This document is the central repository for all information pertaining to debug information in LLVM. It describes the actual format that the LLVM debug information takes, which is useful for those interested in creating front-ends or dealing directly with the information. Further, this document provides specific examples of what debug information for C/C++.

The idea of the LLVM debugging information is to capture how the important pieces of the source-language's Abstract Syntax Tree map onto LLVM code. Several design aspects have shaped the solution that appears here. The important ones are:

• Debugging information should have very little impact on the rest of the compiler. No transformations, analyses, or code generators should need to be modified because of debugging information.
• LLVM optimizations should interact in well-defined and easily described ways with the debugging information.
• Because LLVM is designed to support arbitrary programming languages, LLVM-to-LLVM tools should not need to know anything about the semantics of the source-level-language.
• Source-level languages are often widely different from one another. LLVM should not put any restrictions of the flavor of the source-language, and the debugging information should work with any language.
• With code generator support, it should be possible to use an LLVM compiler to compile a program to native machine code and standard debugging formats. This allows compatibility with traditional machine-code level debuggers, like GDB or DBX.

The approach used by the LLVM implementation is to use a small set of intrinsic functions to define a mapping between LLVM program objects and the source-level objects. The description of the source-level program is maintained in LLVM metadata in an implementation-defined format (the C/C++ front-end currently uses working draft 7 of the DWARF 3 standard).

When a program is being debugged, a debugger interacts with the user and turns the stored debug information into source-language specific information. As such, a debugger must be aware of the source-language, and is thus tied to a specific language or family of languages.

The role of debug information is to provide meta information normally stripped away during the compilation process. This meta information provides an LLVM user a relationship between generated code and the original program source code.

Currently, debug information is consumed by the DwarfWriter to produce dwarf information used by the gdb debugger. Other targets could use the same information to produce stabs or other debug forms.

It would also be reasonable to use debug information to feed profiling tools for analysis of generated code, or, tools for reconstructing the original source from generated code.

TODO - expound a bit more.

An extremely high priority of LLVM debugging information is to make it interact well with optimizations and analysis. In particular, the LLVM debug information provides the following guarantees:

• LLVM debug information always provides information to accurately read the source-level state of the program, regardless of which LLVM optimizations have been run, and without any modification to the optimizations themselves. However, some optimizations may impact the ability to modify the current state of the program with a debugger, such as setting program variables, or calling functions that have been deleted.
• LLVM optimizations gracefully interact with debugging information. If they are not aware of debug information, they are automatically disabled as necessary in the cases that would invalidate the debug info. This retains the LLVM features, making it easy to write new transformations.
• As desired, LLVM optimizations can be upgraded to be aware of the LLVM debugging information, allowing them to update the debugging information as they perform aggressive optimizations. This means that, with effort, the LLVM optimizers could optimize debug code just as well as non-debug code.
• LLVM debug information does not prevent many important optimizations from happening (for example inlining, basic block reordering/merging/cleanup, tail duplication, etc), further reducing the amount of the compiler that eventually is "aware" of debugging information.
• LLVM debug information is automatically optimized along with the rest of the program, using existing facilities. For example, duplicate information is automatically merged by the linker, and unused information is automatically removed.

Basically, the debug information allows you to compile a program with "-O0 -g" and get full debug information, allowing you to arbitrarily modify the program as it executes from a debugger. Compiling a program with "-O3 -g" gives you full debug information that is always available and accurate for reading (e.g., you get accurate stack traces despite tail call elimination and inlining), but you might lose the ability to modify the program and call functions where were optimized out of the program, or inlined away completely.

LLVM test suite provides a framework to test optimizer's handling of debugging information. It can be run like this:

% cd llvm/projects/test-suite/MultiSource/Benchmarks  # or some other level
% make TEST=dbgopt

This will test impact of debugging information on optimization passes. If debugging information influences optimization passes then it will be reported as a failure. See TestingGuide for more information on LLVM test infrastructure and how to run various tests.

