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11 <h1>Source Level Debugging with LLVM</h1>
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_files">File descriptors</a></li>
29 <li><a href="#format_global_variables">Global variable descriptors</a></li>
30 <li><a href="#format_subprograms">Subprogram descriptors</a></li>
31 <li><a href="#format_blocks">Block descriptors</a></li>
32 <li><a href="#format_basic_type">Basic type descriptors</a></li>
33 <li><a href="#format_derived_type">Derived type descriptors</a></li>
34 <li><a href="#format_composite_type">Composite type descriptors</a></li>
35 <li><a href="#format_subrange">Subrange descriptors</a></li>
36 <li><a href="#format_enumeration">Enumerator descriptors</a></li>
37 <li><a href="#format_variables">Local variables</a></li>
39 <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>
41 <li><a href="#format_common_declare">llvm.dbg.declare</a></li>
42 <li><a href="#format_common_value">llvm.dbg.value</a></li>
45 <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>
46 <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
48 <li><a href="#ccxx_compile_units">C/C++ source file information</a></li>
49 <li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>
50 <li><a href="#ccxx_subprogram">C/C++ function information</a></li>
51 <li><a href="#ccxx_basic_types">C/C++ basic types</a></li>
52 <li><a href="#ccxx_derived_types">C/C++ derived types</a></li>
53 <li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>
54 <li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>
59 <img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
64 <div class="doc_author">
65 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
66 and <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
70 <!-- *********************************************************************** -->
71 <h2><a name="introduction">Introduction</a></h2>
72 <!-- *********************************************************************** -->
76 <p>This document is the central repository for all information pertaining to
77 debug information in LLVM. It describes the <a href="#format">actual format
78 that the LLVM debug information</a> takes, which is useful for those
79 interested in creating front-ends or dealing directly with the information.
80 Further, this document provides specific examples of what debug information
81 for C/C++ looks like.</p>
83 <!-- ======================================================================= -->
85 <a name="phil">Philosophy behind LLVM debugging information</a>
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 <!-- ======================================================================= -->
135 <a name="consumers">Debug information consumers</a>
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 DwarfDebug 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 <!-- ======================================================================= -->
159 <a name="debugopt">Debugging optimized code</a>
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>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
178 debugging information, allowing them to update the debugging information
179 as they perform aggressive optimizations. This means that, with effort,
180 the LLVM optimizers could optimize debug code just as well as non-debug
183 <li>LLVM debug information does not prevent optimizations from
184 happening (for example inlining, basic block reordering/merging/cleanup,
185 tail duplication, etc).</li>
187 <li>LLVM debug information is automatically optimized along with the rest of
188 the program, using existing facilities. For example, duplicate
189 information is automatically merged by the linker, and unused information
190 is automatically removed.</li>
193 <p>Basically, the debug information allows you to compile a program with
194 "<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily
195 modify the program as it executes from a debugger. Compiling a program with
196 "<tt>-O3 -g</tt>" gives you full debug information that is always available
197 and accurate for reading (e.g., you get accurate stack traces despite tail
198 call elimination and inlining), but you might lose the ability to modify the
199 program and call functions where were optimized out of the program, or
200 inlined away completely.</p>
202 <p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a
203 framework to test optimizer's handling of debugging information. It can be
206 <div class="doc_code">
208 % cd llvm/projects/test-suite/MultiSource/Benchmarks # or some other level
213 <p>This will test impact of debugging information on optimization passes. If
214 debugging information influences optimization passes then it will be reported
215 as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more
216 information on LLVM test infrastructure and how to run various tests.</p>
222 <!-- *********************************************************************** -->
224 <a name="format">Debugging information format</a>
226 <!-- *********************************************************************** -->
230 <p>LLVM debugging information has been carefully designed to make it possible
231 for the optimizer to optimize the program and debugging information without
232 necessarily having to know anything about debugging information. In
233 particular, the use of metadata avoids duplicated debugging information from
234 the beginning, and the global dead code elimination pass automatically
235 deletes debugging information for a function if it decides to delete the
238 <p>To do this, most of the debugging information (descriptors for types,
239 variables, functions, source files, etc) is inserted by the language
240 front-end in the form of LLVM metadata. </p>
242 <p>Debug information is designed to be agnostic about the target debugger and
243 debugging information representation (e.g. DWARF/Stabs/etc). It uses a
244 generic pass to decode the information that represents variables, types,
245 functions, namespaces, etc: this allows for arbitrary source-language
246 semantics and type-systems to be used, as long as there is a module
247 written for the target debugger to interpret the information. </p>
249 <p>To provide basic functionality, the LLVM debugger does have to make some
250 assumptions about the source-level language being debugged, though it keeps
251 these to a minimum. The only common features that the LLVM debugger assumes
252 exist are <a href="#format_files">source files</a>,
253 and <a href="#format_global_variables">program objects</a>. These abstract
254 objects are used by a debugger to form stack traces, show information about
255 local variables, etc.</p>
257 <p>This section of the documentation first describes the representation aspects
258 common to any source-language. The <a href="#ccxx_frontend">next section</a>
259 describes the data layout conventions used by the C and C++ front-ends.</p>
261 <!-- ======================================================================= -->
263 <a name="debug_info_descriptors">Debug information descriptors</a>
268 <p>In consideration of the complexity and volume of debug information, LLVM
269 provides a specification for well formed debug descriptors. </p>
271 <p>Consumers of LLVM debug information expect the descriptors for program
272 objects to start in a canonical format, but the descriptors can include
273 additional information appended at the end that is source-language
274 specific. All LLVM debugging information is versioned, allowing backwards
275 compatibility in the case that the core structures need to change in some
276 way. Also, all debugging information objects start with a tag to indicate
277 what type of object it is. The source-language is allowed to define its own
278 objects, by using unreserved tag numbers. We recommend using with tags in
279 the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base =
282 <p>The fields of debug descriptors used internally by LLVM
283 are restricted to only the simple data types <tt>i32</tt>, <tt>i1</tt>,
284 <tt>float</tt>, <tt>double</tt>, <tt>mdstring</tt> and <tt>mdnode</tt>. </p>
286 <div class="doc_code">
295 <p><a name="LLVMDebugVersion">The first field of a descriptor is always an
296 <tt>i32</tt> containing a tag value identifying the content of the
297 descriptor. The remaining fields are specific to the descriptor. The values
298 of tags are loosely bound to the tag values of DWARF information entries.
