</td>
<td class="right">
<img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
height="369">
</td>
</tr></table>
<div class="doc_author">
<p>Written by <a href="mailto:[email protected]">Chris Lattner</a>
and <a href="mailto:[email protected]">Jim Laskey</a></p>
</div>
<!-- *********************************************************************** -->
<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 the <a href="#format">actual format
that the LLVM debug information</a> takes, which is useful for those
interested in creating front-ends or dealing directly with the information.
Further, this document provides specific examples of what debug information
for C/C++.</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>
<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 and standard debugging
formats. This allows compatibility with traditional machine-code level
debuggers, like GDB or DBX.</li>
</ul>
<p>The approach used by the LLVM implementation is to use a small set
of <a href="#format_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="#ccxx_frontend">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 being debugged, a debugger interacts with the user and
turns the stored debug information into source-language specific information.
As such, a debugger must be aware of the source-language, and is thus tied to
a specific language or family of languages.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="consumers">Debug information consumers</a>
</div>
<div class="doc_text">
<p>The role of debug information is to provide meta information normally
stripped away during the compilation process. This meta information provides
an LLVM user a relationship between generated code and the original program
source code.</p>
<p>Currently, debug information is consumed by the DwarfWriter to produce dwarf
information used by the gdb debugger. Other targets could use the same
information to produce stabs or other debug forms.</p>
<p>It would also be reasonable to use debug information to feed profiling tools
for analysis of generated code, or, tools for reconstructing the original
source from generated code.</p>
<p>TODO - expound a bit more.</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>
<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 functions 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>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 a 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>
<p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a
framework to test optimizer's handling of debugging information. It can be
run like this:</p>
<div class="doc_code">
<pre>
% cd llvm/projects/test-suite/MultiSource/Benchmarks # or some other level
% make TEST=dbgopt
</pre>
</div>
<p>This will test impact of debugging information on optimization passes. If
debugging information influences optimization passes then it will be reported
as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more
information on LLVM test infrastructure and how to run various tests.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="format">Debugging information format</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>Debug information is designed to be agnostic about the target debugger and
debugging information representation (e.g. DWARF/Stabs/etc). It uses a
generic machine debug information pass 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 target debugger to interpret the
information. In addition, debug global variables are declared in
the <tt>"llvm.metadata"</tt> section. All values declared in this section
are stripped away after target debug information is constructed and before
the program object is emitted.</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="#format_compile_units">source files</a>,
and <a href="#format_global_variables">program objects</a>. These abstract
objects are used by a debugger to form stack traces, show information about
local variables, etc.</p>
<p>This section of the documentation first describes the representation aspects
common to any source-language. The <a href="#ccxx_frontend">next section</a>
describes the data layout conventions used by the C and C++ front-ends.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="debug_info_descriptors">Debug information descriptors</a>
</div>
<div class="doc_text">
<p>In consideration of the complexity and volume of debug information, LLVM
provides a specification for well formed debug global variables. The
constant value of each of these globals is one of a limited set of
structures, known as debug descriptors.</p>
<p>Consumers of LLVM debug information expect the descriptors for program
objects to start in a canonical format, but the descriptors can include
additional information appended at the end that is source-language
specific. All LLVM debugging information is versioned, allowing backwards
compatibility in the case that the core structures need to change in some
way. Also, all debugging information objects start with a tag to indicate
what type of object it is. The source-language is allowed to define its own
objects, by using unreserved tag numbers. We recommend using with tags in
the range 0x1000 thru 0x2000 (there is a defined enum DW_TAG_user_base =
0x1000.)</p>
<p>The fields of debug descriptors used internally by LLVM (MachineModuleInfo)
are restricted to only the simple data types <tt>int</tt>, <tt>uint</tt>,
<tt>bool</tt>, <tt>float</tt>, <tt>double</tt>, <tt>i8*</tt> and
<tt>{ }*</tt>. References to arbitrary values are handled using a
<tt>{ }*</tt> and a cast to <tt>{ }*</tt> expression; typically
references to other field descriptors, arrays of descriptors or global
variables.</p>
<div class="doc_code">
<pre>
%llvm.dbg.object.type = type {
uint, ;; A tag
...
}
</pre>
</div>
<p><a name="LLVMDebugVersion">The first field of a descriptor is always an
<tt>uint</tt> containing a tag value identifying the content of the
descriptor. The remaining fields are specific to the descriptor. The values
of tags are loosely bound to the tag values of DWARF information entries.
