Source Level Debugging with LLVM

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 DwarfDebug 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, the use of metadata avoids duplicated debugging 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 i32, i1, float, double, mdstring and mdnode.

!1 = metadata !{
  i32,   ;; A tag
  ...
}

The first field of a descriptor is always an i32 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 = 8 << 16 or 0x80000 or 524288.)

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 compilation unit. File descriptors are defined using this context.

!0 = metadata !{
  i32,       ;; Tag = 41 + LLVMDebugVersion 
             ;; (DW_TAG_file_type)
  metadata,  ;; Source file name
  metadata,  ;; Source file directory (includes trailing slash)
  metadata   ;; Reference to compile unit where defined
}

These descriptors contain information for a file. Global variables and top level functions would be defined using this context.k File descriptors also provide context for source line correspondence.

Each input file is encoded as a separate file descriptor in LLVM debugging information output. Each file descriptor would be defined using a compile unit.

!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 file 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 file 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)
  i32       ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual
  i32       ;; Index into a virtual function
  metadata, ;; indicates which base type contains the vtable pointer for the 
            ;; derived class
  i1        ;; isArtificial
  i1        ;; isOptimized
  Function *;; Pointer to LLVM function
}

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 file 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 file 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
  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 file 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 file 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.declare(metadata, metadata)

This intrinsic provides information about a local element (ex. variable.) The first argument is metadata holding alloca for the variable.. The second argument is the %llvm.dbg.variable containing the description of the variable.

  void %llvm.dbg.value(metadata, i64, metadata)

This intrinsic provides information when a user source variable is set to a new value. The first argument is the new value (wrapped as metadata). The second argument is the offset in the user source variable where the new value is written. The third argument is the %llvm.dbg.variable containing the description of the user source variable.

In many languages, the local variables in functions can have their lifetimes or scopes limited to a subset of a function. In the C family of languages, for example, variables are only live (readable and writable) within the source block that they are defined in. In functional languages, values are only readable after they have been defined. Though this is a very obvious concept, it is non-trivial to model in LLVM, because it has no notion of scoping in this sense, and does not want to be tied to a language's scoping rules.

In order to handle this, the LLVM debug format uses the metadata attached to llvm instructions to encode line number and scoping information. Consider the following C fragment, for example:

1.  void foo() {
2.    int X = 21;
3.    int Y = 22;
4.    {
5.      int Z = 23;
6.      Z = X;
7.    }
8.    X = Y;
9.  }

Compiled to LLVM, this function would be represented like this:

define void @foo() nounwind ssp {
entry:
  %X = alloca i32, align 4                        ; <i32*> [#uses=4]
  %Y = alloca i32, align 4                        ; <i32*> [#uses=4]
  %Z = alloca i32, align 4                        ; <i32*> [#uses=3]
  %0 = bitcast i32* %X to {}*                     ; <{}*> [#uses=1]
  call void @llvm.dbg.declare({}* %0, metadata !0), !dbg !7
  store i32 21, i32* %X, !dbg !8
  %1 = bitcast i32* %Y to {}*                     ; <{}*> [#uses=1]
  call void @llvm.dbg.declare({}* %1, metadata !9), !dbg !10
  store i32 22, i32* %Y, !dbg !11
  %2 = bitcast i32* %Z to {}*                     ; <{}*> [#uses=1]
  call void @llvm.dbg.declare({}* %2, metadata !12), !dbg !14
  store i32 23, i32* %Z, !dbg !15
  %tmp = load i32* %X, !dbg !16                   ; <i32> [#uses=1]
  %tmp1 = load i32* %Y, !dbg !16                  ; <i32> [#uses=1]
  %add = add nsw i32 %tmp, %tmp1, !dbg !16        ; <i32> [#uses=1]
  store i32 %add, i32* %Z, !dbg !16
  %tmp2 = load i32* %Y, !dbg !17                  ; <i32> [#uses=1]
  store i32 %tmp2, i32* %X, !dbg !17
  ret void, !dbg !18
}

declare void @llvm.dbg.declare({}*, metadata) nounwind readnone

!0 = metadata !{i32 459008, metadata !1, metadata !"X", 
                metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]
!1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
!2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo", 
               metadata !"foo", metadata !3, i32 1, metadata !4, 
               i1 false, i1 true}; [DW_TAG_subprogram ]
!3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c", 
                metadata !"/private/tmp", metadata !"clang 1.1", i1 true, 
                i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]
!4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0, 
                i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]
!5 = metadata !{null}
!6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0, 
                i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]
!7 = metadata !{i32 2, i32 7, metadata !1, null}
!8 = metadata !{i32 2, i32 3, metadata !1, null}
!9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3, 
                metadata !6}; [ DW_TAG_auto_variable ]
!10 = metadata !{i32 3, i32 7, metadata !1, null}
!11 = metadata !{i32 3, i32 3, metadata !1, null}
!12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5, 
                 metadata !6}; [ DW_TAG_auto_variable ]
!13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
!14 = metadata !{i32 5, i32 9, metadata !13, null}
!15 = metadata !{i32 5, i32 5, metadata !13, null}
!16 = metadata !{i32 6, i32 5, metadata !13, null}
!17 = metadata !{i32 8, i32 3, metadata !1, null}
!18 = metadata !{i32 9, i32 1, metadata !2, null}