299 However, that does not restrict the use of the information supplied to DWARF
300 targets. To facilitate versioning of debug information, the tag is augmented
301 with the current debug version (LLVMDebugVersion = 8 << 16 or
302 0x80000 or 524288.)</a></p>
304 <p>The details of the various descriptors follow.</p>
306 <!-- ======================================================================= -->
308 <a name="format_compile_units">Compile unit descriptors</a>
313 <div class="doc_code">
316 i32, ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
317 ;; (DW_TAG_compile_unit)
318 i32, ;; Unused field.
319 i32, ;; DWARF language identifier (ex. DW_LANG_C89)
320 metadata, ;; Source file name
321 metadata, ;; Source file directory (includes trailing slash)
322 metadata ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
323 i1, ;; True if this is a main compile unit.
324 i1, ;; True if this is optimized.
326 i32 ;; Runtime version
331 <p>These descriptors contain a source language ID for the file (we use the DWARF
332 3.0 ID numbers, such as <tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>,
333 <tt>DW_LANG_Cobol74</tt>, etc), three strings describing the filename,
334 working directory of the compiler, and an identifier string for the compiler
335 that produced it.</p>
337 <p>Compile unit descriptors provide the root context for objects declared in a
338 specific compilation unit. File descriptors are defined using this context.
339 These descriptors are collected by a named metadata
340 <tt>!llvm.dbg.cu</tt>.
344 <!-- ======================================================================= -->
346 <a name="format_files">File descriptors</a>
351 <div class="doc_code">
354 i32, ;; Tag = 41 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
355 ;; (DW_TAG_file_type)
356 metadata, ;; Source file name
357 metadata, ;; Source file directory (includes trailing slash)
363 <p>These descriptors contain information for a file. Global variables and top
364 level functions would be defined using this context.k File descriptors also
365 provide context for source line correspondence. </p>
367 <p>Each input file is encoded as a separate file descriptor in LLVM debugging
368 information output. </p>
372 <!-- ======================================================================= -->
374 <a name="format_global_variables">Global variable descriptors</a>
379 <div class="doc_code">
382 i32, ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
384 i32, ;; Unused field.
385 metadata, ;; Reference to context descriptor
387 metadata, ;; Display name (fully qualified C++ name)
388 metadata, ;; MIPS linkage name (for C++)
389 metadata, ;; Reference to file where defined
390 i32, ;; Line number where defined
391 metadata, ;; Reference to type descriptor
392 i1, ;; True if the global is local to compile unit (static)
393 i1, ;; True if the global is defined in the compile unit (not extern)
394 {}* ;; Reference to the global variable
399 <p>These descriptors provide debug information about globals variables. The
400 provide details such as name, type and where the variable is defined. All
401 global variables are collected by named metadata <tt>!llvm.dbg.gv</tt>.</p>
405 <!-- ======================================================================= -->
407 <a name="format_subprograms">Subprogram descriptors</a>
412 <div class="doc_code">
415 i32, ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
416 ;; (DW_TAG_subprogram)
417 i32, ;; Unused field.
418 metadata, ;; Reference to context descriptor
420 metadata, ;; Display name (fully qualified C++ name)
421 metadata, ;; MIPS linkage name (for C++)
422 metadata, ;; Reference to file where defined
423 i32, ;; Line number where defined
424 metadata, ;; Reference to type descriptor
425 i1, ;; True if the global is local to compile unit (static)
426 i1, ;; True if the global is defined in the compile unit (not extern)
427 i32, ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual
428 i32, ;; Index into a virtual function
429 metadata, ;; indicates which base type contains the vtable pointer for the
433 Function *,;; Pointer to LLVM function
434 metadata, ;; Lists function template parameters
435 metadata ;; Function declaration descriptor
440 <p>These descriptors provide debug information about functions, methods and
441 subprograms. They provide details such as name, return types and the source
442 location where the subprogram is defined.
443 All subprogram descriptors are collected by a named metadata
444 <tt>!llvm.dbg.sp</tt>.
