(Presented in COSCUP 2022) Debug information is a mapping between the original source code and low-level binary locations. It provides developers powerful insights to diagnose problems (via debuggers) in their code and acts as one of the most important foundations for modern software development. Furthermore, in recent years, we are seeing increasing demands of high quality debug information for highly optimized applications that are otherwise “un-debuggable”. For instance, debugging unoptimized games is generally not feasible since it’s likely to miss every single frame. In this talk, we are going to introduce how debug information works and how compilers generate proper debug info even with extensive levels of optimization enabled. At the end of this talk, you will gain insights into the structure of debug information and learn key compiler engineering knowledge on generating high quality debug info for critical, highly optimized software.

1. Debug INFormation
And Where They Come From
Min-Yih “Min” Hsu, COSCUP 2022

2. Debugging
2
Execution
Program

3. Debugging
2
Execution
Program
Pause

4. Debugging
2
Execution
Program
Pause
Debugger
Developers

5. Debugging
2
Execution
Program
Pause
Debugger
Developers
Source Code
• Source location (e.g. line number)

6. Debugging
2
Execution
Program
Pause
Debugger
Developers
Source Code
• Source location (e.g. line number)
• Variable values

7. Debugging
2
Execution
Program
Pause
Debugger
Developers
Source Code
• Source location (e.g. line number)
• Variable values
• Function call hierarchy

8. Languages & Debugging
3

9. Languages & Debugging
3

10. Languages & Debugging
4
Run & Debug on
Native Binaries

11. Debug Symbols / Info
5
Program
Binary

12. Debug Symbols / Info
5
Program
Binary
Debug Info

13. Debug Symbols / Info
5
Program
Binary
Debug Info
Source Code
Mapping

14. Debug Symbols / Info
5
Program
Binary
Debug Info
Debugger
Consume
Source Code
Mapping

15. Debug Symbols / Info
5
Program
Binary
Debug Info
Debugger
Consume
Source Code
Mapping
Pro
fi
ler

16. Today’s Topic
6
Program
Binary
Debug Info
Source Code
Compiler

17. Today’s Topic
6
Program
Binary
Debug Info
Source Code
Compiler
Debug Info…

18. Today’s Topic
6
Program
Binary
Debug Info
Source Code
Compiler
Debug Info…
… And Where They Come From

19. Learning Debug Info

20. Example Input
8
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
*Host Architecture: x86_64

21. Example Input
8
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
$ cc -g -c demo.c -o demo.o
*Host Architecture: x86_64

22. Example Input
8
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
$ cc -g -c demo.c -o demo.o
$ llvm-dva demo.o …
*Host Architecture: x86_64

23. llvm-dva: Visualizing Debug Info
9
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

24. llvm-dva: Visualizing Debug Info
10
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

25. llvm-dva: Visualizing Debug Info
10
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

26. llvm-dva: Visualizing Debug Info
11
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

27. llvm-dva: Visualizing Debug Info
11
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

28. llvm-dva: Visualizing Debug Info
12
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Line numbers & instructions

29. llvm-dva: Visualizing Debug Info
12
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Line numbers & instructions

30. llvm-dva: Visualizing Debug Info
12
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Where ‘c’ is stored
Line numbers & instructions

31. llvm-dva: Visualizing Debug Info
12
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Where ‘c’ is stored
PC Address Line Number
Assembly
(NOT stored in debug info)
0x1C 5 cmpl $0x0,-0x18(%rbp)
0x20 5 je 0xc
Line numbers & instructions

32. llvm-dva: Visualizing Debug Info
13
{CompileUnit} ‘demo.c'
....
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
....
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
3 {Line}
{Code} 'endbr64'
{Code} 'pushq %rbp'
....
4 {Line}
{Code} 'movl $0x0, -0x8(%rbp)'
{Code} 'movl $0x0, -0x4(%rbp)'
5 {Line}
{Code} 'cmpl $0x0, -0x18(%rbp)'
{Code} 'je 0xc’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