However, that does not restrict the use of the information supplied to DWARF
targets. To facilitate versioning of debug information, the tag is augmented
with the current debug version (LLVMDebugVersion = 4 << 16 or 0x40000 or
262144.)</a></p>
<p>The details of the various descriptors follow.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_compile_units">Compile unit descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_compile_units">llvm.dbg.compile_unit.type</a> = type {
i32, ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_compile_unit)
{ }*, ;; Compile unit anchor = cast = (%<a href="#format_anchors">llvm.dbg.anchor.type</a>* %<a href="#format_compile_units">llvm.dbg.compile_units</a> to { }*)
i32, ;; DWARF language identifier (ex. DW_LANG_C89)
i8*, ;; Source file name
i8*, ;; Source file directory (includes trailing slash)
i8* ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
i1, ;; True if this is a main compile unit.
i1, ;; True if this is optimized.
i8*, ;; Flags
i32 ;; Runtime version
}
</pre>
</div>
<p>These descriptors contain 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.</p>
<p>Compile unit descriptors provide the root context for objects declared in a
specific source file. Global variables and top level functions would be
defined using this context. Compile unit descriptors also provide context
for source line correspondence.</p>
<p>Each input file is encoded as a separate compile unit in LLVM debugging
information output. However, many target specific tool chains prefer to
encode only one compile unit in an object file. In this situation, the LLVM
code generator will include debugging information entities in the compile
unit that is marked as main compile unit. The code generator accepts maximum
one main compile unit per module. If a module does not contain any main
compile unit then the code generator will emit multiple compile units in the
output object file.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_global_variables">Global variable descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_global_variables">llvm.dbg.global_variable.type</a> = type {
i32, ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_variable)
{ }*, ;; Global variable anchor = cast (%<a href="#format_anchors">llvm.dbg.anchor.type</a>* %<a href="#format_global_variables">llvm.dbg.global_variables</a> to { }*),
{ }*, ;; Reference to context descriptor
i8*, ;; Name
i8*, ;; Display name (fully qualified C++ name)
i8*, ;; MIPS linkage name (for C++)
{ }*, ;; Reference to compile unit where defined
i32, ;; Line number where defined
{ }*, ;; Reference to type descriptor
i1, ;; True if the global is local to compile unit (static)
i1, ;; True if the global is defined in the compile unit (not extern)
{ }* ;; Reference to the global variable
}
</pre>
</div>
<p>These descriptors provide debug information about globals variables. The
provide details such as name, type and where the variable is defined.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_subprograms">Subprogram descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_subprograms">llvm.dbg.subprogram.type</a> = type {
i32, ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subprogram)
{ }*, ;; Subprogram anchor = cast (%<a href="#format_anchors">llvm.dbg.anchor.type</a>* %<a href="#format_subprograms">llvm.dbg.subprograms</a> to { }*),
{ }*, ;; Reference to context descriptor
i8*, ;; Name
i8*, ;; Display name (fully qualified C++ name)
i8*, ;; MIPS linkage name (for C++)
{ }*, ;; Reference to compile unit where defined
i32, ;; Line number where defined
{ }*, ;; Reference to type descriptor
i1, ;; True if the global is local to compile unit (static)
i1 ;; True if the global is defined in the compile unit (not extern)
}
</pre>
</div>
<p>These descriptors provide debug information about functions, methods and
subprograms. They provide details such as name, return types and the source
location where the subprogram is defined.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_blocks">Block descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_blocks">llvm.dbg.block</a> = type {
i32, ;; Tag = 13 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
{ }* ;; Reference to context descriptor
}
</pre>
</div>
<p>These descriptors provide debug information about nested blocks within a
subprogram. The array of member descriptors is used to define local
variables and deeper nested blocks.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_basic_type">Basic type descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_basic_type">llvm.dbg.basictype.type</a> = type {
i32, ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_base_type)
{ }*, ;; Reference to context (typically a compile unit)
i8*, ;; Name (may be "" for anonymous types)
{ }*, ;; Reference to compile unit where defined (may be NULL)
i32, ;; Line number where defined (may be 0)
i64, ;; Size in bits
i64, ;; Alignment in bits
i64, ;; Offset in bits
i32, ;; Flags
i32 ;; DWARF type encoding
}
</pre>
</div>
<p>These descriptors define primitive types used in the code. Example int, bool
