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11 <div class="doc_title">Source Level Debugging with LLVM</div>
13 <table class="layout" style="width:100%">
17 <li><a href="#introduction">Introduction</a>
19 <li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
20 <li><a href="#consumers">Debug information consumers</a></li>
21 <li><a href="#debugopt">Debugging optimized code</a></li>
23 <li><a href="#format">Debugging information format</a>
25 <li><a href="#debug_info_descriptors">Debug information descriptors</a>
27 <li><a href="#format_compile_units">Compile unit descriptors</a></li>
28 <li><a href="#format_global_variables">Global variable descriptors</a></li>
29 <li><a href="#format_subprograms">Subprogram descriptors</a></li>
30 <li><a href="#format_blocks">Block descriptors</a></li>
31 <li><a href="#format_basic_type">Basic type descriptors</a></li>
32 <li><a href="#format_derived_type">Derived type descriptors</a></li>
33 <li><a href="#format_composite_type">Composite type descriptors</a></li>
34 <li><a href="#format_subrange">Subrange descriptors</a></li>
35 <li><a href="#format_enumeration">Enumerator descriptors</a></li>
36 <li><a href="#format_variables">Local variables</a></li>
38 <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>
40 <li><a href="#format_common_declare">llvm.dbg.declare</a></li>
43 <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>
44 <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
46 <li><a href="#ccxx_compile_units">C/C++ source file information</a></li>
47 <li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>
48 <li><a href="#ccxx_subprogram">C/C++ function information</a></li>
49 <li><a href="#ccxx_basic_types">C/C++ basic types</a></li>
50 <li><a href="#ccxx_derived_types">C/C++ derived types</a></li>
51 <li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>
52 <li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>
57 <img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
62 <div class="doc_author">
63 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
64 and <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
68 <!-- *********************************************************************** -->
69 <div class="doc_section"><a name="introduction">Introduction</a></div>
70 <!-- *********************************************************************** -->
72 <div class="doc_text">
74 <p>This document is the central repository for all information pertaining to
75 debug information in LLVM. It describes the <a href="#format">actual format
76 that the LLVM debug information</a> takes, which is useful for those
77 interested in creating front-ends or dealing directly with the information.
78 Further, this document provides specific examples of what debug information
83 <!-- ======================================================================= -->
84 <div class="doc_subsection">
85 <a name="phil">Philosophy behind LLVM debugging information</a>
88 <div class="doc_text">
90 <p>The idea of the LLVM debugging information is to capture how the important
91 pieces of the source-language's Abstract Syntax Tree map onto LLVM code.
92 Several design aspects have shaped the solution that appears here. The
93 important ones are:</p>
96 <li>Debugging information should have very little impact on the rest of the
97 compiler. No transformations, analyses, or code generators should need to
98 be modified because of debugging information.</li>
100 <li>LLVM optimizations should interact in <a href="#debugopt">well-defined and
101 easily described ways</a> with the debugging information.</li>
103 <li>Because LLVM is designed to support arbitrary programming languages,
104 LLVM-to-LLVM tools should not need to know anything about the semantics of
105 the source-level-language.</li>
107 <li>Source-level languages are often <b>widely</b> different from one another.
108 LLVM should not put any restrictions of the flavor of the source-language,
109 and the debugging information should work with any language.</li>
111 <li>With code generator support, it should be possible to use an LLVM compiler
112 to compile a program to native machine code and standard debugging
113 formats. This allows compatibility with traditional machine-code level
114 debuggers, like GDB or DBX.</li>
117 <p>The approach used by the LLVM implementation is to use a small set
118 of <a href="#format_common_intrinsics">intrinsic functions</a> to define a
119 mapping between LLVM program objects and the source-level objects. The
120 description of the source-level program is maintained in LLVM metadata
121 in an <a href="#ccxx_frontend">implementation-defined format</a>
122 (the C/C++ front-end currently uses working draft 7 of
123 the <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3
126 <p>When a program is being debugged, a debugger interacts with the user and
127 turns the stored debug information into source-language specific information.
128 As such, a debugger must be aware of the source-language, and is thus tied to
129 a specific language or family of languages.</p>
133 <!-- ======================================================================= -->
134 <div class="doc_subsection">
135 <a name="consumers">Debug information consumers</a>
138 <div class="doc_text">
140 <p>The role of debug information is to provide meta information normally
141 stripped away during the compilation process. This meta information provides
142 an LLVM user a relationship between generated code and the original program
145 <p>Currently, debug information is consumed by the DwarfWriter to produce dwarf
146 information used by the gdb debugger. Other targets could use the same
147 information to produce stabs or other debug forms.</p>
149 <p>It would also be reasonable to use debug information to feed profiling tools
150 for analysis of generated code, or, tools for reconstructing the original
151 source from generated code.</p>
153 <p>TODO - expound a bit more.</p>
157 <!-- ======================================================================= -->
158 <div class="doc_subsection">
159 <a name="debugopt">Debugging optimized code</a>
162 <div class="doc_text">
164 <p>An extremely high priority of LLVM debugging information is to make it
165 interact well with optimizations and analysis. In particular, the LLVM debug
166 information provides the following guarantees:</p>
169 <li>LLVM debug information <b>always provides information to accurately read
170 the source-level state of the program</b>, regardless of which LLVM
171 optimizations have been run, and without any modification to the
172 optimizations themselves. However, some optimizations may impact the
173 ability to modify the current state of the program with a debugger, such
174 as setting program variables, or calling functions that have been
177 <li>LLVM optimizations gracefully interact with debugging information. If