LLVM debugging information has been carefully designed to make it possible for the optimizer to optimize the program and debugging information without necessarily having to know anything about debugging information. In particular, te use of metadadta avoids duplicated dubgging information from the beginning, and the global dead code elimination pass automatically deletes debugging information for a function if it decides to delete the function.

To do this, most of the debugging information (descriptors for types, variables, functions, source files, etc) is inserted by the language front-end in the form of LLVM metadata.

Debug information is designed to be agnostic about the target debugger and debugging information representation (e.g. DWARF/Stabs/etc). It uses a generic pass to decode the information that represents variables, types, functions, namespaces, etc: this allows for arbitrary source-language semantics and type-systems to be used, as long as there is a module written for the target debugger to interpret the information.

To provide basic functionality, the LLVM debugger does have to make some assumptions about the source-level language being debugged, though it keeps these to a minimum. The only common features that the LLVM debugger assumes exist are source files, and program objects. These abstract objects are used by a debugger to form stack traces, show information about local variables, etc.

This section of the documentation first describes the representation aspects common to any source-language. The next section describes the data layout conventions used by the C and C++ front-ends.

In consideration of the complexity and volume of debug information, LLVM provides a specification for well formed debug descriptors.

Consumers of LLVM debug information expect the descriptors for program objects to start in a canonical format, but the descriptors can include additional information appended at the end that is source-language specific. All LLVM debugging information is versioned, allowing backwards compatibility in the case that the core structures need to change in some way. Also, all debugging information objects start with a tag to indicate what type of object it is. The source-language is allowed to define its own objects, by using unreserved tag numbers. We recommend using with tags in the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base = 0x1000.)

The fields of debug descriptors used internally by LLVM are restricted to only the simple data types int, uint, bool, float, double, mdstring and mdnode.

!1 = metadata !{
  uint,   ;; A tag
  ...
}

The first field of a descriptor is always an uint containing a tag value identifying the content of the descriptor. The remaining fields are specific to the descriptor. The values of tags are loosely bound to the tag values of DWARF information entries. However, that does not restrict the use of the information supplied to DWARF targets. To facilitate versioning of debug information, the tag is augmented with the current debug version (LLVMDebugVersion = 7 << 16 or 0x70000 or 458752.)

The details of the various descriptors follow.

!0 = metadata !{
  i32,       ;; Tag = 17 + LLVMDebugVersion 
             ;; (DW_TAG_compile_unit)
  i32,       ;; Unused field. 
  i32,       ;; DWARF language identifier (ex. DW_LANG_C89) 
  metadata,  ;; Source file name
  metadata,  ;; Source file directory (includes trailing slash)
  metadata   ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
  i1,        ;; True if this is a main compile unit. 
  i1,        ;; True if this is optimized.
  metadata,  ;; Flags
  i32        ;; Runtime version
}

These descriptors contain a source language ID for the file (we use the DWARF 3.0 ID numbers, such as DW_LANG_C89, DW_LANG_C_plus_plus, DW_LANG_Cobol74, etc), three strings describing the filename, working directory of the compiler, and an identifier string for the compiler that produced it.

Compile unit descriptors provide the root context for objects declared in a specific source file. Global variables and top level functions would be defined using this context. Compile unit descriptors also provide context for source line correspondence.

Each input file is encoded as a separate compile unit in LLVM debugging information output. However, many target specific tool chains prefer to encode only one compile unit in an object file. In this situation, the LLVM code generator will include debugging information entities in the compile unit that is marked as main compile unit. The code generator accepts maximum one main compile unit per module. If a module does not contain any main compile unit then the code generator will emit multiple compile units in the output object file.

!1 = metadata !{
  i32,      ;; Tag = 52 + LLVMDebugVersion 
            ;; (DW_TAG_variable)
  i32,      ;; Unused field.
  metadata, ;; Reference to context descriptor
  metadata, ;; Name
  metadata, ;; Display name (fully qualified C++ name)
  metadata, ;; MIPS linkage name (for C++)
  metadata, ;; Reference to compile unit where defined
  i32,      ;; Line number where defined
  metadata, ;; Reference to type descriptor
  i1,       ;; True if the global is local to compile unit (static)
  i1,       ;; True if the global is defined in the compile unit (not extern)
  {  }*     ;; Reference to the global variable
}

These descriptors provide debug information about globals variables. The provide details such as name, type and where the variable is defined.