33. llvm-dva: Visualizing Debug Info
14
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Variable locations

34. llvm-dva: Visualizing Debug Info
14
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Variable locations

35. llvm-dva: Visualizing Debug Info
15
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Variable locations
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
Current Stack Frame
Lo Address
Hi Address

36. llvm-dva: Visualizing Debug Info
15
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Variable locations
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
Current Stack Frame
Lo Address
Hi Address
fbreg

37. llvm-dva: Visualizing Debug Info
15
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Variable locations
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
-24
Current Stack Frame
Lo Address
Hi Address
fbreg

38. llvm-dva: Visualizing Debug Info
15
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Variable locations
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
-24
-36
Current Stack Frame
Lo Address
Hi Address
fbreg

39. llvm-dva: Visualizing Debug Info
15
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Variable locations
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
-24
-36
-40
Current Stack Frame
Lo Address
Hi Address
fbreg

40. llvm-dva: Visualizing Debug Info
16
{CompileUnit} ‘demo.c'
1 {Struct} 'Point'
1 {Member} public 'x' -> 'int'
{Location}
{Entry} offset 0
1 {Member} public 'y' -> 'int'
{Location}
{Entry} offset 4
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Type Layout
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
Current Stack Frame
Lo Address
Hi Address

41. llvm-dva: Visualizing Debug Info
16
{CompileUnit} ‘demo.c'
1 {Struct} 'Point'
1 {Member} public 'x' -> 'int'
{Location}
{Entry} offset 0
1 {Member} public 'y' -> 'int'
{Location}
{Entry} offset 4
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Type Layout
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
Current Stack Frame
Lo Address
Hi Address

42. llvm-dva: Visualizing Debug Info
16
{CompileUnit} ‘demo.c'
1 {Struct} 'Point'
1 {Member} public 'x' -> 'int'
{Location}
{Entry} offset 0
1 {Member} public 'y' -> 'int'
{Location}
{Entry} offset 4
3 {Function} extern not_inlined 'foo' -> 'int'
3 {Parameter} 'k' -> 'int'
{Location}
{Entry} fbreg -36
3 {Parameter} 'c' -> 'int'
{Location}
{Entry} fbreg -40
4 {Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
Type Layout
Variable ‘point’
Variable ‘k’
Variable ‘c’
Previous Frame Ptr
Return Address
Current Stack Frame
Lo Address
Hi Address
Field ‘x’
Field ‘y’
+0
+4

43. Other Common Debug Info Properties
17

44. Other Common Debug Info Properties
• Scopes
17

45. Other Common Debug Info Properties
• Scopes
• Advanced type information
• Type aliases
• Type hierarchy
17

46. Debug Info Standards
18

47. Debug Info Standards
18
DWARF CodeView

48. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix

49. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix
• Supported by GNU & LLVM toolchain

50. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix
• Supported by GNU & LLVM toolchain
• Container formats: DWO, DWP, dSYM

51. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix
• Supported by GNU & LLVM toolchain
• Container formats: DWO, DWP, dSYM
• Default format in Windows (MSVC)

52. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix
• Supported by GNU & LLVM toolchain
• Container formats: DWO, DWP, dSYM
• Default format in Windows (MSVC)
• Supported by MSVC & LLVM toolchain

53. Debug Info Standards
18
DWARF CodeView
• Default format in Linux, Apple platforms,
most of the Unix
• Supported by GNU & LLVM toolchain
• Container formats: DWO, DWP, dSYM
• Default format in Windows (MSVC)
• Supported by MSVC & LLVM toolchain
• Container format: PDB