and float. The context provides the scope of the type, which is usually the
top level. Since basic types are not usually user defined the compile unit
and line number can be left as NULL and 0. The size, alignment and offset
are expressed in bits and can be 64 bit values. The alignment is used to
round the offset when embedded in a
<a href="#format_composite_type">composite type</a> (example to keep float
doubles on 64 bit boundaries.) The offset is the bit offset if embedded in
a <a href="#format_composite_type">composite type</a>.</p>
<p>The type encoding provides the details of the type. The values are typically
one of the following:</p>
<div class="doc_code">
<pre>
DW_ATE_address = 1
DW_ATE_boolean = 2
DW_ATE_float = 4
DW_ATE_signed = 5
DW_ATE_signed_char = 6
DW_ATE_unsigned = 7
DW_ATE_unsigned_char = 8
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_derived_type">Derived type descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_derived_type">llvm.dbg.derivedtype.type</a> = type {
i32, ;; Tag (see below)
{ }*, ;; Reference to context
i8*, ;; Name (may be "" for anonymous types)
{ }*, ;; Reference to compile unit where defined (may be NULL)
i32, ;; Line number where defined (may be 0)
i32, ;; Size in bits
i32, ;; Alignment in bits
i32, ;; Offset in bits
{ }* ;; Reference to type derived from
}
</pre>
</div>
<p>These descriptors are used to define types derived from other types. The
value of the tag varies depending on the meaning. The following are possible
tag values:</p>
<div class="doc_code">
<pre>
DW_TAG_formal_parameter = 5
DW_TAG_member = 13
DW_TAG_pointer_type = 15
DW_TAG_reference_type = 16
DW_TAG_typedef = 22
DW_TAG_const_type = 38
DW_TAG_volatile_type = 53
DW_TAG_restrict_type = 55
</pre>
</div>
<p><tt>DW_TAG_member</tt> is used to define a member of
a <a href="#format_composite_type">composite type</a>
or <a href="#format_subprograms">subprogram</a>. The type of the member is
the <a href="#format_derived_type">derived
type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which
is a formal argument of a subprogram.</p>
<p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>
<p><tt>DW_TAG_pointer_type</tt>,<tt>DW_TAG_reference_type</tt>,
<tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt>
and <tt>DW_TAG_restrict_type</tt> are used to qualify
the <a href="#format_derived_type">derived type</a>. </p>
<p><a href="#format_derived_type">Derived type</a> location can be determined
from the compile unit and line number. The size, alignment and offset are
expressed in bits and can be 64 bit values. The alignment is used to round
the offset when embedded in a <a href="#format_composite_type">composite
type</a> (example to keep float doubles on 64 bit boundaries.) The offset is
the bit offset if embedded in a <a href="#format_composite_type">composite
type</a>.</p>
<p>Note that the <tt>void *</tt> type is expressed as a
<tt>llvm.dbg.derivedtype.type</tt> with tag of <tt>DW_TAG_pointer_type</tt>
and <tt>NULL</tt> derived type.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_composite_type">Composite type descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_composite_type">llvm.dbg.compositetype.type</a> = type {
i32, ;; Tag (see below)
{ }*, ;; Reference to context
i8*, ;; Name (may be "" for anonymous types)
{ }*, ;; Reference to compile unit where defined (may be NULL)
i32, ;; Line number where defined (may be 0)
i64, ;; Size in bits
i64, ;; Alignment in bits
i64, ;; Offset in bits
i32, ;; Flags
{ }*, ;; Reference to type derived from
{ }*, ;; Reference to array of member descriptors
i32 ;; Runtime languages
}
</pre>
</div>
<p>These descriptors are used to define types that are composed of 0 or more
elements. The value of the tag varies depending on the meaning. The following
are possible tag values:</p>
<div class="doc_code">
<pre>
DW_TAG_array_type = 1
DW_TAG_enumeration_type = 4
DW_TAG_structure_type = 19
DW_TAG_union_type = 23
DW_TAG_vector_type = 259
DW_TAG_subroutine_type = 21
DW_TAG_inheritance = 28
</pre>
</div>
<p>The vector flag indicates that an array type is a native packed vector.</p>
<p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types
(tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange
descriptors</a>, each representing the range of subscripts at that level of
indexing.</p>
<p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are
<a href="#format_enumeration">enumerator descriptors</a>, each representing
the definition of enumeration value for the set.</p>
<p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag
= <tt>DW_TAG_union_type</tt>) types are any one of
the <a href="#format_basic_type">basic</a>,
<a href="#format_derived_type">derived</a>
or <a href="#format_composite_type">composite</a> type descriptors, each
representing a field member of the structure or union.</p>
<p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors
provide information about base classes, static members and member
functions. If a member is a <a href="#format_derived_type">derived type
descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type