!2 = metadata !{
  i32,      ;; Tag = 46 + LLVMDebugVersion
            ;; (DW_TAG_subprogram)
  i32,      ;; Unused field.
  metadata, ;; Reference to context descriptor
  metadata, ;; Name
  metadata, ;; Display name (fully qualified C++ name)
  metadata, ;; MIPS linkage name (for C++)
  metadata, ;; Reference to compile unit where defined
  i32,      ;; Line number where defined
  metadata, ;; Reference to type descriptor
  i1,       ;; True if the global is local to compile unit (static)
  i1        ;; True if the global is defined in the compile unit (not extern)
}

These descriptors provide debug information about functions, methods and subprograms. They provide details such as name, return types and the source location where the subprogram is defined.

!3 = metadata !{
  i32,     ;; Tag = 13 + LLVMDebugVersion (DW_TAG_lexical_block)
  metadata ;; Reference to context descriptor
}

These descriptors provide debug information about nested blocks within a subprogram. The array of member descriptors is used to define local variables and deeper nested blocks.

!4 = metadata !{
  i32,      ;; Tag = 36 + LLVMDebugVersion 
            ;; (DW_TAG_base_type)
  metadata, ;; Reference to context (typically a compile unit)
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to compile unit where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i64,      ;; Size in bits
  i64,      ;; Alignment in bits
  i64,      ;; Offset in bits
  i32,      ;; Flags
  i32       ;; DWARF type encoding
}

These descriptors define primitive types used in the code. Example int, bool and float. The context provides the scope of the type, which is usually the top level. Since basic types are not usually user defined the compile unit and line number can be left as NULL and 0. The size, alignment and offset are expressed in bits and can be 64 bit values. The alignment is used to round the offset when embedded in a composite type (example to keep float doubles on 64 bit boundaries.) The offset is the bit offset if embedded in a composite type.

The type encoding provides the details of the type. The values are typically one of the following:

DW_ATE_address       = 1
DW_ATE_boolean       = 2
DW_ATE_float         = 4
DW_ATE_signed        = 5
DW_ATE_signed_char   = 6
DW_ATE_unsigned      = 7
DW_ATE_unsigned_char = 8

!5 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Reference to context
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to compile unit where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i32,      ;; Size in bits
  i32,      ;; Alignment in bits
  i32,      ;; Offset in bits
  metadata  ;; Reference to type derived from
}

These descriptors are used to define types derived from other types. The value of the tag varies depending on the meaning. The following are possible tag values:

DW_TAG_formal_parameter = 5
DW_TAG_member           = 13
DW_TAG_pointer_type     = 15
DW_TAG_reference_type   = 16
DW_TAG_typedef          = 22
DW_TAG_const_type       = 38
DW_TAG_volatile_type    = 53
DW_TAG_restrict_type    = 55

DW_TAG_member is used to define a member of a composite type or subprogram. The type of the member is the derived type. DW_TAG_formal_parameter is used to define a member which is a formal argument of a subprogram.

DW_TAG_typedef is used to provide a name for the derived type.

DW_TAG_pointer_type,DW_TAG_reference_type, DW_TAG_const_type, DW_TAG_volatile_type and DW_TAG_restrict_type are used to qualify the derived type.

Derived type location can be determined from the compile unit and line number. The size, alignment and offset are expressed in bits and can be 64 bit values. The alignment is used to round the offset when embedded in a composite type (example to keep float doubles on 64 bit boundaries.) The offset is the bit offset if embedded in a composite type.

Note that the void * type is expressed as a llvm.dbg.derivedtype.type with tag of DW_TAG_pointer_type and NULL derived type.