54. 19
DWARF CodeView
Abstraction
llvm-dva
Logical View

55. Generating Debug Info

56. Compilation Pipeline: A Crash Course
21
Source Code
Native Code
(e.g. *.o
fi
les)

57. Compilation Pipeline: A Crash Course
21
Source Code
AST
Parse
Native Code
(e.g. *.o
fi
les)

58. Compilation Pipeline: A Crash Course
21
Source Code
AST
Parse
Intermediate
Representation
(IR)
Native Code
(e.g. *.o
fi
les)

59. Compilation Pipeline: A Crash Course
21
Source Code
AST
Parse
Intermediate
Representation
(IR)
Another
Intermediate
Representation
Native Code
(e.g. *.o
fi
les)

60. Debug Info in a Compilation Pipeline: Highlights
22
Source Code
AST
Parse
Intermediate
Representation
(IR)
Another
Intermediate
Representation
Native Code
(e.g. *.o
fi
les)

61. Debug Info in a Compilation Pipeline: Highlights
22
Source Code
AST
Parse
Intermediate
Representation
(IR)
Another
Intermediate
Representation
Native Code
(e.g. *.o
fi
les)
• How to “carry” debug info in IR ?

62. Debug Info in a Compilation Pipeline: Highlights
22
Source Code
AST
Parse
Intermediate
Representation
(IR)
Another
Intermediate
Representation
Native Code
(e.g. *.o
fi
les)
• How to “carry” debug info in IR ?
• Correctly translate from source to debug info in IR

63. Debug Info in a Compilation Pipeline: Highlights
22
Source Code
AST
Parse
Intermediate
Representation
(IR)
Another
Intermediate
Representation
Native Code
(e.g. *.o
fi
les)
• How to “carry” debug info in IR ?
• Correctly translate from source to debug info in IR
• Preserve debug info across transformations (e.g. optimizations)

64. Case Study: LLVM / Clang
23
void foo() {
int x = 9;
int y = 4;
}
C Source (foo.c)

65. Case Study: LLVM / Clang
23
define void @foo() {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1
store i32 4, i32* %2
ret void
}
void foo() {
int x = 9;
int y = 4;
}
C Source (foo.c)
LLVM IR (foo.ll)
$ clang -emit-llvm -S foo.c -o foo.ll

66. Case Study: LLVM / Clang
23
define void @foo() {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1
store i32 4, i32* %2
ret void
}
void foo() {
int x = 9;
int y = 4;
}
C Source (foo.c)
LLVM IR (foo.ll)
$ clang -emit-llvm -S foo.c -o foo.ll
Allocating stack space

67. Case Study: LLVM / Clang
23
define void @foo() {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1
store i32 4, i32* %2
ret void
}
void foo() {
int x = 9;
int y = 4;
}
C Source (foo.c)
LLVM IR (foo.ll)
$ clang -emit-llvm -S foo.c -o foo.ll

68. 24
$ clang -g -emit-llvm -S foo.c -o foo.ll

69. Debug Info in LLVM IR
24
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR (foo.ll)
$ clang -g -emit-llvm -S foo.c -o foo.ll

70. Debug Info in LLVM IR
25
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1, !dbg !13
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
void foo() {
int x = 9;
int y = 4;
}
C Source
1
2
3
4
LLVM IR

71. Debug Info in LLVM IR
25
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1, !dbg !13
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
void foo() {
int x = 9;
int y = 4;
}
C Source
1
2
3
4
LLVM IR

72. Debug Info in LLVM IR
25
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1, !dbg !13
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
void foo() {
int x = 9;
int y = 4;
}
C Source
1
2
3
4
LLVM IR
!13 = !DILocation(line: 2, column: 7, ...) At the bottom of IR
fi
le

73. Debug Info in LLVM IR
25
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
store i32 9, i32* %1, !dbg !13
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
void foo() {
int x = 9;
int y = 4;
}
C Source
1
2
3
4
LLVM IR
!13 = !DILocation(line: 2, column: 7, ...)
!15 = !DILocation(line: 3, column: 7, ...)
At the bottom of IR
fi
le

74. Debug Info in LLVM IR
26
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR

75. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR

76. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)

77. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)

78. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)
Associated

79. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)
Associated
representing variable’s location during runtime

80. Debug Info in LLVM IR
27
define void @foo() !dbg !7 {
%1 = alloca i32
%2 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …), !dbg !13
store i32 9, i32* %1, !dbg !13
call void @llvm.dbg.declare(metadata i32* %2, metadata !14, …), !dbg !15
store i32 4, i32* %2, !dbg !15
ret void, !dbg !16
}
LLVM IR
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)
!14 = !DILocalVariable(name: "y", ..., line: 3, ...)