represents a base class. If the member of is
a <a href="#format_global_variables">global variable descriptor</a> then it
represents a static member. And, if the member is
a <a href="#format_subprograms">subprogram descriptor</a> then it represents
a member function. For static members and member
functions, <tt>getName()</tt> returns the members link or the C++ mangled
name. <tt>getDisplayName()</tt> the simplied version of the name.</p>
<p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type
elements is the return type for the subroutine. The remaining elements are
the formal arguments to the subroutine.</p>
<p><a href="#format_composite_type">Composite type</a> location can be
determined from the compile unit and line number. The size, alignment and
offset are expressed in bits and can be 64 bit values. The alignment is used
to round the offset when embedded in
a <a href="#format_composite_type">composite type</a> (as an example, to keep
float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
in a <a href="#format_composite_type">composite type</a>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_subrange">Subrange descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_subrange">llvm.dbg.subrange.type</a> = type {
i32, ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)
i64, ;; Low value
i64 ;; High value
}
</pre>
</div>
<p>These descriptors are used to define ranges of array subscripts for an array
<a href="#format_composite_type">composite type</a>. The low value defines
the lower bounds typically zero for C/C++. The high value is the upper
bounds. Values are 64 bit. High - low + 1 is the size of the array. If low
== high the array will be unbounded.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_enumeration">Enumerator descriptors</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_enumeration">llvm.dbg.enumerator.type</a> = type {
i32, ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_enumerator)
i8*, ;; Name
i64 ;; Value
}
</pre>
</div>
<p>These descriptors are used to define members of an
enumeration <a href="#format_composite_type">composite type</a>, it
associates the name to the value.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_variables">Local variables</a>
</div>
<div class="doc_text">
<div class="doc_code">
<pre>
%<a href="#format_variables">llvm.dbg.variable.type</a> = type {
i32, ;; Tag (see below)
{ }*, ;; Context
i8*, ;; Name
{ }*, ;; Reference to compile unit where defined
i32, ;; Line number where defined
{ }* ;; Type descriptor
}
</pre>
</div>
<p>These descriptors are used to define variables local to a sub program. The
value of the tag depends on the usage of the variable:</p>
<div class="doc_code">
<pre>
DW_TAG_auto_variable = 256
DW_TAG_arg_variable = 257
DW_TAG_return_variable = 258
</pre>
</div>
<p>An auto variable is any variable declared in the body of the function. An
argument variable is any variable that appears as a formal argument to the
function. A return variable is used to track the result of a function and
has no source correspondent.</p>
<p>The context is either the subprogram or block where the variable is defined.
Name the source variable name. Compile unit and line indicate where the
variable was defined. Type descriptor defines the declared type of the
variable.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="format_common_intrinsics">Debugger intrinsic functions</a>
</div>
<div class="doc_text">
<p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to
provide debug information at various points in generated code.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_common_stoppoint">llvm.dbg.stoppoint</a>
</div>
<div class="doc_text">
<pre>
void %<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint, uint, { }* )
</pre>
<p>This intrinsic is used to provide correspondence between the source file and
the generated code. The first argument is the line number (base 1), second
argument is the column number (0 if unknown) and the third argument the
source <tt>%<a href="#format_compile_units">llvm.dbg.compile_unit</a>*</tt>
cast to a <tt>{ }*</tt>. Code following a call to this intrinsic will
have been defined in close proximity of the line, column and file. This
information holds until the next call
to <tt>%<a href="#format_common_stoppoint">lvm.dbg.stoppoint</a></tt>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_common_func_start">llvm.dbg.func.start</a>
</div>
<div class="doc_text">
<pre>
void %<a href="#format_common_func_start">llvm.dbg.func.start</a>( { }* )
</pre>
<p>This intrinsic is used to link the debug information
in <tt>%<a href="#format_subprograms">llvm.dbg.subprogram</a></tt> to the
function. It defines the beginning of the function's declarative region
(scope). It also implies a call to
%<tt><a href="#format_common_stoppoint">llvm.dbg.stoppoint</a></tt> which
defines a source line "stop point". The intrinsic should be called early in
the function after the all the alloca instructions. It should be paired off
with a closing
<tt>%<a href="#format_common_region_end">llvm.dbg.region.end</a></tt>.