This example illustrates a few important details about LLVM debugging information. In particular, it shows how the llvm.dbg.declare intrinsic and location information, which are attached to an instruction, are applied together to allow a debugger to analyze the relationship between statements, variable definitions, and the code used to implement the function.

call void @llvm.dbg.declare({}* %0, metadata !0), !dbg !7

The first intrinsic %llvm.dbg.declare encodes debugging information for the variable X. The metadata !dbg !7 attached to the intrinsic provides scope information for the variable X.

!7 = metadata !{i32 2, i32 7, metadata !1, null}
!1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
!2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", 
                metadata !"foo", metadata !"foo", metadata !3, i32 1, 
                metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]

Here !7 is metadata providing location information. It has four fields: line number, column number, scope, and original scope. The original scope represents inline location if this instruction is inlined inside a caller, and is null otherwise. In this example, scope is encoded by !1. !1 represents a lexical block inside the scope !2, where !2 is a subprogram descriptor. This way the location information attached to the intrinsics indicates that the variable X is declared at line number 2 at a function level scope in function foo.

Now lets take another example.

call void @llvm.dbg.declare({}* %2, metadata !12), !dbg !14

The second intrinsic %llvm.dbg.declare encodes debugging information for variable Z. The metadata !dbg !14 attached to the intrinsic provides scope information for the variable Z.

!13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
!14 = metadata !{i32 5, i32 9, metadata !13, null}

Here !14 indicates that Z is declared at line number 5 and column number 9 inside of lexical scope !13. The lexical scope itself resides inside of lexical scope !1 described above.

The scope information attached with each instruction provides a straightforward way to find instructions covered by a scope.

The C and C++ front-ends represent information about the program in a format that is effectively identical to DWARF 3.0 in terms of information content. This allows code generators to trivially support native debuggers by generating standard dwarf information, and contains enough information for non-dwarf targets to translate it as needed.

This section describes the forms used to represent C and C++ programs. Other languages could pattern themselves after this (which itself is tuned to representing programs in the same way that DWARF 3 does), or they could choose to provide completely different forms if they don't fit into the DWARF model. As support for debugging information gets added to the various LLVM source-language front-ends, the information used should be documented here.

The following sections provide examples of various C/C++ constructs and the debug information that would best describe those constructs.

Given the source files MySource.cpp and MyHeader.h located in the directory /Users/mine/sources, the following code:

#include "MyHeader.h"

int main(int argc, char *argv[]) {
  return 0;
}

a C/C++ front-end would generate the following descriptors:

...
;;
;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".
;;
!2 = metadata !{
  i32 524305,    ;; Tag
  i32 0,         ;; Unused
  i32 4,         ;; Language Id
  metadata !"MySource.cpp", 
  metadata !"/Users/mine/sources", 
  metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)", 
  i1 true,       ;; Main Compile Unit
  i1 false,      ;; Optimized compile unit
  metadata !"",  ;; Compiler flags
  i32 0}         ;; Runtime version

;;
;; Define the file for the file "/Users/mine/sources/MySource.cpp".
;;
!1 = metadata !{
  i32 524329,    ;; Tag
  metadata !"MySource.cpp", 
  metadata !"/Users/mine/sources", 
  metadata !2    ;; Compile unit
}

;;
;; Define the file for the file "/Users/mine/sources/Myheader.h"
;;
!3 = metadata !{
  i32 524329,    ;; Tag
  metadata !"Myheader.h"
  metadata !"/Users/mine/sources", 
  metadata !2    ;; Compile unit
}

...

llvm::Instruction provides easy access to metadata attached with an instruction. One can extract line number information encoded in LLVM IR using Instruction::getMetadata() and DILocation::getLineNumber().

 if (MDNode *N = I->getMetadata("dbg")) {  // Here I is an LLVM instruction
   DILocation Loc(N);                      // DILocation is in DebugInfo.h
   unsigned Line = Loc.getLineNumber();
   StringRef File = Loc.getFilename();
   StringRef Dir = Loc.getDirectory();
 }

Given an integer global variable declared as follows:

int MyGlobal = 100;

a C/C++ front-end would generate the following descriptors:

;;
;; Define the global itself.
;;
%MyGlobal = global int 100
...
;;
;; List of debug info of globals
;;
!llvm.dbg.gv = !{!0}