81. Visualizing Debug Info
28
$ cc -g -c foo.c -o foo.o
$ llvm-dva foo.o …

82. Visualizing Debug Info
29
{CompileUnit} ‘foo.c’
1 {Function} extern not_inlined ‘foo’…
2 {Variable} 'x' -> 'int'
{Location}
{Entry} fbreg -4
3 {Variable} 'y' -> 'int'
{Location}
{Entry} fbreg -8
1 {Line}
{Code} 'pushq %rbp'
{Code} 'movq %rsp, %rbp'
2 {Line}
{Code} 'movl $0x9, -0x4(%rbp)'
3 {Line}
{Code} 'movl $0x4, -0x8(%rbp)'
4 {Line}
{Code} 'popq %rbp'
{Code} 'retq'
4 {Line}

83. Visualizing Debug Info
29
{CompileUnit} ‘foo.c’
1 {Function} extern not_inlined ‘foo’…
2 {Variable} 'x' -> 'int'
{Location}
{Entry} fbreg -4
3 {Variable} 'y' -> 'int'
{Location}
{Entry} fbreg -8
1 {Line}
{Code} 'pushq %rbp'
{Code} 'movq %rsp, %rbp'
2 {Line}
{Code} 'movl $0x9, -0x4(%rbp)'
3 {Line}
{Code} 'movl $0x4, -0x8(%rbp)'
4 {Line}
{Code} 'popq %rbp'
{Code} 'retq'
4 {Line}
store i32 9, i32* %1, !dbg !13
!13 = !DILocation(line: 2, column: 7, ...)

84. Visualizing Debug Info
29
{CompileUnit} ‘foo.c’
1 {Function} extern not_inlined ‘foo’…
2 {Variable} 'x' -> 'int'
{Location}
{Entry} fbreg -4
3 {Variable} 'y' -> 'int'
{Location}
{Entry} fbreg -8
1 {Line}
{Code} 'pushq %rbp'
{Code} 'movq %rsp, %rbp'
2 {Line}
{Code} 'movl $0x9, -0x4(%rbp)'
3 {Line}
{Code} 'movl $0x4, -0x8(%rbp)'
4 {Line}
{Code} 'popq %rbp'
{Code} 'retq'
4 {Line}
store i32 9, i32* %1, !dbg !13
!13 = !DILocation(line: 2, column: 7, ...)

85. Visualizing Debug Info
29
{CompileUnit} ‘foo.c’
1 {Function} extern not_inlined ‘foo’…
2 {Variable} 'x' -> 'int'
{Location}
{Entry} fbreg -4
3 {Variable} 'y' -> 'int'
{Location}
{Entry} fbreg -8
1 {Line}
{Code} 'pushq %rbp'
{Code} 'movq %rsp, %rbp'
2 {Line}
{Code} 'movl $0x9, -0x4(%rbp)'
3 {Line}
{Code} 'movl $0x4, -0x8(%rbp)'
4 {Line}
{Code} 'popq %rbp'
{Code} 'retq'
4 {Line}
%1 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …)
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)
store i32 9, i32* %1, !dbg !13
!13 = !DILocation(line: 2, column: 7, ...)