178 they are not aware of debug information, they are automatically disabled
179 as necessary in the cases that would invalidate the debug info. This
180 retains the LLVM features, making it easy to write new
181 transformations.</li>
183 <li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
184 debugging information, allowing them to update the debugging information
185 as they perform aggressive optimizations. This means that, with effort,
186 the LLVM optimizers could optimize debug code just as well as non-debug
189 <li>LLVM debug information does not prevent many important optimizations from
190 happening (for example inlining, basic block reordering/merging/cleanup,
191 tail duplication, etc), further reducing the amount of the compiler that
192 eventually is "aware" of debugging information.</li>
194 <li>LLVM debug information is automatically optimized along with the rest of
195 the program, using existing facilities. For example, duplicate
196 information is automatically merged by the linker, and unused information
197 is automatically removed.</li>
200 <p>Basically, the debug information allows you to compile a program with
201 "<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily
202 modify the program as it executes from a debugger. Compiling a program with
203 "<tt>-O3 -g</tt>" gives you full debug information that is always available
204 and accurate for reading (e.g., you get accurate stack traces despite tail
205 call elimination and inlining), but you might lose the ability to modify the
206 program and call functions where were optimized out of the program, or
207 inlined away completely.</p>
209 <p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a
210 framework to test optimizer's handling of debugging information. It can be
213 <div class="doc_code">
215 % cd llvm/projects/test-suite/MultiSource/Benchmarks # or some other level
220 <p>This will test impact of debugging information on optimization passes. If
221 debugging information influences optimization passes then it will be reported
222 as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more
223 information on LLVM test infrastructure and how to run various tests.</p>
227 <!-- *********************************************************************** -->
228 <div class="doc_section">
229 <a name="format">Debugging information format</a>
231 <!-- *********************************************************************** -->
233 <div class="doc_text">
235 <p>LLVM debugging information has been carefully designed to make it possible
236 for the optimizer to optimize the program and debugging information without
237 necessarily having to know anything about debugging information. In
238 particular, te use of metadadta avoids duplicated dubgging information from
239 the beginning, and the global dead code elimination pass automatically
240 deletes debugging information for a function if it decides to delete the
243 <p>To do this, most of the debugging information (descriptors for types,
244 variables, functions, source files, etc) is inserted by the language
245 front-end in the form of LLVM metadata. </p>
247 <p>Debug information is designed to be agnostic about the target debugger and
248 debugging information representation (e.g. DWARF/Stabs/etc). It uses a
249 generic pass to decode the information that represents variables, types,
250 functions, namespaces, etc: this allows for arbitrary source-language
251 semantics and type-systems to be used, as long as there is a module
252 written for the target debugger to interpret the information. </p>
254 <p>To provide basic functionality, the LLVM debugger does have to make some
255 assumptions about the source-level language being debugged, though it keeps
256 these to a minimum. The only common features that the LLVM debugger assumes
257 exist are <a href="#format_compile_units">source files</a>,
258 and <a href="#format_global_variables">program objects</a>. These abstract
259 objects are used by a debugger to form stack traces, show information about
260 local variables, etc.</p>
262 <p>This section of the documentation first describes the representation aspects
263 common to any source-language. The <a href="#ccxx_frontend">next section</a>
264 describes the data layout conventions used by the C and C++ front-ends.</p>
268 <!-- ======================================================================= -->
269 <div class="doc_subsection">
270 <a name="debug_info_descriptors">Debug information descriptors</a>
273 <div class="doc_text">
275 <p>In consideration of the complexity and volume of debug information, LLVM
276 provides a specification for well formed debug descriptors. </p>
278 <p>Consumers of LLVM debug information expect the descriptors for program
279 objects to start in a canonical format, but the descriptors can include
280 additional information appended at the end that is source-language
281 specific. All LLVM debugging information is versioned, allowing backwards
282 compatibility in the case that the core structures need to change in some
283 way. Also, all debugging information objects start with a tag to indicate
284 what type of object it is. The source-language is allowed to define its own
285 objects, by using unreserved tag numbers. We recommend using with tags in
286 the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base =
289 <p>The fields of debug descriptors used internally by LLVM
290 are restricted to only the simple data types <tt>int</tt>, <tt>uint</tt>,
291 <tt>bool</tt>, <tt>float</tt>, <tt>double</tt>, <tt>mdstring</tt> and
292 <tt>mdnode</tt>. </p>
294 <div class="doc_code">
303 <p><a name="LLVMDebugVersion">The first field of a descriptor is always an
304 <tt>uint</tt> containing a tag value identifying the content of the
305 descriptor. The remaining fields are specific to the descriptor. The values
306 of tags are loosely bound to the tag values of DWARF information entries.
307 However, that does not restrict the use of the information supplied to DWARF
308 targets. To facilitate versioning of debug information, the tag is augmented
309 with the current debug version (LLVMDebugVersion = 7 << 16 or 0x70000 or
312 <p>The details of the various descriptors follow.</p>
316 <!-- ======================================================================= -->
317 <div class="doc_subsubsection">
318 <a name="format_compile_units">Compile unit descriptors</a>
321 <div class="doc_text">
323 <div class="doc_code">
326 i32, ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
327 ;; (DW_TAG_compile_unit)
328 i32, ;; Unused field.
329 i32, ;; DWARF language identifier (ex. DW_LANG_C89)
330 metadata, ;; Source file name
331 metadata, ;; Source file directory (includes trailing slash)
332 metadata ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
333 i1, ;; True if this is a main compile unit.
334 i1, ;; True if this is optimized.