The function's single argument is
the <tt>%<a href="#format_subprograms">llvm.dbg.subprogram.type</a></tt>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_common_region_start">llvm.dbg.region.start</a>
</div>
<div class="doc_text">
<pre>
void %<a href="#format_common_region_start">llvm.dbg.region.start</a>( { }* )
</pre>
<p>This intrinsic is used to define the beginning of a declarative scope (ex.
block) for local language elements. It should be paired off with a closing
<tt>%<a href="#format_common_region_end">llvm.dbg.region.end</a></tt>. The
function's single argument is
the <tt>%<a href="#format_blocks">llvm.dbg.block</a></tt> which is
starting.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_common_region_end">llvm.dbg.region.end</a>
</div>
<div class="doc_text">
<pre>
void %<a href="#format_common_region_end">llvm.dbg.region.end</a>( { }* )
</pre>
<p>This intrinsic is used to define the end of a declarative scope (ex. block)
for local language elements. It should be paired off with an
opening <tt>%<a href="#format_common_region_start">llvm.dbg.region.start</a></tt>
or <tt>%<a href="#format_common_func_start">llvm.dbg.func.start</a></tt>.
The function's single argument is either
the <tt>%<a href="#format_blocks">llvm.dbg.block</a></tt> or
the <tt>%<a href="#format_subprograms">llvm.dbg.subprogram.type</a></tt>
which is ending.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="format_common_declare">llvm.dbg.declare</a>
</div>
<div class="doc_text">
<pre>
void %<a href="#format_common_declare">llvm.dbg.declare</a>( { } *, { }* )
</pre>
<p>This intrinsic provides information about a local element (ex. variable.) The
first argument is the alloca for the variable, cast to a <tt>{ }*</tt>. The
second argument is
the <tt>%<a href="#format_variables">llvm.dbg.variable</a></tt> containing
the description of the variable, also cast to a <tt>{ }*</tt>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="format_common_stoppoints">
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 <a href="#format_common_stoppoint">%<tt>llvm.dbg.stoppoint</tt></a>
intrinsic function at every point in the program where a debugger should be
able to inspect the program (these correspond to places a 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
evaluated), but it is recommended to only put them after every source
statement that includes executable code.</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,
code duplication transformations, or basic-block reordering
transformations.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="format_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>
<div class="doc_code">
<pre>
1. void foo() {
2. int X = ...;
3. int Y = ...;
4. {
5. int Z = ...;
6. ...
7. }
8. ...
9. }
</pre>
</div>
<p>Compiled to LLVM, this function would be represented like this:</p>
<div class="doc_code">
<pre>
void %foo() {
entry:
%X = alloca int
%Y = alloca int
%Z = alloca int
...
call void @<a href="#format_common_func_start">llvm.dbg.func.start</a>( %<a href="#format_subprograms">llvm.dbg.subprogram.type</a>* @llvm.dbg.subprogram )
call void @<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint 2, uint 2, %<a href="#format_compile_units">llvm.dbg.compile_unit</a>* @llvm.dbg.compile_unit )
call void @<a href="#format_common_declare">llvm.dbg.declare</a>({}* %X, ...)
call void @<a href="#format_common_declare">llvm.dbg.declare</a>({}* %Y, ...)
<i>;; Evaluate expression on line 2, assigning to X.</i>
call void @<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint 3, uint 2, %<a href="#format_compile_units">llvm.dbg.compile_unit</a>* @llvm.dbg.compile_unit )
<i>;; Evaluate expression on line 3, assigning to Y.</i>
call void @<a href="#format_common_stoppoint">llvm.region.start</a>()
call void @<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint 5, uint 4, %<a href="#format_compile_units">llvm.dbg.compile_unit</a>* @llvm.dbg.compile_unit )
call void @<a href="#format_common_declare">llvm.dbg.declare</a>({}* %X, ...)
<i>;; Evaluate expression on line 5, assigning to Z.</i>
call void @<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint 7, uint 2, %<a href="#format_compile_units">llvm.dbg.compile_unit</a>* @llvm.dbg.compile_unit )
call void @<a href="#format_common_region_end">llvm.region.end</a>()
call void @<a href="#format_common_stoppoint">llvm.dbg.stoppoint</a>( uint 9, uint 2, %<a href="#format_compile_units">llvm.dbg.compile_unit</a>* @llvm.dbg.compile_unit )
call void @<a href="#format_common_region_end">llvm.region.end</a>()
ret void
}
</pre>
</div>
<p>This example illustrates a few important details about the LLVM debugging
information. In particular, it shows how the various intrinsics are applied
together to allow a debugger to analyze the relationship between statements,
variable definitions, and the code used to implement the function.</p>
<p>The first
intrinsic <tt>%<a href="#format_common_func_start">llvm.dbg.func.start</a></tt>
provides a link with the <a href="#format_subprograms">subprogram
descriptor</a> containing the details of this function. This call also
defines the beginning of the function region, bounded by
the <tt>%<a href="#format_common_region_end">llvm.region.end</a></tt> at the
end of the function. This region is used to bracket the lifetime of