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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++ looks like.
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:
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:
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 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
metadata ;; List of enums types
metadata ;; List of retained types
metadata ;; List of subprograms
metadata ;; List of global variables
}
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. These descriptors are collected by a named metadata !llvm.dbg.cu. Compile unit descriptor keeps track of subprograms, global variables and type information.
!0 = metadata !{
i32, ;; Tag = 41 + LLVMDebugVersion
;; (DW_TAG_file_type)
metadata, ;; Source file name
metadata, ;; Source file directory (includes trailing slash)
metadata ;; Unused
}
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.
!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. All global variables are collected by named metadata !llvm.dbg.gv.
!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
i32, ;; Flags - Artifical, Private, Protected, Explicit, Prototyped.
i1, ;; isOptimized
Function *,;; Pointer to LLVM function
metadata, ;; Lists function template parameters
metadata ;; Function declaration descriptor
metadata ;; List of function variables
}
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 = 11 + LLVMDebugVersion (DW_TAG_lexical_block)
metadata,;; Reference to context descriptor
i32, ;; Line number
i32, ;; Column number
metadata,;; Reference to source file
i32 ;; Unique ID to identify blocks from a template function
}
This descriptor provides debug information about nested blocks within a subprogram. The line number and column numbers are used to dinstinguish two lexical blocks at same depth.
!3 = metadata !{
i32, ;; Tag = 11 + LLVMDebugVersion (DW_TAG_lexical_block)
metadata ;; Reference to the scope we're annotating with a file change
metadata,;; Reference to the file the scope is enclosed in.
}
This descriptor provides a wrapper around a lexical scope to handle file changes in the middle of a lexical block.
!4 = metadata !{
i32, ;; Tag = 36 + LLVMDebugVersion
;; (DW_TAG_base_type)
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
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 context 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
i32, ;; Flags to encode attributes, e.g. private
metadata, ;; Reference to type derived from
metadata, ;; (optional) Name of the Objective C property associated with
;; Objective-C an ivar
metadata, ;; (optional) Name of the Objective C property getter selector.
metadata, ;; (optional) Name of the Objective C property setter selector.
i32 ;; (optional) Objective C property attributes.
}
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 context 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 type derived from NULL.
!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. All enumeration type descriptors are collected by named metadata !llvm.dbg.enum.
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 context 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.
!42 = metadata !{
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 bounds are not included in generated debugging information.
!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, ;; 24 bit - Line number where defined
;; 8 bit - Argument number. 1 indicates 1st argument.
metadata, ;; Type descriptor
i32, ;; flags
metadata ;; (optional) Reference to inline location
}
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. Context 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 (e.g., variable). The first argument is metadata holding the alloca for the variable. The second argument is metadata containing a 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 metadata containing a 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(metadata !{i32 * %X}, metadata !0), !dbg !7
store i32 21, i32* %X, !dbg !8
%1 = bitcast i32* %Y to {}* ; <{}*> [#uses=1]
call void @llvm.dbg.declare(metadata !{i32 * %Y}, metadata !9), !dbg !10
store i32 22, i32* %Y, !dbg !11
%2 = bitcast i32* %Z to {}* ; <{}*> [#uses=1]
call void @llvm.dbg.declare(metadata !{i32 * %Z}, 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, 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(metadata, 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(metadata, 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
i32 0, ;; Virtuality attribute, e.g. pure virtual function
i32 0, ;; Index into virtual table for C++ methods
i32 0, ;; Type that holds virtual table.
i32 0, ;; Flags
i1 false, ;; True if this function is optimized
Function *, ;; Pointer to llvm::Function
null ;; Function template parameters
}
;;
;; 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}
Given the following as an example of C/C++ enumeration type:
enum Trees {
Spruce = 100,
Oak = 200,
Maple = 300
};
a C/C++ front-end would generate the following descriptors:
;;
;; Define composite type for enum Trees
;;
!2 = metadata !{
i32 524292, ;; Tag
metadata !1, ;; Context
metadata !"Trees", ;; Name
metadata !1, ;; File
i32 1, ;; Line number
i64 32, ;; Size in bits
i64 32, ;; Align in bits
i64 0, ;; Offset in bits
i32 0, ;; Flags
null, ;; Derived From type
metadata !3, ;; Elements
i32 0 ;; Runtime language
}
;;
;; Define the array of enumerators used by composite type Trees.
;;
!3 = metadata !{metadata !4, metadata !5, metadata !6}
;;
;; Define Spruce enumerator.
;;
!4 = metadata !{i32 524328, metadata !"Spruce", i64 100}
;;
;; Define Oak enumerator.
;;
!5 = metadata !{i32 524328, metadata !"Oak", i64 200}
;;
;; Define Maple enumerator.
;;
!6 = metadata !{i32 524328, metadata !"Maple", i64 300}
Objective C provides a simpler way to declare and define accessor methods using declared properties. The language provides features to declare a property and to let compiler synthesize accessor methods.