336 i32 ;; Runtime version
341 <p>These descriptors contain a source language ID for the file (we use the DWARF
342 3.0 ID numbers, such as <tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>,
343 <tt>DW_LANG_Cobol74</tt>, etc), three strings describing the filename,
344 working directory of the compiler, and an identifier string for the compiler
345 that produced it.</p>
347 <p>Compile unit descriptors provide the root context for objects declared in a
348 specific source file. Global variables and top level functions would be
349 defined using this context. Compile unit descriptors also provide context
350 for source line correspondence.</p>
352 <p>Each input file is encoded as a separate compile unit in LLVM debugging
353 information output. However, many target specific tool chains prefer to
354 encode only one compile unit in an object file. In this situation, the LLVM
355 code generator will include debugging information entities in the compile
356 unit that is marked as main compile unit. The code generator accepts maximum
357 one main compile unit per module. If a module does not contain any main
358 compile unit then the code generator will emit multiple compile units in the
359 output object file.</p>
363 <!-- ======================================================================= -->
364 <div class="doc_subsubsection">
365 <a name="format_global_variables">Global variable descriptors</a>
368 <div class="doc_text">
370 <div class="doc_code">
373 i32, ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
375 i32, ;; Unused field.
376 metadata, ;; Reference to context descriptor
378 metadata, ;; Display name (fully qualified C++ name)
379 metadata, ;; MIPS linkage name (for C++)
380 metadata, ;; Reference to compile unit where defined
381 i32, ;; Line number where defined
382 metadata, ;; Reference to type descriptor
383 i1, ;; True if the global is local to compile unit (static)
384 i1, ;; True if the global is defined in the compile unit (not extern)
385 { }* ;; Reference to the global variable
390 <p>These descriptors provide debug information about globals variables. The
391 provide details such as name, type and where the variable is defined.</p>
395 <!-- ======================================================================= -->
396 <div class="doc_subsubsection">
397 <a name="format_subprograms">Subprogram descriptors</a>
400 <div class="doc_text">
402 <div class="doc_code">
405 i32, ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
406 ;; (DW_TAG_subprogram)
407 i32, ;; Unused field.
408 metadata, ;; Reference to context descriptor
410 metadata, ;; Display name (fully qualified C++ name)
411 metadata, ;; MIPS linkage name (for C++)
412 metadata, ;; Reference to compile unit where defined
413 i32, ;; Line number where defined
414 metadata, ;; Reference to type descriptor
415 i1, ;; True if the global is local to compile unit (static)
416 i1 ;; True if the global is defined in the compile unit (not extern)
421 <p>These descriptors provide debug information about functions, methods and
422 subprograms. They provide details such as name, return types and the source
423 location where the subprogram is defined.</p>
427 <!-- ======================================================================= -->
428 <div class="doc_subsubsection">
429 <a name="format_blocks">Block descriptors</a>
432 <div class="doc_text">
434 <div class="doc_code">
437 i32, ;; Tag = 13 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
438 metadata ;; Reference to context descriptor
443 <p>These descriptors provide debug information about nested blocks within a
444 subprogram. The array of member descriptors is used to define local
445 variables and deeper nested blocks.</p>
449 <!-- ======================================================================= -->
450 <div class="doc_subsubsection">
451 <a name="format_basic_type">Basic type descriptors</a>
454 <div class="doc_text">
456 <div class="doc_code">
459 i32, ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
460 ;; (DW_TAG_base_type)
461 metadata, ;; Reference to context (typically a compile unit)
462 metadata, ;; Name (may be "" for anonymous types)
463 metadata, ;; Reference to compile unit where defined (may be NULL)
464 i32, ;; Line number where defined (may be 0)
466 i64, ;; Alignment in bits
467 i64, ;; Offset in bits
469 i32 ;; DWARF type encoding
474 <p>These descriptors define primitive types used in the code. Example int, bool
475 and float. The context provides the scope of the type, which is usually the
476 top level. Since basic types are not usually user defined the compile unit
477 and line number can be left as NULL and 0. The size, alignment and offset
478 are expressed in bits and can be 64 bit values. The alignment is used to
479 round the offset when embedded in a
480 <a href="#format_composite_type">composite type</a> (example to keep float
481 doubles on 64 bit boundaries.) The offset is the bit offset if embedded in
482 a <a href="#format_composite_type">composite type</a>.</p>
484 <p>The type encoding provides the details of the type. The values are typically
485 one of the following:</p>
487 <div class="doc_code">
493 DW_ATE_signed_char = 6
495 DW_ATE_unsigned_char = 8
501 <!-- ======================================================================= -->
502 <div class="doc_subsubsection">
503 <a name="format_derived_type">Derived type descriptors</a>
506 <div class="doc_text">
508 <div class="doc_code">
511 i32, ;; Tag (see below)
512 metadata, ;; Reference to context
513 metadata, ;; Name (may be "" for anonymous types)
514 metadata, ;; Reference to compile unit where defined (may be NULL)
515 i32, ;; Line number where defined (may be 0)
517 i32, ;; Alignment in bits
518 i32, ;; Offset in bits
519 metadata ;; Reference to type derived from
524 <p>These descriptors are used to define types derived from other types. The
525 value of the tag varies depending on the meaning. The following are possible
528 <div class="doc_code">
530 DW_TAG_formal_parameter = 5
532 DW_TAG_pointer_type = 15
533 DW_TAG_reference_type = 16
535 DW_TAG_const_type = 38
536 DW_TAG_volatile_type = 53
537 DW_TAG_restrict_type = 55
541 <p><tt>DW_TAG_member</tt> is used to define a member of
542 a <a href="#format_composite_type">composite type</a>
543 or <a href="#format_subprograms">subprogram</a>. The type of the member is
544 the <a href="#format_derived_type">derived
545 type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which
546 is a formal argument of a subprogram.</p>