!6 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Reference to context
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to compile unit where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i64,      ;; Size in bits
  i64,      ;; Alignment in bits
  i64,      ;; Offset in bits
  i32,      ;; Flags
  metadata, ;; Reference to type derived from
  metadata, ;; Reference to array of member descriptors
  i32       ;; Runtime languages
}

These descriptors are used to define types that are composed of 0 or more elements. The value of the tag varies depending on the meaning. The following are possible tag values:

DW_TAG_array_type       = 1
DW_TAG_enumeration_type = 4
DW_TAG_structure_type   = 19
DW_TAG_union_type       = 23
DW_TAG_vector_type      = 259
DW_TAG_subroutine_type  = 21
DW_TAG_inheritance      = 28

The vector flag indicates that an array type is a native packed vector.

The members of array types (tag = DW_TAG_array_type) or vector types (tag = DW_TAG_vector_type) are subrange descriptors, each representing the range of subscripts at that level of indexing.

The members of enumeration types (tag = DW_TAG_enumeration_type) are enumerator descriptors, each representing the definition of enumeration value for the set.

The members of structure (tag = DW_TAG_structure_type) or union (tag = DW_TAG_union_type) types are any one of the basic, derived or composite type descriptors, each representing a field member of the structure or union.

For C++ classes (tag = DW_TAG_structure_type), member descriptors provide information about base classes, static members and member functions. If a member is a derived type descriptor and has a tag of DW_TAG_inheritance, then the type represents a base class. If the member of is a global variable descriptor then it represents a static member. And, if the member is a subprogram descriptor then it represents a member function. For static members and member functions, getName() returns the members link or the C++ mangled name. getDisplayName() the simplied version of the name.

The first member of subroutine (tag = DW_TAG_subroutine_type) type elements is the return type for the subroutine. The remaining elements are the formal arguments to the subroutine.

Composite type location can be determined from the compile unit and line number. The size, alignment and offset are expressed in bits and can be 64 bit values. The alignment is used to round the offset when embedded in a composite type (as an example, to keep float doubles on 64 bit boundaries.) The offset is the bit offset if embedded in a composite type.

%llvm.dbg.subrange.type = type {
  i32,    ;; Tag = 33 + LLVMDebugVersion (DW_TAG_subrange_type)
  i64,    ;; Low value
  i64     ;; High value
}

These descriptors are used to define ranges of array subscripts for an array composite type. The low value defines the lower bounds typically zero for C/C++. The high value is the upper bounds. Values are 64 bit. High - low + 1 is the size of the array. If low == high the array will be unbounded.

!6 = metadata !{
  i32,      ;; Tag = 40 + LLVMDebugVersion 
            ;; (DW_TAG_enumerator)
  metadata, ;; Name
  i64       ;; Value
}

These descriptors are used to define members of an enumeration composite type, it associates the name to the value.

!7 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Context
  metadata, ;; Name
  metadata, ;; Reference to compile unit where defined
  i32,      ;; Line number where defined
  metadata  ;; Type descriptor
}

These descriptors are used to define variables local to a sub program. The value of the tag depends on the usage of the variable:

DW_TAG_auto_variable   = 256
DW_TAG_arg_variable    = 257
DW_TAG_return_variable = 258

An auto variable is any variable declared in the body of the function. An argument variable is any variable that appears as a formal argument to the function. A return variable is used to track the result of a function and has no source correspondent.

The context is either the subprogram or block where the variable is defined. Name the source variable name. Compile unit and line indicate where the variable was defined. Type descriptor defines the declared type of the variable.

LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to provide debug information at various points in generated code.

  void %llvm.dbg.stoppoint( uint, uint, metadata)

This intrinsic is used to provide correspondence between the source file and the generated code. The first argument is the line number (base 1), second argument is the column number (0 if unknown) and the third argument the source %llvm.dbg.compile_unit. Code following a call to this intrinsic will have been defined in close proximity of the line, column and file. This information holds until the next call to %lvm.dbg.stoppoint.