86. Visualizing Debug Info
29
{CompileUnit} ‘foo.c’
1 {Function} extern not_inlined ‘foo’…
2 {Variable} 'x' -> 'int'
{Location}
{Entry} fbreg -4
3 {Variable} 'y' -> 'int'
{Location}
{Entry} fbreg -8
1 {Line}
{Code} 'pushq %rbp'
{Code} 'movq %rsp, %rbp'
2 {Line}
{Code} 'movl $0x9, -0x4(%rbp)'
3 {Line}
{Code} 'movl $0x4, -0x8(%rbp)'
4 {Line}
{Code} 'popq %rbp'
{Code} 'retq'
4 {Line}
%1 = alloca i32
call void @llvm.dbg.declare(metadata i32* %1, metadata !11, …)
!11 = !DILocalVariable(name: "x", ..., line: 2, ...)
store i32 9, i32* %1, !dbg !13
!13 = !DILocation(line: 2, column: 7, ...)

87. 30

88. 30
…And Developers Lives a Happy Debu
gg
ing Life Ever Since

89. 30
…And Developers Lives a Happy Debu
gg
ing Life Ever Since

90. Debug Info in Optimized Binaries

91. Debug Info in Optimized Binaries
32
$ cc -g -O2 -c foo.c

92. Debug Info in Optimized Binaries
32
$ cc -g -O2 -c foo.c

93. Debug Info in Optimized Binaries
32
$ cc -g -O2 -c foo.c
Why?

94. Debugging Optimized Programs
33

95. Debugging Optimized Programs
• Games
• Di
ffi
cult (if not nearly impossible) to debug low-FPS games
33

96. Debugging Optimized Programs
• Games
• Di
ffi
cult (if not nearly impossible) to debug low-FPS games
• Embedded systems
• Size optimization is usually a hard requirement
33

97. Debugging Optimized Programs
• Games
• Di
ffi
cult (if not nearly impossible) to debug low-FPS games
• Embedded systems
• Size optimization is usually a hard requirement
• Easier debugging on release binaries
• E.g. Using core
fi
les directly from customers
33

98. Recall: Early Example Code
34
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
$ clang -g foo.c -emit-llvm -S

99. 35
define i32 @foo(i32 %0, i32 %1) !dbg !7 {
%3 = alloca i32
%4 = alloca i32
%5 = alloca %struct.Point
store i32 %0, i32* %3
call void @llvm.dbg.declare(metadata i32* %3, metadata !12, ...), !dbg !13
store i32 %1, i32* %4
call void @llvm.dbg.declare(metadata i32* %4, metadata !14, ...), !dbg !15
call void @llvm.dbg.declare(metadata %struct.Point* %5, metadata !16, ...), !dbg !21
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

100. 36
define i32 @foo(i32 %0, i32 %1) !dbg !7 {
%3 = alloca i32
%4 = alloca i32
%5 = alloca %struct.Point
store i32 %0, i32* %3
call void @llvm.dbg.declare(metadata i32* %3, metadata !12, ...), !dbg !13
store i32 %1, i32* %4
call void @llvm.dbg.declare(metadata i32* %4, metadata !14, ...), !dbg !15
call void @llvm.dbg.declare(metadata %struct.Point* %5, metadata !16, ...), !dbg !21
...
}
Allocating stack space for ‘k’, ‘c’, and ‘point’
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

101. 37
define i32 @foo(i32 %0, i32 %1) !dbg !7 {
%3 = alloca i32
%4 = alloca i32
%5 = alloca %struct.Point
store i32 %0, i32* %3
call void @llvm.dbg.declare(metadata i32* %3, metadata !12, ...), !dbg !13
store i32 %1, i32* %4
call void @llvm.dbg.declare(metadata i32* %4, metadata !14, ...), !dbg !15
call void @llvm.dbg.declare(metadata %struct.Point* %5, metadata !16, ...), !dbg !21
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Associating stack spaces with the source variables

102. Optimized Example Code
38
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
$ clang -O2 foo.c -emit-llvm -S

103. 39
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

104. 39
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Values are not put on stack anymore!