548 <p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>
550 <p><tt>DW_TAG_pointer_type</tt>,<tt>DW_TAG_reference_type</tt>,
551 <tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt>
552 and <tt>DW_TAG_restrict_type</tt> are used to qualify
553 the <a href="#format_derived_type">derived type</a>. </p>
555 <p><a href="#format_derived_type">Derived type</a> location can be determined
556 from the compile unit and line number. The size, alignment and offset are
557 expressed in bits and can be 64 bit values. The alignment is used to round
558 the offset when embedded in a <a href="#format_composite_type">composite
559 type</a> (example to keep float doubles on 64 bit boundaries.) The offset is
560 the bit offset if embedded in a <a href="#format_composite_type">composite
563 <p>Note that the <tt>void *</tt> type is expressed as a
564 <tt>llvm.dbg.derivedtype.type</tt> with tag of <tt>DW_TAG_pointer_type</tt>
565 and <tt>NULL</tt> derived type.</p>
569 <!-- ======================================================================= -->
570 <div class="doc_subsubsection">
571 <a name="format_composite_type">Composite type descriptors</a>
574 <div class="doc_text">
576 <div class="doc_code">
579 i32, ;; Tag (see below)
580 metadata, ;; Reference to context
581 metadata, ;; Name (may be "" for anonymous types)
582 metadata, ;; Reference to compile unit where defined (may be NULL)
583 i32, ;; Line number where defined (may be 0)
585 i64, ;; Alignment in bits
586 i64, ;; Offset in bits
588 metadata, ;; Reference to type derived from
589 metadata, ;; Reference to array of member descriptors
590 i32 ;; Runtime languages
595 <p>These descriptors are used to define types that are composed of 0 or more
596 elements. The value of the tag varies depending on the meaning. The following
597 are possible tag values:</p>
599 <div class="doc_code">
601 DW_TAG_array_type = 1
602 DW_TAG_enumeration_type = 4
603 DW_TAG_structure_type = 19
604 DW_TAG_union_type = 23
605 DW_TAG_vector_type = 259
606 DW_TAG_subroutine_type = 21
607 DW_TAG_inheritance = 28
611 <p>The vector flag indicates that an array type is a native packed vector.</p>
613 <p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types
614 (tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange
615 descriptors</a>, each representing the range of subscripts at that level of
618 <p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are
619 <a href="#format_enumeration">enumerator descriptors</a>, each representing
620 the definition of enumeration value for the set.</p>
622 <p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag
623 = <tt>DW_TAG_union_type</tt>) types are any one of
624 the <a href="#format_basic_type">basic</a>,
625 <a href="#format_derived_type">derived</a>
626 or <a href="#format_composite_type">composite</a> type descriptors, each
627 representing a field member of the structure or union.</p>
629 <p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors
630 provide information about base classes, static members and member
631 functions. If a member is a <a href="#format_derived_type">derived type
632 descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type
633 represents a base class. If the member of is
634 a <a href="#format_global_variables">global variable descriptor</a> then it
635 represents a static member. And, if the member is
636 a <a href="#format_subprograms">subprogram descriptor</a> then it represents
637 a member function. For static members and member
638 functions, <tt>getName()</tt> returns the members link or the C++ mangled
639 name. <tt>getDisplayName()</tt> the simplied version of the name.</p>
641 <p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type
642 elements is the return type for the subroutine. The remaining elements are
643 the formal arguments to the subroutine.</p>
645 <p><a href="#format_composite_type">Composite type</a> location can be
646 determined from the compile unit and line number. The size, alignment and
647 offset are expressed in bits and can be 64 bit values. The alignment is used
648 to round the offset when embedded in
649 a <a href="#format_composite_type">composite type</a> (as an example, to keep
650 float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
651 in a <a href="#format_composite_type">composite type</a>.</p>
655 <!-- ======================================================================= -->
656 <div class="doc_subsubsection">
657 <a name="format_subrange">Subrange descriptors</a>
660 <div class="doc_text">
662 <div class="doc_code">
664 %<a href="#format_subrange">llvm.dbg.subrange.type</a> = type {
665 i32, ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)
672 <p>These descriptors are used to define ranges of array subscripts for an array
673 <a href="#format_composite_type">composite type</a>. The low value defines
674 the lower bounds typically zero for C/C++. The high value is the upper
675 bounds. Values are 64 bit. High - low + 1 is the size of the array. If low
676 == high the array will be unbounded.</p>
680 <!-- ======================================================================= -->
681 <div class="doc_subsubsection">
682 <a name="format_enumeration">Enumerator descriptors</a>
685 <div class="doc_text">
687 <div class="doc_code">
690 i32, ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
691 ;; (DW_TAG_enumerator)
698 <p>These descriptors are used to define members of an
699 enumeration <a href="#format_composite_type">composite type</a>, it
700 associates the name to the value.</p>
704 <!-- ======================================================================= -->
705 <div class="doc_subsubsection">
706 <a name="format_variables">Local variables</a>
709 <div class="doc_text">
711 <div class="doc_code">
714 i32, ;; Tag (see below)
717 metadata, ;; Reference to compile unit where defined
718 i32, ;; Line number where defined
719 metadata ;; Type descriptor
724 <p>These descriptors are used to define variables local to a sub program. The
725 value of the tag depends on the usage of the variable:</p>
727 <div class="doc_code">
729 DW_TAG_auto_variable = 256
730 DW_TAG_arg_variable = 257
731 DW_TAG_return_variable = 258
735 <p>An auto variable is any variable declared in the body of the function. An
736 argument variable is any variable that appears as a formal argument to the
737 function. A return variable is used to track the result of a function and
738 has no source correspondent.</p>
740 <p>The context is either the subprogram or block where the variable is defined.