449 <!-- ======================================================================= -->
451 <a name="format_blocks">Block descriptors</a>
456 <div class="doc_code">
459 i32, ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
460 metadata,;; Reference to context descriptor
462 i32, ;; Column number
463 metadata,;; Reference to source file
464 i32 ;; Unique ID to identify blocks from a template function
469 <p>These descriptors provide debug information about nested blocks within a
470 subprogram. The line number and column numbers are used to dinstinguish
471 two lexical blocks at same depth. </p>
475 <!-- ======================================================================= -->
477 <a name="format_basic_type">Basic type descriptors</a>
482 <div class="doc_code">
485 i32, ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
486 ;; (DW_TAG_base_type)
487 metadata, ;; Reference to context
488 metadata, ;; Name (may be "" for anonymous types)
489 metadata, ;; Reference to file where defined (may be NULL)
490 i32, ;; Line number where defined (may be 0)
492 i64, ;; Alignment in bits
493 i64, ;; Offset in bits
495 i32 ;; DWARF type encoding
500 <p>These descriptors define primitive types used in the code. Example int, bool
501 and float. The context provides the scope of the type, which is usually the
502 top level. Since basic types are not usually user defined the context
503 and line number can be left as NULL and 0. The size, alignment and offset
504 are expressed in bits and can be 64 bit values. The alignment is used to
505 round the offset when embedded in a
506 <a href="#format_composite_type">composite type</a> (example to keep float
507 doubles on 64 bit boundaries.) The offset is the bit offset if embedded in
508 a <a href="#format_composite_type">composite type</a>.</p>
510 <p>The type encoding provides the details of the type. The values are typically
511 one of the following:</p>
513 <div class="doc_code">
519 DW_ATE_signed_char = 6
521 DW_ATE_unsigned_char = 8
527 <!-- ======================================================================= -->
529 <a name="format_derived_type">Derived type descriptors</a>
534 <div class="doc_code">
537 i32, ;; Tag (see below)
538 metadata, ;; Reference to context
539 metadata, ;; Name (may be "" for anonymous types)
540 metadata, ;; Reference to file where defined (may be NULL)
541 i32, ;; Line number where defined (may be 0)
543 i64, ;; Alignment in bits
544 i64, ;; Offset in bits
545 metadata, ;; Reference to type derived from
546 metadata, ;; (optional) Name of the Objective C property assoicated with
547 ;; Objective-C an ivar
548 metadata, ;; (optional) Name of the Objective C property getter selector.
549 metadata, ;; (optional) Name of the Objective C property setter selector.
550 i32 ;; (optional) Objective C property attributes.
555 <p>These descriptors are used to define types derived from other types. The
556 value of the tag varies depending on the meaning. The following are possible
559 <div class="doc_code">
561 DW_TAG_formal_parameter = 5
563 DW_TAG_pointer_type = 15
564 DW_TAG_reference_type = 16
566 DW_TAG_const_type = 38
567 DW_TAG_volatile_type = 53
568 DW_TAG_restrict_type = 55
572 <p><tt>DW_TAG_member</tt> is used to define a member of
573 a <a href="#format_composite_type">composite type</a>
574 or <a href="#format_subprograms">subprogram</a>. The type of the member is
575 the <a href="#format_derived_type">derived
576 type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which
577 is a formal argument of a subprogram.</p>
579 <p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>
581 <p><tt>DW_TAG_pointer_type</tt>,<tt>DW_TAG_reference_type</tt>,
582 <tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt>
583 and <tt>DW_TAG_restrict_type</tt> are used to qualify
584 the <a href="#format_derived_type">derived type</a>. </p>
586 <p><a href="#format_derived_type">Derived type</a> location can be determined
587 from the context and line number. The size, alignment and offset are
588 expressed in bits and can be 64 bit values. The alignment is used to round
589 the offset when embedded in a <a href="#format_composite_type">composite
590 type</a> (example to keep float doubles on 64 bit boundaries.) The offset is
591 the bit offset if embedded in a <a href="#format_composite_type">composite
594 <p>Note that the <tt>void *</tt> type is expressed as a type derived from NULL.