;;
;; Define the global variable descriptor.  Note the reference to the global
;; variable anchor and the global variable itself.
;;
!0 = metadata !{
  i32 524340,              ;; Tag
  i32 0,                   ;; Unused
  metadata !1,             ;; Context
  metadata !"MyGlobal",    ;; Name
  metadata !"MyGlobal",    ;; Display Name
  metadata !"MyGlobal",    ;; Linkage Name
  metadata !3,             ;; Compile Unit
  i32 1,                   ;; Line Number
  metadata !4,             ;; Type
  i1 false,                ;; Is a local variable
  i1 true,                 ;; Is this a definition
  i32* @MyGlobal           ;; The global variable
}

;;
;; Define the basic type of 32 bit signed integer.  Note that since int is an
;; intrinsic type the source file is NULL and line 0.
;;    
!4 = metadata !{
  i32 524324,              ;; Tag
  metadata !1,             ;; Context
  metadata !"int",         ;; Name
  metadata !1,             ;; File
  i32 0,                   ;; Line number
  i64 32,                  ;; Size in Bits
  i64 32,                  ;; Align in Bits
  i64 0,                   ;; Offset in Bits
  i32 0,                   ;; Flags
  i32 5                    ;; Encoding
}

Given a function declared as follows:

int main(int argc, char *argv[]) {
  return 0;
}

a C/C++ front-end would generate the following descriptors:

;;
;; Define the anchor for subprograms.  Note that the second field of the
;; anchor is 46, which is the same as the tag for subprograms
;; (46 = DW_TAG_subprogram.)
;;
!6 = metadata !{
  i32 524334,        ;; Tag
  i32 0,             ;; Unused
  metadata !1,       ;; Context
  metadata !"main",  ;; Name
  metadata !"main",  ;; Display name
  metadata !"main",  ;; Linkage name
  metadata !1,       ;; File
  i32 1,             ;; Line number
  metadata !4,       ;; Type
  i1 false,          ;; Is local 
  i1 true            ;; Is definition
}
;;
;; Define the subprogram itself.
;;
define i32 @main(i32 %argc, i8** %argv) {
...
}

The following are the basic type descriptors for C/C++ core types:

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"bool",  ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 2              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"char",  ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 6              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned char", 
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 8              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"short int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 16,            ;; Size in Bits
  i64 16,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"short unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 16,            ;; Size in Bits
  i64 16,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"int",   ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"long long int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"long long unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"float",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 4              ;; Encoding
}

!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"double",;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 4              ;; Encoding
}

Given the following as an example of C/C++ derived type:

typedef const int *IntPtr;

a C/C++ front-end would generate the following descriptors:

;;
;; Define the typedef "IntPtr".
;;
!2 = metadata !{
  i32 524310,          ;; Tag
  metadata !1,         ;; Context
  metadata !"IntPtr",  ;; Name
  metadata !3,         ;; File
  i32 0,               ;; Line number
  i64 0,               ;; Size in bits
  i64 0,               ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !4          ;; Derived From type
}

;;
;; Define the pointer type.
;;
!4 = metadata !{
  i32 524303,          ;; Tag
  metadata !1,         ;; Context
  metadata !"",        ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 64,              ;; Size in bits
  i64 64,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !5          ;; Derived From type
}
;;
;; Define the const type.
;;
!5 = metadata !{
  i32 524326,          ;; Tag
  metadata !1,         ;; Context
  metadata !"",        ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 32,              ;; Size in bits
  i64 32,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !6          ;; Derived From type
}
;;
;; Define the int type.
;;
!6 = metadata !{
  i32 524324,          ;; Tag
  metadata !1,         ;; Context
  metadata !"int",     ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 32,              ;; Size in bits
  i64 32,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  5                    ;; Encoding
}

Given the following as an example of C/C++ struct type:

struct Color {
  unsigned Red;
  unsigned Green;
  unsigned Blue;
};

a C/C++ front-end would generate the following descriptors:

;;
;; Define basic type for unsigned int.
;;
!5 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}
;;
;; Define composite type for struct Color.
;;
!2 = metadata !{
  i32 524307,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Color", ;; Name
  metadata !1,       ;; Compile unit
  i32 1,             ;; Line number
  i64 96,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 0,             ;; Offset in bits
  i32 0,             ;; Flags
  null,              ;; Derived From
  metadata !3,       ;; Elements
  i32 0              ;; Runtime Language
}

;;
;; Define the Red field.
;;
!4 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Red",   ;; Name
  metadata !1,       ;; File
  i32 2,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 0,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the Green field.
;;
!6 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Green", ;; Name
  metadata !1,       ;; File
  i32 3,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 32,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the Blue field.
;;
!7 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Blue",  ;; Name
  metadata !1,       ;; File
  i32 4,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 64,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the array of fields used by the composite type Color.
;;
!3 = metadata !{metadata !4, metadata !6, metadata !7}