741 Name the source variable name. Compile unit and line indicate where the
742 variable was defined. Type descriptor defines the declared type of the
747 <!-- ======================================================================= -->
748 <div class="doc_subsection">
749 <a name="format_common_intrinsics">Debugger intrinsic functions</a>
752 <div class="doc_text">
754 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to
755 provide debug information at various points in generated code.</p>
759 <!-- ======================================================================= -->
760 <div class="doc_subsubsection">
761 <a name="format_common_declare">llvm.dbg.declare</a>
764 <div class="doc_text">
766 void %<a href="#format_common_declare">llvm.dbg.declare</a>( { } *, metadata )
769 <p>This intrinsic provides information about a local element (ex. variable.) The
770 first argument is the alloca for the variable, cast to a <tt>{ }*</tt>. The
772 the <tt>%<a href="#format_variables">llvm.dbg.variable</a></tt> containing
773 the description of the variable. </p>
777 <!-- ======================================================================= -->
778 <div class="doc_subsection">
779 <a name="format_common_lifetime">Object lifetimes and scoping</a>
782 <div class="doc_text">
783 <p>In many languages, the local variables in functions can have their lifetime
784 or scope limited to a subset of a function. In the C family of languages,
785 for example, variables are only live (readable and writable) within the
786 source block that they are defined in. In functional languages, values are
787 only readable after they have been defined. Though this is a very obvious
788 concept, it is also non-trivial to model in LLVM, because it has no notion of
789 scoping in this sense, and does not want to be tied to a language's scoping
792 <p>In order to handle this, the LLVM debug format uses the metadata attached
793 with llvm instructions to encode line nuber and scoping information.
794 Consider the following C fragment, for example:</p>
796 <div class="doc_code">
810 <p>Compiled to LLVM, this function would be represented like this:</p>
812 <div class="doc_code">
816 %X = alloca i32, align 4 ; <i32*> [#uses=4]
817 %Y = alloca i32, align 4 ; <i32*> [#uses=4]
818 %Z = alloca i32, align 4 ; <i32*> [#uses=3]
819 %0 = bitcast i32* %X to { }* ; <{ }*> [#uses=1]
820 call void @llvm.dbg.declare({ }* %0, metadata !0), !dbg !7
821 store i32 21, i32* %X, !dbg !8
822 %1 = bitcast i32* %Y to { }* ; <{ }*> [#uses=1]
823 call void @llvm.dbg.declare({ }* %1, metadata !9), !dbg !10
824 store i32 22, i32* %Y, !dbg !11
825 %2 = bitcast i32* %Z to { }* ; <{ }*> [#uses=1]
826 call void @llvm.dbg.declare({ }* %2, metadata !12), !dbg !14
827 store i32 23, i32* %Z, !dbg !15
828 %tmp = load i32* %X, !dbg !16 ; <i32> [#uses=1]
829 %tmp1 = load i32* %Y, !dbg !16 ; <i32> [#uses=1]
830 %add = add nsw i32 %tmp, %tmp1, !dbg !16 ; <i32> [#uses=1]
831 store i32 %add, i32* %Z, !dbg !16
832 %tmp2 = load i32* %Y, !dbg !17 ; <i32> [#uses=1]
833 store i32 %tmp2, i32* %X, !dbg !17
837 declare void @llvm.dbg.declare({ }*, metadata) nounwind readnone
839 !0 = metadata !{i32 459008, metadata !1, metadata !"X",
840 metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]
841 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
842 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo",
843 metadata !"foo", metadata !3, i32 1, metadata !4,
844 i1 false, i1 true}; [DW_TAG_subprogram ]
845 !3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c",
846 metadata !"/private/tmp", metadata !"clang 1.1", i1 true,
847 i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]
848 !4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0,
849 i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]
850 !5 = metadata !{null}
851 !6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0,
852 i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]
853 !7 = metadata !{i32 2, i32 7, metadata !1, null}
854 !8 = metadata !{i32 2, i32 3, metadata !1, null}
855 !9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3,
856 metadata !6}; [ DW_TAG_auto_variable ]
857 !10 = metadata !{i32 3, i32 7, metadata !1, null}
858 !11 = metadata !{i32 3, i32 3, metadata !1, null}
859 !12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5,
860 metadata !6}; [ DW_TAG_auto_variable ]
861 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
862 !14 = metadata !{i32 5, i32 9, metadata !13, null}
863 !15 = metadata !{i32 5, i32 5, metadata !13, null}
864 !16 = metadata !{i32 6, i32 5, metadata !13, null}
865 !17 = metadata !{i32 8, i32 3, metadata !1, null}
866 !18 = metadata !{i32 9, i32 1, metadata !2, null}
870 <p>This example illustrates a few important details about the LLVM debugging
871 information. In particular, it shows how the llvm.dbg.declare intrinsic
872 and location information, attached with an instruction, are applied
873 together to allow a debugger to analyze the relationship between statements,
874 variable definitions, and the code used to implement the function.</p>
876 <div class="doc_code">
878 call void @llvm.dbg.declare({ }* %0, metadata !0), !dbg !7
881 <p>This first intrinsic
882 <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
883 encodes debugging information for variable <tt>X</tt>. The metadata,
884 <tt>!dbg !7</tt> attached with the intrinsic provides scope information for
885 the variable <tt>X</tt>. </p>
886 <div class="doc_code">
888 !7 = metadata !{i32 2, i32 7, metadata !1, null}
889 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
890 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo",
891 metadata !"foo", metadata !"foo", metadata !3, i32 1,
892 metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]
896 <p> Here <tt>!7</tt> is a metadata providing location information. It has four
897 fields : line number, column number, scope and original scope. The original
898 scope represents inline location if this instruction is inlined inside
899 a caller. It is null otherwise. In this example scope is encoded by
900 <tt>!1</tt>. <tt>!1</tt> represents a lexical block inside the scope
901 <tt>!2</tt>, where <tt>!2</tt> is a
902 <a href="#format_subprograms">subprogram descriptor</a>.