The debugger lets developer inspect Objective C interfaces and their instance variables and class variables. However, the debugger does not know anything about the properties defined in Objective C interfaces. The debugger consumes information generated by compiler in DWARF format. The format does not support encoding of Objective C properties. This proposal describes DWARF extensions to encode Objective C properties, which the debugger can use to let developers inspect Objective C properties.
Objective C properties exist separately from class members. A property can be defined only by "setter" and "getter" selectors, and be calculated anew on each access. Or a property can just be a direct access to some declared ivar. Finally it can have an ivar "automatically synthesized" for it by the compiler, in which case the property can be referred to in user code directly using the standard C dereference syntax as well as through the property "dot" syntax, but there is no entry in the @interface declaration corresponding to this ivar.
To facilitate debugging, these properties we will add a new DWARF TAG into the DW_TAG_structure_type definition for the class to hold the description of a given property, and a set of DWARF attributes that provide said description. The property tag will also contain the name and declared type of the property.
If there is a related ivar, there will also be a DWARF property attribute placed in the DW_TAG_member DIE for that ivar referring back to the property TAG for that property. And in the case where the compiler synthesizes the ivar directly, the compiler is expected to generate a DW_TAG_member for that ivar (with the DW_AT_artificial set to 1), whose name will be the name used to access this ivar directly in code, and with the property attribute pointing back to the property it is backing.
The following examples will serve as illustration for our discussion:
@interface I1 {
int n2;
}
@property int p1;
@property int p2;
@end
@implementation I1
@synthesize p1;
@synthesize p2 = n2;
@end
This produces the following DWARF (this is a "pseudo dwarfdump" output):
0x00000100: TAG_structure_type [7] *
AT_APPLE_runtime_class( 0x10 )
AT_name( "I1" )
AT_decl_file( "Objc_Property.m" )
AT_decl_line( 3 )
0x00000110 TAG_APPLE_property
AT_name ( "p1" )
AT_type ( {0x00000150} ( int ) )
0x00000120: TAG_APPLE_property
AT_name ( "p2" )
AT_type ( {0x00000150} ( int ) )
0x00000130: TAG_member [8]
AT_name( "_p1" )
AT_APPLE_property ( {0x00000110} "p1" )
AT_type( {0x00000150} ( int ) )
AT_artificial ( 0x1 )
0x00000140: TAG_member [8]
AT_name( "n2" )
AT_APPLE_property ( {0x00000120} "p2" )
AT_type( {0x00000150} ( int ) )
0x00000150: AT_type( ( int ) )
Note, the current convention is that the name of the ivar for an auto-synthesized property is the name of the property from which it derives with an underscore prepended, as is shown in the example. But we actually don't need to know this convention, since we are given the name of the ivar directly.
Also, it is common practice in ObjC to have different property declarations in the @interface and @implementation - e.g. to provide a read-only property in the interface,and a read-write interface in the implementation. In that case, the compiler should emit whichever property declaration will be in force in the current translation unit.
Developers can decorate a property with attributes which are encoded using DW_AT_APPLE_property_attribute.
@property (readonly, nonatomic) int pr;
Which produces a property tag:
TAG_APPLE_property [8]
AT_name( "pr" )
AT_type ( {0x00000147} (int) )
AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic)
The setter and getter method names are attached to the property using DW_AT_APPLE_property_setter and DW_AT_APPLE_property_getter attributes.
@interface I1
@property (setter=myOwnP3Setter:) int p3;
-(void)myOwnP3Setter:(int)a;
@end
@implementation I1
@synthesize p3;
-(void)myOwnP3Setter:(int)a{ }
@end
The DWARF for this would be:
0x000003bd: TAG_structure_type [7] *
AT_APPLE_runtime_class( 0x10 )
AT_name( "I1" )
AT_decl_file( "Objc_Property.m" )
AT_decl_line( 3 )
0x000003cd TAG_APPLE_property
AT_name ( "p3" )
AT_APPLE_property_setter ( "myOwnP3Setter:" )
AT_type( {0x00000147} ( int ) )
0x000003f3: TAG_member [8]
AT_name( "_p3" )
AT_type ( {0x00000147} ( int ) )
AT_APPLE_property ( {0x000003cd} )
AT_artificial ( 0x1 )
| TAG | Value |
|---|---|
| DW_TAG_APPLE_property | 0x4200 |
| Attribute | Value | Classes |
|---|---|---|
| DW_AT_APPLE_property | 0x3fed | Reference |
| DW_AT_APPLE_property_getter | 0x3fe9 | String |
| DW_AT_APPLE_property_setter | 0x3fea | String |
| DW_AT_APPLE_property_attribute | 0x3feb | Constant |
| Name | Value |
|---|---|
| DW_AT_APPLE_PROPERTY_readonly | 0x1 |
| DW_AT_APPLE_PROPERTY_readwrite | 0x2 |
| DW_AT_APPLE_PROPERTY_assign | 0x4 |
| DW_AT_APPLE_PROPERTY_retain | 0x8 |
| DW_AT_APPLE_PROPERTY_copy | 0x10 |
| DW_AT_APPLE_PROPERTY_nonatomic | 0x20 |