599 <!-- ======================================================================= -->
601 <a name="format_composite_type">Composite type descriptors</a>
606 <div class="doc_code">
609 i32, ;; Tag (see below)
610 metadata, ;; Reference to context
611 metadata, ;; Name (may be "" for anonymous types)
612 metadata, ;; Reference to file where defined (may be NULL)
613 i32, ;; Line number where defined (may be 0)
615 i64, ;; Alignment in bits
616 i64, ;; Offset in bits
618 metadata, ;; Reference to type derived from
619 metadata, ;; Reference to array of member descriptors
620 i32 ;; Runtime languages
625 <p>These descriptors are used to define types that are composed of 0 or more
626 elements. The value of the tag varies depending on the meaning. The following
627 are possible tag values:</p>
629 <div class="doc_code">
631 DW_TAG_array_type = 1
632 DW_TAG_enumeration_type = 4
633 DW_TAG_structure_type = 19
634 DW_TAG_union_type = 23
635 DW_TAG_vector_type = 259
636 DW_TAG_subroutine_type = 21
637 DW_TAG_inheritance = 28
641 <p>The vector flag indicates that an array type is a native packed vector.</p>
643 <p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types
644 (tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange
645 descriptors</a>, each representing the range of subscripts at that level of
648 <p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are
649 <a href="#format_enumeration">enumerator descriptors</a>, each representing
650 the definition of enumeration value for the set. All enumeration type
651 descriptors are collected by named metadata <tt>!llvm.dbg.enum</tt>.</p>
653 <p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag
654 = <tt>DW_TAG_union_type</tt>) types are any one of
655 the <a href="#format_basic_type">basic</a>,
656 <a href="#format_derived_type">derived</a>
657 or <a href="#format_composite_type">composite</a> type descriptors, each
658 representing a field member of the structure or union.</p>
660 <p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors
661 provide information about base classes, static members and member
662 functions. If a member is a <a href="#format_derived_type">derived type
663 descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type
664 represents a base class. If the member of is
665 a <a href="#format_global_variables">global variable descriptor</a> then it
666 represents a static member. And, if the member is
667 a <a href="#format_subprograms">subprogram descriptor</a> then it represents
668 a member function. For static members and member
669 functions, <tt>getName()</tt> returns the members link or the C++ mangled
670 name. <tt>getDisplayName()</tt> the simplied version of the name.</p>
672 <p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type
673 elements is the return type for the subroutine. The remaining elements are
674 the formal arguments to the subroutine.</p>
676 <p><a href="#format_composite_type">Composite type</a> location can be
677 determined from the context and line number. The size, alignment and
678 offset are expressed in bits and can be 64 bit values. The alignment is used
679 to round the offset when embedded in
680 a <a href="#format_composite_type">composite type</a> (as an example, to keep
681 float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
682 in a <a href="#format_composite_type">composite type</a>.</p>
686 <!-- ======================================================================= -->
688 <a name="format_subrange">Subrange descriptors</a>
693 <div class="doc_code">
696 i32, ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)
703 <p>These descriptors are used to define ranges of array subscripts for an array
704 <a href="#format_composite_type">composite type</a>. The low value defines
705 the lower bounds typically zero for C/C++. The high value is the upper
706 bounds. Values are 64 bit. High - low + 1 is the size of the array. If low
707 > high the array bounds are not included in generated debugging information.
712 <!-- ======================================================================= -->
714 <a name="format_enumeration">Enumerator descriptors</a>
719 <div class="doc_code">
722 i32, ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
723 ;; (DW_TAG_enumerator)
730 <p>These descriptors are used to define members of an
731 enumeration <a href="#format_composite_type">composite type</a>, it
732 associates the name to the value.</p>
736 <!-- ======================================================================= -->
738 <a name="format_variables">Local variables</a>
743 <div class="doc_code">
746 i32, ;; Tag (see below)
749 metadata, ;; Reference to file where defined
750 i32, ;; 24 bit - Line number where defined
751 ;; 8 bit - Argument number. 1 indicates 1st argument.
752 metadata, ;; Type descriptor
754 metadata ;; (optional) Reference to inline location
759 <p>These descriptors are used to define variables local to a sub program. The
760 value of the tag depends on the usage of the variable:</p>
762 <div class="doc_code">
764 DW_TAG_auto_variable = 256
765 DW_TAG_arg_variable = 257
766 DW_TAG_return_variable = 258
770 <p>An auto variable is any variable declared in the body of the function. An
771 argument variable is any variable that appears as a formal argument to the
772 function. A return variable is used to track the result of a function and
773 has no source correspondent.</p>
775 <p>The context is either the subprogram or block where the variable is defined.
776 Name the source variable name. Context and line indicate where the
777 variable was defined. Type descriptor defines the declared type of the
784 <!-- ======================================================================= -->
786 <a name="format_common_intrinsics">Debugger intrinsic functions</a>
791 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to
792 provide debug information at various points in generated code.</p>
794 <!-- ======================================================================= -->
796 <a name="format_common_declare">llvm.dbg.declare</a>
801 void %<a href="#format_common_declare">llvm.dbg.declare</a>(metadata, metadata)
804 <p>This intrinsic provides information about a local element (ex. variable.) The
805 first argument is metadata holding alloca for the variable. The
806 second argument is metadata containing description of the variable. </p>
809 <!-- ======================================================================= -->
811 <a name="format_common_value">llvm.dbg.value</a>
816 void %<a href="#format_common_value">llvm.dbg.value</a>(metadata, i64, metadata)
819 <p>This intrinsic provides information when a user source variable is set to a
820 new value. The first argument is the new value (wrapped as metadata). The
821 second argument is the offset in the user source variable where the new value
822 is written. The third argument is metadata containing description of the
823 user source variable. </p>
828 <!-- ======================================================================= -->
830 <a name="format_common_lifetime">Object lifetimes and scoping</a>
834 <p>In many languages, the local variables in functions can have their lifetimes
835 or scopes limited to a subset of a function. In the C family of languages,
836 for example, variables are only live (readable and writable) within the
837 source block that they are defined in. In functional languages, values are
838 only readable after they have been defined. Though this is a very obvious
839 concept, it is non-trivial to model in LLVM, because it has no notion of