903 This way the location information attched with the intrinsics indicates
904 that the variable <tt>X</tt> is declared at line number 2 at a function level
905 scope in function <tt>foo</tt>.</p>
907 <p>Now lets take another example.</p>
909 <div class="doc_code">
911 call void @llvm.dbg.declare({ }* %2, metadata !12), !dbg !14
915 <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
916 encodes debugging information for variable <tt>Z</tt>. The metadata,
917 <tt>!dbg !14</tt> attached with the intrinsic provides scope information for
918 the variable <tt>Z</tt>. </p>
919 <div class="doc_code">
921 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
922 !14 = metadata !{i32 5, i32 9, metadata !13, null}
926 <p> Here <tt>!14</tt> indicates that <tt>Z</tt> is declaread at line number 5,
927 column number 9 inside a lexical scope <tt>!13</tt>. This lexical scope
928 itself resides inside lexcial scope <tt>!1</tt> described above.</p>
930 <p>The scope information attached with each instruction provides a straight
931 forward way to find instructions covered by a scope. </p>
934 <!-- *********************************************************************** -->
935 <div class="doc_section">
936 <a name="ccxx_frontend">C/C++ front-end specific debug information</a>
938 <!-- *********************************************************************** -->
940 <div class="doc_text">
942 <p>The C and C++ front-ends represent information about the program in a format
943 that is effectively identical
944 to <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in
945 terms of information content. This allows code generators to trivially
946 support native debuggers by generating standard dwarf information, and
947 contains enough information for non-dwarf targets to translate it as
950 <p>This section describes the forms used to represent C and C++ programs. Other
951 languages could pattern themselves after this (which itself is tuned to
952 representing programs in the same way that DWARF 3 does), or they could
953 choose to provide completely different forms if they don't fit into the DWARF
954 model. As support for debugging information gets added to the various LLVM
955 source-language front-ends, the information used should be documented
958 <p>The following sections provide examples of various C/C++ constructs and the
959 debug information that would best describe those constructs.</p>
963 <!-- ======================================================================= -->
964 <div class="doc_subsection">
965 <a name="ccxx_compile_units">C/C++ source file information</a>
968 <div class="doc_text">
970 <p>Given the source files <tt>MySource.cpp</tt> and <tt>MyHeader.h</tt> located
971 in the directory <tt>/Users/mine/sources</tt>, the following code:</p>
973 <div class="doc_code">
975 #include "MyHeader.h"
977 int main(int argc, char *argv[]) {
983 <p>a C/C++ front-end would generate the following descriptors:</p>
985 <div class="doc_code">
989 ;; Define the compile unit for the source file "/Users/mine/sources/MySource.cpp".
994 i32 4, ;; Language Id
995 metadata !"MySource.cpp",
996 metadata !"/Users/mine/sources",
997 metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",
998 i1 true, ;; Main Compile Unit
999 i1 false, ;; Optimized compile unit
1000 metadata !"", ;; Compiler flags
1001 i32 0} ;; Runtime version
1004 ;; Define the compile unit for the header file "/Users/mine/sources/MyHeader.h".
1009 i32 4, ;; Language Id
1010 metadata !"MyHeader.h",
1011 metadata !"/Users/mine/sources",
1012 metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",
1013 i1 false, ;; Main Compile Unit
1014 i1 false, ;; Optimized compile unit
1015 metadata !"", ;; Compiler flags
1016 i32 0} ;; Runtime version
1024 <!-- ======================================================================= -->
1025 <div class="doc_subsection">
1026 <a name="ccxx_global_variable">C/C++ global variable information</a>
1029 <div class="doc_text">
1031 <p>Given an integer global variable declared as follows:</p>
1033 <div class="doc_code">
1039 <p>a C/C++ front-end would generate the following descriptors:</p>
1041 <div class="doc_code">
1044 ;; Define the global itself.
1046 %MyGlobal = global int 100
1049 ;; List of debug info of globals
1051 !llvm.dbg.gv = !{!0}
1054 ;; Define the global variable descriptor. Note the reference to the global
1055 ;; variable anchor and the global variable itself.
1060 metadata !1, ;; Context
1061 metadata !"MyGlobal", ;; Name
1062 metadata !"MyGlobal", ;; Display Name
1063 metadata !"MyGlobal", ;; Linkage Name
1064 metadata !1, ;; Compile Unit
1065 i32 1, ;; Line Number
1066 metadata !2, ;; Type
1067 i1 false, ;; Is a local variable
1068 i1 true, ;; Is this a definition
1069 i32* @MyGlobal ;; The global variable
1073 ;; Define the basic type of 32 bit signed integer. Note that since int is an
1074 ;; intrinsic type the source file is NULL and line 0.
1078 metadata !1, ;; Context
1079 metadata !"int", ;; Name
1080 metadata !1, ;; Compile Unit
1081 i32 0, ;; Line number
1082 i64 32, ;; Size in Bits
1083 i64 32, ;; Align in Bits
1084 i64 0, ;; Offset in Bits
1094 <!-- ======================================================================= -->
1095 <div class="doc_subsection">
1096 <a name="ccxx_subprogram">C/C++ function information</a>
1099 <div class="doc_text">
1101 <p>Given a function declared as follows:</p>
1103 <div class="doc_code">
1105 int main(int argc, char *argv[]) {
1111 <p>a C/C++ front-end would generate the following descriptors:</p>
1113 <div class="doc_code">
1116 ;; Define the anchor for subprograms. Note that the second field of the
1117 ;; anchor is 46, which is the same as the tag for subprograms
1118 ;; (46 = DW_TAG_subprogram.)
1123 metadata !1, ;; Context
1124 metadata !"main", ;; Name
1125 metadata !"main", ;; Display name
1126 metadata !"main", ;; Linkage name
1127 metadata !1, ;; Compile unit
1128 i32 1, ;; Line number
1129 metadata !2, ;; Type
1130 i1 false, ;; Is local
1131 i1 true ;; Is definition
1134 ;; Define the subprogram itself.