105. 40
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Prompt:
What is the runtime location/value of source
variable ‘point’ ?

106. 40
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Prompt:
What is the runtime location/value of source
variable ‘point’ ?
(gdb) print point
<your answer>
(gdb)

107. 40
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Prompt:
What is the runtime location/value of source
variable ‘point’ ?
(gdb) print point
<your answer>
(gdb)
?

108. 41
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Which instructions should I annotate
with source line 4, 6, and 7 ?

109. 41
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Which instructions should I annotate
with source line 4, 6, and 7 ?
?

110. Other Common Challenges
42
Updating Scopes
Function Inlining

111. Preserving Debug Info in Optimized Code
• Most modern compilers preserve debug information as part of the code
transformations (e.g. optimizations)
43

112. Preserving Debug Info in Optimized Code
• Most modern compilers preserve debug information as part of the code
transformations (e.g. optimizations)
• Challenges
43

113. Preserving Debug Info in Optimized Code
• Most modern compilers preserve debug information as part of the code
transformations (e.g. optimizations)
• Challenges
• It’s easy for compiler developers to forget to handle debug info
43

114. Preserving Debug Info in Optimized Code
• Most modern compilers preserve debug information as part of the code
transformations (e.g. optimizations)
• Challenges
• It’s easy for compiler developers to forget to handle debug info
• It’s not possible to (faithfully) map optimized code back to source
locations in every cases
43

115. Preserving Debug Info in Optimized Code
• Most modern compilers preserve debug information as part of the code
transformations (e.g. optimizations)
• Challenges
• It’s easy for compiler developers to forget to handle debug info
• It’s not possible to (faithfully) map optimized code back to source
locations in every cases
• Debug info in optimized binaries is preserved on a best-e
ff
ort basis
43

116. Case Study:
How LLVM Mitigates These Issues

117. Recall: Optimized Code Example
45
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Prompt:
What is the runtime location/value of source
variable ‘point’ ?
(gdb) print point
<your answer>
(gdb)
?

118. Optimized Example Code w/ Debug Info
46
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
$ clang -O2 -g foo.c -emit-llvm -S

119. Preserving Variable Locations / Values in LLVM
47
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
call void @llvm.dbg.value(metadata i32 %1, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32,32))
call void @llvm.dbg.value(metadata i32 %4, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0,32))
%5 = add nsw i32 %4, %1
ret i32 %5
}

120. Preserving Variable Locations / Values in LLVM
48
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
}

121. Preserving Variable Locations / Values in LLVM
48
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
} !13 = !DILocalVariable(name: "k", arg: 1, ..., line: 3, type: !11)
!14 = !DILocalVariable(name: "c", arg: 2, ..., line: 3, type: !11)

122. Preserving Variable Locations / Values in LLVM
49
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
} !13 = !DILocalVariable(name: "k", arg: 1, ..., line: 3, type: !11)
!14 = !DILocalVariable(name: "c", arg: 2, ..., line: 3, type: !11)
LLVM Intrinsic Name Description
llvm.dbg.declare
Specify the (memory) location for a source variable.
Only a single occurrence per source variable is allowed
llvm.dbg.value
Designate a (runtime) value to a source variable
Can have multiple occurrences for a source variable
(akin to updating di
ff
erent values on a source variable)

123. Preserving Variable Locations / Values in LLVM
49
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
} !13 = !DILocalVariable(name: "k", arg: 1, ..., line: 3, type: !11)
!14 = !DILocalVariable(name: "c", arg: 2, ..., line: 3, type: !11)
LLVM Intrinsic Name Description
llvm.dbg.declare
Specify the (memory) location for a source variable.
Only a single occurrence per source variable is allowed
llvm.dbg.value
Designate a (runtime) value to a source variable
Can have multiple occurrences for a source variable
(akin to updating di
ff
erent values on a source variable)