840 scoping in this sense, and does not want to be tied to a language's scoping
843 <p>In order to handle this, the LLVM debug format uses the metadata attached to
844 llvm instructions to encode line number and scoping information. Consider
845 the following C fragment, for example:</p>
847 <div class="doc_code">
861 <p>Compiled to LLVM, this function would be represented like this:</p>
863 <div class="doc_code">
865 define void @foo() nounwind ssp {
867 %X = alloca i32, align 4 ; <i32*> [#uses=4]
868 %Y = alloca i32, align 4 ; <i32*> [#uses=4]
869 %Z = alloca i32, align 4 ; <i32*> [#uses=3]
870 %0 = bitcast i32* %X to {}* ; <{}*> [#uses=1]
871 call void @llvm.dbg.declare(metadata !{i32 * %X}, metadata !0), !dbg !7
872 store i32 21, i32* %X, !dbg !8
873 %1 = bitcast i32* %Y to {}* ; <{}*> [#uses=1]
874 call void @llvm.dbg.declare(metadata !{i32 * %Y}, metadata !9), !dbg !10
875 store i32 22, i32* %Y, !dbg !11
876 %2 = bitcast i32* %Z to {}* ; <{}*> [#uses=1]
877 call void @llvm.dbg.declare(metadata !{i32 * %Z}, metadata !12), !dbg !14
878 store i32 23, i32* %Z, !dbg !15
879 %tmp = load i32* %X, !dbg !16 ; <i32> [#uses=1]
880 %tmp1 = load i32* %Y, !dbg !16 ; <i32> [#uses=1]
881 %add = add nsw i32 %tmp, %tmp1, !dbg !16 ; <i32> [#uses=1]
882 store i32 %add, i32* %Z, !dbg !16
883 %tmp2 = load i32* %Y, !dbg !17 ; <i32> [#uses=1]
884 store i32 %tmp2, i32* %X, !dbg !17
888 declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone
890 !0 = metadata !{i32 459008, metadata !1, metadata !"X",
891 metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]
892 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
893 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo",
894 metadata !"foo", metadata !3, i32 1, metadata !4,
895 i1 false, i1 true}; [DW_TAG_subprogram ]
896 !3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c",
897 metadata !"/private/tmp", metadata !"clang 1.1", i1 true,
898 i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]
899 !4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0,
900 i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]
901 !5 = metadata !{null}
902 !6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0,
903 i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]
904 !7 = metadata !{i32 2, i32 7, metadata !1, null}
905 !8 = metadata !{i32 2, i32 3, metadata !1, null}
906 !9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3,
907 metadata !6}; [ DW_TAG_auto_variable ]
908 !10 = metadata !{i32 3, i32 7, metadata !1, null}
909 !11 = metadata !{i32 3, i32 3, metadata !1, null}
910 !12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5,
911 metadata !6}; [ DW_TAG_auto_variable ]
912 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
913 !14 = metadata !{i32 5, i32 9, metadata !13, null}
914 !15 = metadata !{i32 5, i32 5, metadata !13, null}
915 !16 = metadata !{i32 6, i32 5, metadata !13, null}
916 !17 = metadata !{i32 8, i32 3, metadata !1, null}
917 !18 = metadata !{i32 9, i32 1, metadata !2, null}
921 <p>This example illustrates a few important details about LLVM debugging
922 information. In particular, it shows how the <tt>llvm.dbg.declare</tt>
923 intrinsic and location information, which are attached to an instruction,
924 are applied together to allow a debugger to analyze the relationship between
925 statements, variable definitions, and the code used to implement the
928 <div class="doc_code">
930 call void @llvm.dbg.declare(metadata, metadata !0), !dbg !7
934 <p>The first intrinsic
935 <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
936 encodes debugging information for the variable <tt>X</tt>. The metadata
937 <tt>!dbg !7</tt> attached to the intrinsic provides scope information for the
938 variable <tt>X</tt>.</p>
940 <div class="doc_code">
942 !7 = metadata !{i32 2, i32 7, metadata !1, null}
943 !1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
944 !2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo",
945 metadata !"foo", metadata !"foo", metadata !3, i32 1,
946 metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]
950 <p>Here <tt>!7</tt> is metadata providing location information. It has four
951 fields: line number, column number, scope, and original scope. The original
952 scope represents inline location if this instruction is inlined inside a
953 caller, and is null otherwise. In this example, scope is encoded by
954 <tt>!1</tt>. <tt>!1</tt> represents a lexical block inside the scope
955 <tt>!2</tt>, where <tt>!2</tt> is a
956 <a href="#format_subprograms">subprogram descriptor</a>. This way the
957 location information attached to the intrinsics indicates that the
958 variable <tt>X</tt> is declared at line number 2 at a function level scope in
959 function <tt>foo</tt>.</p>
961 <p>Now lets take another example.</p>
963 <div class="doc_code">
965 call void @llvm.dbg.declare(metadata, metadata !12), !dbg !14
969 <p>The second intrinsic
970 <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
971 encodes debugging information for variable <tt>Z</tt>. The metadata
972 <tt>!dbg !14</tt> attached to the intrinsic provides scope information for
973 the variable <tt>Z</tt>.</p>
975 <div class="doc_code">
977 !13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
978 !14 = metadata !{i32 5, i32 9, metadata !13, null}
982 <p>Here <tt>!14</tt> indicates that <tt>Z</tt> is declared at line number 5 and
983 column number 9 inside of lexical scope <tt>!13</tt>. The lexical scope
984 itself resides inside of lexical scope <tt>!1</tt> described above.</p>
986 <p>The scope information attached with each instruction provides a
987 straightforward way to find instructions covered by a scope.</p>
993 <!-- *********************************************************************** -->
995 <a name="ccxx_frontend">C/C++ front-end specific debug information</a>
997 <!-- *********************************************************************** -->
1001 <p>The C and C++ front-ends represent information about the program in a format
1002 that is effectively identical
1003 to <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in
1004 terms of information content. This allows code generators to trivially
1005 support native debuggers by generating standard dwarf information, and
1006 contains enough information for non-dwarf targets to translate it as
1009 <p>This section describes the forms used to represent C and C++ programs. Other
1010 languages could pattern themselves after this (which itself is tuned to
1011 representing programs in the same way that DWARF 3 does), or they could
1012 choose to provide completely different forms if they don't fit into the DWARF
1013 model. As support for debugging information gets added to the various LLVM
1014 source-language front-ends, the information used should be documented
1017 <p>The following sections provide examples of various C/C++ constructs and the
1018 debug information that would best describe those constructs.</p>
1020 <!-- ======================================================================= -->
1022 <a name="ccxx_compile_units">C/C++ source file information</a>
1027 <p>Given the source files <tt>MySource.cpp</tt> and <tt>MyHeader.h</tt> located
1028 in the directory <tt>/Users/mine/sources</tt>, the following code:</p>
1030 <div class="doc_code">
1032 #include "MyHeader.h"
1034 int main(int argc, char *argv[]) {
1040 <p>a C/C++ front-end would generate the following descriptors:</p>
1042 <div class="doc_code">
1046 ;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".