1136 define i32 @main(i32 %argc, i8** %argv) {
1144 <!-- ======================================================================= -->
1145 <div class="doc_subsection">
1146 <a name="ccxx_basic_types">C/C++ basic types</a>
1149 <div class="doc_text">
1151 <p>The following are the basic type descriptors for C/C++ core types:</p>
1155 <!-- ======================================================================= -->
1156 <div class="doc_subsubsection">
1157 <a name="ccxx_basic_type_bool">bool</a>
1160 <div class="doc_text">
1162 <div class="doc_code">
1166 metadata !1, ;; Context
1167 metadata !"bool", ;; Name
1168 metadata !1, ;; Compile Unit
1169 i32 0, ;; Line number
1170 i64 8, ;; Size in Bits
1171 i64 8, ;; Align in Bits
1172 i64 0, ;; Offset in Bits
1181 <!-- ======================================================================= -->
1182 <div class="doc_subsubsection">
1183 <a name="ccxx_basic_char">char</a>
1186 <div class="doc_text">
1188 <div class="doc_code">
1192 metadata !1, ;; Context
1193 metadata !"char", ;; Name
1194 metadata !1, ;; Compile Unit
1195 i32 0, ;; Line number
1196 i64 8, ;; Size in Bits
1197 i64 8, ;; Align in Bits
1198 i64 0, ;; Offset in Bits
1207 <!-- ======================================================================= -->
1208 <div class="doc_subsubsection">
1209 <a name="ccxx_basic_unsigned_char">unsigned char</a>
1212 <div class="doc_text">
1214 <div class="doc_code">
1218 metadata !1, ;; Context
1219 metadata !"unsigned char",
1220 metadata !1, ;; Compile Unit
1221 i32 0, ;; Line number
1222 i64 8, ;; Size in Bits
1223 i64 8, ;; Align in Bits
1224 i64 0, ;; Offset in Bits
1233 <!-- ======================================================================= -->
1234 <div class="doc_subsubsection">
1235 <a name="ccxx_basic_short">short</a>
1238 <div class="doc_text">
1240 <div class="doc_code">
1244 metadata !1, ;; Context
1245 metadata !"short int",
1246 metadata !1, ;; Compile Unit
1247 i32 0, ;; Line number
1248 i64 16, ;; Size in Bits
1249 i64 16, ;; Align in Bits
1250 i64 0, ;; Offset in Bits
1259 <!-- ======================================================================= -->
1260 <div class="doc_subsubsection">
1261 <a name="ccxx_basic_unsigned_short">unsigned short</a>
1264 <div class="doc_text">
1266 <div class="doc_code">
1270 metadata !1, ;; Context
1271 metadata !"short unsigned int",
1272 metadata !1, ;; Compile Unit
1273 i32 0, ;; Line number
1274 i64 16, ;; Size in Bits
1275 i64 16, ;; Align in Bits
1276 i64 0, ;; Offset in Bits
1285 <!-- ======================================================================= -->
1286 <div class="doc_subsubsection">
1287 <a name="ccxx_basic_int">int</a>
1290 <div class="doc_text">
1292 <div class="doc_code">
1296 metadata !1, ;; Context
1297 metadata !"int", ;; Name
1298 metadata !1, ;; Compile Unit
1299 i32 0, ;; Line number
1300 i64 32, ;; Size in Bits
1301 i64 32, ;; Align in Bits
1302 i64 0, ;; Offset in Bits
1310 <!-- ======================================================================= -->
1311 <div class="doc_subsubsection">
1312 <a name="ccxx_basic_unsigned_int">unsigned int</a>
1315 <div class="doc_text">
1317 <div class="doc_code">
1321 metadata !1, ;; Context
1322 metadata !"unsigned int",
1323 metadata !1, ;; Compile Unit
1324 i32 0, ;; Line number
1325 i64 32, ;; Size in Bits
1326 i64 32, ;; Align in Bits
1327 i64 0, ;; Offset in Bits
1336 <!-- ======================================================================= -->
1337 <div class="doc_subsubsection">
1338 <a name="ccxx_basic_long_long">long long</a>
1341 <div class="doc_text">
1343 <div class="doc_code">
1347 metadata !1, ;; Context
1348 metadata !"long long int",
1349 metadata !1, ;; Compile Unit
1350 i32 0, ;; Line number
1351 i64 64, ;; Size in Bits
1352 i64 64, ;; Align in Bits
1353 i64 0, ;; Offset in Bits
1362 <!-- ======================================================================= -->
1363 <div class="doc_subsubsection">
1364 <a name="ccxx_basic_unsigned_long_long">unsigned long long</a>
1367 <div class="doc_text">
1369 <div class="doc_code">
1373 metadata !1, ;; Context
1374 metadata !"long long unsigned int",
1375 metadata !1, ;; Compile Unit
1376 i32 0, ;; Line number
1377 i64 64, ;; Size in Bits
1378 i64 64, ;; Align in Bits
1379 i64 0, ;; Offset in Bits
1388 <!-- ======================================================================= -->
1389 <div class="doc_subsubsection">
1390 <a name="ccxx_basic_float">float</a>
1393 <div class="doc_text">
1395 <div class="doc_code">
1399 metadata !1, ;; Context
1401 metadata !1, ;; Compile Unit
1402 i32 0, ;; Line number
1403 i64 32, ;; Size in Bits
1404 i64 32, ;; Align in Bits
1405 i64 0, ;; Offset in Bits
1414 <!-- ======================================================================= -->
1415 <div class="doc_subsubsection">
1416 <a name="ccxx_basic_double">double</a>
1419 <div class="doc_text">
1421 <div class="doc_code">
1425 metadata !1, ;; Context
1426 metadata !"double",;; Name
1427 metadata !1, ;; Compile Unit
1428 i32 0, ;; Line number
1429 i64 64, ;; Size in Bits
1430 i64 64, ;; Align in Bits
1431 i64 0, ;; Offset in Bits
1440 <!-- ======================================================================= -->
1441 <div class="doc_subsection">
1442 <a name="ccxx_derived_types">C/C++ derived types</a>
1445 <div class="doc_text">
1447 <p>Given the following as an example of C/C++ derived type:</p>
1449 <div class="doc_code">
1451 typedef const int *IntPtr;
1455 <p>a C/C++ front-end would generate the following descriptors:</p>
1457 <div class="doc_code">
1460 ;; Define the typedef "IntPtr".