124. Preserving Variable Locations / Values in LLVM
49
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
} !13 = !DILocalVariable(name: "k", arg: 1, ..., line: 3, type: !11)
!14 = !DILocalVariable(name: "c", arg: 2, ..., line: 3, type: !11)
LLVM Intrinsic Name Description
llvm.dbg.declare
Specify the (memory) location for a source variable.
Only a single occurrence per source variable is allowed
llvm.dbg.value
Designate a (runtime) value to a source variable
Can have multiple occurrences for a source variable
(akin to updating di
ff
erent values on a source variable)

125. Preserving Variable Locations / Values in LLVM
49
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata …)
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
…
} !13 = !DILocalVariable(name: "k", arg: 1, ..., line: 3, type: !11)
!14 = !DILocalVariable(name: "c", arg: 2, ..., line: 3, type: !11)

126. Preserving Variable Locations / Values in LLVM
50
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

127. Preserving Variable Locations / Values in LLVM
50
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)

128. Preserving Variable Locations / Values in LLVM
50
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
Why this is not “point.x” or “point.y” ?

129. Preserving Variable Locations / Values in LLVM
50
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
Why this is not “point.x” or “point.y” ?

130. Preserving Variable Locations / Values in LLVM
51
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A fragment of source variable “point” has value 0.

131. Preserving Variable Locations / Values in LLVM
51
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A fragment of source variable “point” has value 0.

132. Preserving Variable Locations / Values in LLVM
51
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A fragment of source variable “point” has value 0.

133. Preserving Variable Locations / Values in LLVM
51
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A fragment of source variable “point” has value 0.

134. Preserving Variable Locations / Values in LLVM
52
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A 32-bit, o
ff
set 0 fragment of source variable “point” has value 0.

135. Preserving Variable Locations / Values in LLVM
52
define i32 @foo(i32 %0, i32 %1) {
...
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
...
}
!15 = !DILocalVariable(name: "point", ..., line: 4, type: !16)
A 32-bit, o
ff
set 0 fragment of source variable “point” has value 0.
(i.e. the “point.x”
fi
eld)

136. Preserving Variable Locations / Values in LLVM
53
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
call void @llvm.dbg.value(metadata i32 %1, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32,32))
call void @llvm.dbg.value(metadata i32 %4, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0,32))
%5 = add nsw i32 %4, %1
ret i32 %5
}
First, “point.x” & “point.y” were initialized to zeros…

137. Preserving Variable Locations / Values in LLVM
54
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
call void @llvm.dbg.value(metadata i32 %1, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32,32))
call void @llvm.dbg.value(metadata i32 %4, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0,32))
%5 = add nsw i32 %4, %1
ret i32 %5
}
After these two instructions…

138. Preserving Variable Locations / Values in LLVM
55
define i32 @foo(i32 %0, i32 %1) {
call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 %1, metadata !14, metadata !DIExpression())
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32))
call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 32))
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
call void @llvm.dbg.value(metadata i32 %1, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 32,32))
call void @llvm.dbg.value(metadata i32 %4, metadata !15, metadata !DIExpression(DW_OP_LLVM_fragment, 0,32))
%5 = add nsw i32 %4, %1
ret i32 %5
}
“point.x” & “point.y” now have values %4 and %1, respectively

139. Recall: Optimized Code Example
56
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
Which instructions should I annotate
with source line 4, 6, and 7 ?