1051 i32 4, ;; Language Id
1052 metadata !"MySource.cpp",
1053 metadata !"/Users/mine/sources",
1054 metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",
1055 i1 true, ;; Main Compile Unit
1056 i1 false, ;; Optimized compile unit
1057 metadata !"", ;; Compiler flags
1058 i32 0} ;; Runtime version
1061 ;; Define the file for the file "/Users/mine/sources/MySource.cpp".
1065 metadata !"MySource.cpp",
1066 metadata !"/Users/mine/sources",
1067 metadata !2 ;; Compile unit
1071 ;; Define the file for the file "/Users/mine/sources/Myheader.h"
1075 metadata !"Myheader.h"
1076 metadata !"/Users/mine/sources",
1077 metadata !2 ;; Compile unit
1084 <p>llvm::Instruction provides easy access to metadata attached with an
1085 instruction. One can extract line number information encoded in LLVM IR
1086 using <tt>Instruction::getMetadata()</tt> and
1087 <tt>DILocation::getLineNumber()</tt>.
1089 if (MDNode *N = I->getMetadata("dbg")) { // Here I is an LLVM instruction
1090 DILocation Loc(N); // DILocation is in DebugInfo.h
1091 unsigned Line = Loc.getLineNumber();
1092 StringRef File = Loc.getFilename();
1093 StringRef Dir = Loc.getDirectory();
1098 <!-- ======================================================================= -->
1100 <a name="ccxx_global_variable">C/C++ global variable information</a>
1105 <p>Given an integer global variable declared as follows:</p>
1107 <div class="doc_code">
1113 <p>a C/C++ front-end would generate the following descriptors:</p>
1115 <div class="doc_code">
1118 ;; Define the global itself.
1120 %MyGlobal = global int 100
1123 ;; List of debug info of globals
1125 !llvm.dbg.gv = !{!0}
1128 ;; Define the global variable descriptor. Note the reference to the global
1129 ;; variable anchor and the global variable itself.
1134 metadata !1, ;; Context
1135 metadata !"MyGlobal", ;; Name
1136 metadata !"MyGlobal", ;; Display Name
1137 metadata !"MyGlobal", ;; Linkage Name
1138 metadata !3, ;; Compile Unit
1139 i32 1, ;; Line Number
1140 metadata !4, ;; Type
1141 i1 false, ;; Is a local variable
1142 i1 true, ;; Is this a definition
1143 i32* @MyGlobal ;; The global variable
1147 ;; Define the basic type of 32 bit signed integer. Note that since int is an
1148 ;; intrinsic type the source file is NULL and line 0.
1152 metadata !1, ;; Context
1153 metadata !"int", ;; Name
1154 metadata !1, ;; File
1155 i32 0, ;; Line number
1156 i64 32, ;; Size in Bits
1157 i64 32, ;; Align in Bits
1158 i64 0, ;; Offset in Bits
1168 <!-- ======================================================================= -->
1170 <a name="ccxx_subprogram">C/C++ function information</a>
1175 <p>Given a function declared as follows:</p>
1177 <div class="doc_code">
1179 int main(int argc, char *argv[]) {
1185 <p>a C/C++ front-end would generate the following descriptors:</p>
1187 <div class="doc_code">
1190 ;; Define the anchor for subprograms. Note that the second field of the
1191 ;; anchor is 46, which is the same as the tag for subprograms
1192 ;; (46 = DW_TAG_subprogram.)
1197 metadata !1, ;; Context
1198 metadata !"main", ;; Name
1199 metadata !"main", ;; Display name
1200 metadata !"main", ;; Linkage name
1201 metadata !1, ;; File
1202 i32 1, ;; Line number
1203 metadata !4, ;; Type
1204 i1 false, ;; Is local
1205 i1 true, ;; Is definition
1206 i32 0, ;; Virtuality attribute, e.g. pure virtual function
1207 i32 0, ;; Index into virtual table for C++ methods
1208 i32 0, ;; Type that holds virtual table.