1464 metadata !1, ;; Context
1465 metadata !"IntPtr", ;; Name
1466 metadata !3, ;; Compile unit
1467 i32 0, ;; Line number
1468 i64 0, ;; Size in bits
1469 i64 0, ;; Align in bits
1470 i64 0, ;; Offset in bits
1472 metadata !4 ;; Derived From type
1476 ;; Define the pointer type.
1480 metadata !1, ;; Context
1481 metadata !"", ;; Name
1482 metadata !1, ;; Compile unit
1483 i32 0, ;; Line number
1484 i64 64, ;; Size in bits
1485 i64 64, ;; Align in bits
1486 i64 0, ;; Offset in bits
1488 metadata !5 ;; Derived From type
1491 ;; Define the const type.
1495 metadata !1, ;; Context
1496 metadata !"", ;; Name
1497 metadata !1, ;; Compile unit
1498 i32 0, ;; Line number
1499 i64 32, ;; Size in bits
1500 i64 32, ;; Align in bits
1501 i64 0, ;; Offset in bits
1503 metadata !6 ;; Derived From type
1506 ;; Define the int type.
1510 metadata !1, ;; Context
1511 metadata !"int", ;; Name
1512 metadata !1, ;; Compile unit
1513 i32 0, ;; Line number
1514 i64 32, ;; Size in bits
1515 i64 32, ;; Align in bits
1516 i64 0, ;; Offset in bits
1525 <!-- ======================================================================= -->
1526 <div class="doc_subsection">
1527 <a name="ccxx_composite_types">C/C++ struct/union types</a>
1530 <div class="doc_text">
1532 <p>Given the following as an example of C/C++ struct type:</p>
1534 <div class="doc_code">
1544 <p>a C/C++ front-end would generate the following descriptors:</p>
1546 <div class="doc_code">
1549 ;; Define basic type for unsigned int.
1553 metadata !1, ;; Context
1554 metadata !"unsigned int",
1555 metadata !1, ;; Compile Unit
1556 i32 0, ;; Line number
1557 i64 32, ;; Size in Bits
1558 i64 32, ;; Align in Bits
1559 i64 0, ;; Offset in Bits
1564 ;; Define composite type for struct Color.
1568 metadata !1, ;; Context
1569 metadata !"Color", ;; Name
1570 metadata !1, ;; Compile unit
1571 i32 1, ;; Line number
1572 i64 96, ;; Size in bits
1573 i64 32, ;; Align in bits
1574 i64 0, ;; Offset in bits
1576 null, ;; Derived From
1577 metadata !3, ;; Elements
1578 i32 0 ;; Runtime Language
1582 ;; Define the Red field.
1586 metadata !1, ;; Context
1587 metadata !"Red", ;; Name
1588 metadata !1, ;; Compile Unit
1589 i32 2, ;; Line number
1590 i64 32, ;; Size in bits
1591 i64 32, ;; Align in bits
1592 i64 0, ;; Offset in bits
1594 metadata !5 ;; Derived From type
1598 ;; Define the Green field.
1602 metadata !1, ;; Context
1603 metadata !"Green", ;; Name
1604 metadata !1, ;; Compile Unit
1605 i32 3, ;; Line number
1606 i64 32, ;; Size in bits
1607 i64 32, ;; Align in bits
1608 i64 32, ;; Offset in bits
1610 metadata !5 ;; Derived From type
1614 ;; Define the Blue field.
1618 metadata !1, ;; Context
1619 metadata !"Blue", ;; Name
1620 metadata !1, ;; Compile Unit
1621 i32 4, ;; Line number
1622 i64 32, ;; Size in bits
1623 i64 32, ;; Align in bits
1624 i64 64, ;; Offset in bits
1626 metadata !5 ;; Derived From type
1630 ;; Define the array of fields used by the composite type Color.
1632 !3 = metadata !{metadata !4, metadata !6, metadata !7}
1638 <!-- ======================================================================= -->
1639 <div class="doc_subsection">
1640 <a name="ccxx_enumeration_types">C/C++ enumeration types</a>
1643 <div class="doc_text">
1645 <p>Given the following as an example of C/C++ enumeration type:</p>
1647 <div class="doc_code">
1657 <p>a C/C++ front-end would generate the following descriptors:</p>
1659 <div class="doc_code">
1662 ;; Define composite type for enum Trees
1666 metadata !1, ;; Context
1667 metadata !"Trees", ;; Name
1668 metadata !1, ;; Compile unit
1669 i32 1, ;; Line number
1670 i64 32, ;; Size in bits
1671 i64 32, ;; Align in bits
1672 i64 0, ;; Offset in bits
1674 null, ;; Derived From type
1675 metadata !3, ;; Elements
1676 i32 0 ;; Runtime language
1680 ;; Define the array of enumerators used by composite type Trees.
1682 !3 = metadata !{metadata !4, metadata !5, metadata !6}
1685 ;; Define Spruce enumerator.
1687 !4 = metadata !{i32 458792, metadata !"Spruce", i64 100}
1690 ;; Define Oak enumerator.
1692 !5 = metadata !{i32 458792, metadata !"Oak", i64 200}
1695 ;; Define Maple enumerator.
1697 !6 = metadata !{i32 458792, metadata !"Maple", i64 300}
1704 <!-- *********************************************************************** -->
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