1210 i1 false, ;; True if this function is optimized
1211 Function *, ;; Pointer to llvm::Function
1212 null ;; Function template parameters
1215 ;; Define the subprogram itself.
1217 define i32 @main(i32 %argc, i8** %argv) {
1225 <!-- ======================================================================= -->
1227 <a name="ccxx_basic_types">C/C++ basic types</a>
1232 <p>The following are the basic type descriptors for C/C++ core types:</p>
1234 <!-- ======================================================================= -->
1236 <a name="ccxx_basic_type_bool">bool</a>
1241 <div class="doc_code">
1245 metadata !1, ;; Context
1246 metadata !"bool", ;; Name
1247 metadata !1, ;; File
1248 i32 0, ;; Line number
1249 i64 8, ;; Size in Bits
1250 i64 8, ;; Align in Bits
1251 i64 0, ;; Offset in Bits
1260 <!-- ======================================================================= -->
1262 <a name="ccxx_basic_char">char</a>
1267 <div class="doc_code">
1271 metadata !1, ;; Context
1272 metadata !"char", ;; Name
1273 metadata !1, ;; File
1274 i32 0, ;; Line number
1275 i64 8, ;; Size in Bits
1276 i64 8, ;; Align in Bits
1277 i64 0, ;; Offset in Bits
1286 <!-- ======================================================================= -->
1288 <a name="ccxx_basic_unsigned_char">unsigned char</a>
1293 <div class="doc_code">
1297 metadata !1, ;; Context
1298 metadata !"unsigned char",
1299 metadata !1, ;; File
1300 i32 0, ;; Line number
1301 i64 8, ;; Size in Bits
1302 i64 8, ;; Align in Bits
1303 i64 0, ;; Offset in Bits
1312 <!-- ======================================================================= -->
1314 <a name="ccxx_basic_short">short</a>
1319 <div class="doc_code">
1323 metadata !1, ;; Context
1324 metadata !"short int",
1325 metadata !1, ;; File
1326 i32 0, ;; Line number
1327 i64 16, ;; Size in Bits
1328 i64 16, ;; Align in Bits
1329 i64 0, ;; Offset in Bits
1338 <!-- ======================================================================= -->
1340 <a name="ccxx_basic_unsigned_short">unsigned short</a>
1345 <div class="doc_code">
1349 metadata !1, ;; Context
1350 metadata !"short unsigned int",
1351 metadata !1, ;; File
1352 i32 0, ;; Line number
1353 i64 16, ;; Size in Bits
1354 i64 16, ;; Align in Bits
1355 i64 0, ;; Offset in Bits
1364 <!-- ======================================================================= -->
1366 <a name="ccxx_basic_int">int</a>
1371 <div class="doc_code">
1375 metadata !1, ;; Context
1376 metadata !"int", ;; Name
1377 metadata !1, ;; File
1378 i32 0, ;; Line number
1379 i64 32, ;; Size in Bits
1380 i64 32, ;; Align in Bits
1381 i64 0, ;; Offset in Bits
1389 <!-- ======================================================================= -->
1391 <a name="ccxx_basic_unsigned_int">unsigned int</a>
1396 <div class="doc_code">
1400 metadata !1, ;; Context
1401 metadata !"unsigned int",
1402 metadata !1, ;; File
1403 i32 0, ;; Line number
1404 i64 32, ;; Size in Bits
1405 i64 32, ;; Align in Bits
1406 i64 0, ;; Offset in Bits
1415 <!-- ======================================================================= -->
1417 <a name="ccxx_basic_long_long">long long</a>
1422 <div class="doc_code">
1426 metadata !1, ;; Context
1427 metadata !"long long int",
1428 metadata !1, ;; File
1429 i32 0, ;; Line number
1430 i64 64, ;; Size in Bits
1431 i64 64, ;; Align in Bits
1432 i64 0, ;; Offset in Bits
1441 <!-- ======================================================================= -->
1443 <a name="ccxx_basic_unsigned_long_long">unsigned long long</a>
1448 <div class="doc_code">
1452 metadata !1, ;; Context
1453 metadata !"long long unsigned int",
1454 metadata !1, ;; File
1455 i32 0, ;; Line number
1456 i64 64, ;; Size in Bits
1457 i64 64, ;; Align in Bits
1458 i64 0, ;; Offset in Bits
1467 <!-- ======================================================================= -->
1469 <a name="ccxx_basic_float">float</a>
1474 <div class="doc_code">
1478 metadata !1, ;; Context
1480 metadata !1, ;; File
1481 i32 0, ;; Line number
1482 i64 32, ;; Size in Bits
1483 i64 32, ;; Align in Bits
1484 i64 0, ;; Offset in Bits
1493 <!-- ======================================================================= -->
1495 <a name="ccxx_basic_double">double</a>
1500 <div class="doc_code">
1504 metadata !1, ;; Context
1505 metadata !"double",;; Name
1506 metadata !1, ;; File
1507 i32 0, ;; Line number
1508 i64 64, ;; Size in Bits
1509 i64 64, ;; Align in Bits
1510 i64 0, ;; Offset in Bits
1521 <!-- ======================================================================= -->
1523 <a name="ccxx_derived_types">C/C++ derived types</a>
1528 <p>Given the following as an example of C/C++ derived type:</p>
1530 <div class="doc_code">
1532 typedef const int *IntPtr;
1536 <p>a C/C++ front-end would generate the following descriptors:</p>
1538 <div class="doc_code">
1541 ;; Define the typedef "IntPtr".
1545 metadata !1, ;; Context
1546 metadata !"IntPtr", ;; Name
1547 metadata !3, ;; File
1548 i32 0, ;; Line number
1549 i64 0, ;; Size in bits
1550 i64 0, ;; Align in bits
1551 i64 0, ;; Offset in bits
1553 metadata !4 ;; Derived From type
1557 ;; Define the pointer type.
1561 metadata !1, ;; Context
1562 metadata !"", ;; Name
1563 metadata !1, ;; File
1564 i32 0, ;; Line number
1565 i64 64, ;; Size in bits
1566 i64 64, ;; Align in bits
1567 i64 0, ;; Offset in bits
1569 metadata !5 ;; Derived From type
1572 ;; Define the const type.
1576 metadata !1, ;; Context
1577 metadata !"", ;; Name
1578 metadata !1, ;; File
1579 i32 0, ;; Line number
1580 i64 32, ;; Size in bits
1581 i64 32, ;; Align in bits
1582 i64 0, ;; Offset in bits
1584 metadata !6 ;; Derived From type
1587 ;; Define the int type.
1591 metadata !1, ;; Context
1592 metadata !"int", ;; Name
1593 metadata !1, ;; File
1594 i32 0, ;; Line number
1595 i64 32, ;; Size in bits
1596 i64 32, ;; Align in bits
1597 i64 0, ;; Offset in bits
1606 <!-- ======================================================================= -->
1608 <a name="ccxx_composite_types">C/C++ struct/union types</a>
1613 <p>Given the following as an example of C/C++ struct type:</p>
1615 <div class="doc_code">
1625 <p>a C/C++ front-end would generate the following descriptors:</p>
1627 <div class="doc_code">
1630 ;; Define basic type for unsigned int.
1634 metadata !1, ;; Context
1635 metadata !"unsigned int",
1636 metadata !1, ;; File
1637 i32 0, ;; Line number
1638 i64 32, ;; Size in Bits
1639 i64 32, ;; Align in Bits
1640 i64 0, ;; Offset in Bits
1645 ;; Define composite type for struct Color.
1649 metadata !1, ;; Context
1650 metadata !"Color", ;; Name
1651 metadata !1, ;; Compile unit
1652 i32 1, ;; Line number
1653 i64 96, ;; Size in bits
1654 i64 32, ;; Align in bits
1655 i64 0, ;; Offset in bits
1657 null, ;; Derived From
1658 metadata !3, ;; Elements
1659 i32 0 ;; Runtime Language
1663 ;; Define the Red field.
1667 metadata !1, ;; Context
1668 metadata !"Red", ;; Name
1669 metadata !1, ;; File
1670 i32 2, ;; Line number
1671 i64 32, ;; Size in bits
1672 i64 32, ;; Align in bits
1673 i64 0, ;; Offset in bits
1675 metadata !5 ;; Derived From type
1679 ;; Define the Green field.
1683 metadata !1, ;; Context
1684 metadata !"Green", ;; Name
1685 metadata !1, ;; File
1686 i32 3, ;; Line number
1687 i64 32, ;; Size in bits
1688 i64 32, ;; Align in bits
1689 i64 32, ;; Offset in bits
1691 metadata !5 ;; Derived From type
1695 ;; Define the Blue field.
1699 metadata !1, ;; Context
1700 metadata !"Blue", ;; Name
1701 metadata !1, ;; File
1702 i32 4, ;; Line number
1703 i64 32, ;; Size in bits
1704 i64 32, ;; Align in bits
1705 i64 64, ;; Offset in bits
1707 metadata !5 ;; Derived From type
1711 ;; Define the array of fields used by the composite type Color.
1713 !3 = metadata !{metadata !4, metadata !6, metadata !7}
1719 <!-- ======================================================================= -->
1721 <a name="ccxx_enumeration_types">C/C++ enumeration types</a>
1726 <p>Given the following as an example of C/C++ enumeration type:</p>
1728 <div class="doc_code">
1738 <p>a C/C++ front-end would generate the following descriptors:</p>
1740 <div class="doc_code">
1743 ;; Define composite type for enum Trees
1747 metadata !1, ;; Context
1748 metadata !"Trees", ;; Name
1749 metadata !1, ;; File
1750 i32 1, ;; Line number
1751 i64 32, ;; Size in bits
1752 i64 32, ;; Align in bits
1753 i64 0, ;; Offset in bits
1755 null, ;; Derived From type
1756 metadata !3, ;; Elements
1757 i32 0 ;; Runtime language
1761 ;; Define the array of enumerators used by composite type Trees.
1763 !3 = metadata !{metadata !4, metadata !5, metadata !6}
1766 ;; Define Spruce enumerator.
1768 !4 = metadata !{i32 524328, metadata !"Spruce", i64 100}
1771 ;; Define Oak enumerator.
1773 !5 = metadata !{i32 524328, metadata !"Oak", i64 200}
1776 ;; Define Maple enumerator.
1778 !6 = metadata !{i32 524328, metadata !"Maple", i64 300}
1787 <!-- *********************************************************************** -->
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1797 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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