140. Preserving Debug Locations in LLVM
57
Principles
Actions

141. Preserving Debug Locations in LLVM
57
Principles
Actions
Keep

142. Preserving Debug Locations in LLVM
57
Principles
Actions
Keep Merge

143. Preserving Debug Locations in LLVM
57
Principles
Actions
Keep Merge Delete

144. Preserving Debug Locations in LLVM
57
Principles
• Don’t create misleading debug locations that are only correct in some cases
Actions
Keep Merge Delete

145. Preserving Debug Locations in LLVM
57
Principles
• Don’t create misleading debug locations that are only correct in some cases
• If you’re not sure, just drop the debug locations
Actions
Keep Merge Delete

146. Preserving Debug Locations in LLVM
57
Principles
• Don’t create misleading debug locations that are only correct in some cases
• If you’re not sure, just drop the debug locations
• Otherwise, preserve as much debug locations as possible
Actions
Keep Merge Delete

147. Debug Locations in our Example Code
58
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

148. Debug Locations in our Example Code
59
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10

149. Debug Locations in our Example Code
59
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
line 6 will only hit conditionally

150. Debug Locations in our Example Code
59
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
line 6 will only hit conditionally
“Don’t create misleading debug locations
that are only correct in some cases”

151. Debug Locations in our Example Code
59
define i32 @foo(i32 %0, i32 %1) {
%3 = icmp eq i32 %1, 0
%4 = select i1 %3, i32 0, i32 %0
%5 = add nsw i32 %4, %1
ret i32 %5
}
struct Point { int x, y; };
int foo(int k, int c) {
struct Point point = {x: 0, y: 0};
if (c) {
point.x = k;
point.y = c;
}
return point.x + point.y;
}
1
2
3
4
5
6
7
8
9
10
line 6 will only hit conditionally
“Don’t create misleading debug locations
that are only correct in some cases”
Delete

152. Preserving Debug Locations in LLVM
60
Prantl and Kumar, US LLVM Dev Meeting 2020
Guidelines for updating debug locations in code transformations
https://tinyurl.com/llvmdebuginfo
Full write-up:

153. Preserving Debug Info in LLVM: Automatically
61

154. Preserving Debug Info in LLVM: Automatically
61
• Common transformation APIs will help you to keep / merge debug
location underlying

155. Preserving Debug Info in LLVM: Automatically
61
• Common transformation APIs will help you to keep / merge debug
location underlying
• e.g. Replace All Uses With (RAUW)

156. Preserving Debug Info in LLVM: Automatically
61
• Common transformation APIs will help you to keep / merge debug
location underlying
• e.g. Replace All Uses With (RAUW)
• Handy debug info utilities to make debug info manipulations easier

157. Preserving Debug Info in LLVM: Automatically
61
• Common transformation APIs will help you to keep / merge debug
location underlying
• e.g. Replace All Uses With (RAUW)
• Handy debug info utilities to make debug info manipulations easier
• salvageDebugInfo helps you to generate llvm.dbg.value intrinsics

158. Preserving Debug Info in LLVM: Automatically
61
• Common transformation APIs will help you to keep / merge debug
location underlying
• e.g. Replace All Uses With (RAUW)
• Handy debug info utilities to make debug info manipulations easier
• salvageDebugInfo helps you to generate llvm.dbg.value intrinsics
• Instruction::applyMergedLocation helps you to merge debug
locations

159. Summary
62

160. Summary
• We learned how line source locations (e.g. line number) and variable
locations are represented in debug info
62

161. Summary
• We learned how line source locations (e.g. line number) and variable
locations are represented in debug info
• We learned how debug info is stored in LLVM IR
62

162. Summary
• We learned how line source locations (e.g. line number) and variable
locations are represented in debug info
• We learned how debug info is stored in LLVM IR
• The challenges of debug info in optimized binaries, and how LLVM
mitigates those issues
62

163. Contact
63
Email: minyihh@uci.edu
GitHub: mshockwave
LinkedIn: https://www.linkedin.com/in/bekketmcclane/

164. Thank You!
Q&A

165. Appendix

166. llvm-dva
• LLVM DVA is still in the process of upstreaming to LLVM
• RFC: https://discourse.llvm.org/t/llvm-dev-rfc-llvm-dva-debug-
information-visual-analyzer/62570
• You can, however, build it with this patch: https://reviews.llvm.org/D88661
• The llvm-dva command I used in this slides:
• llvm-dva --attribute=location,format --output-sort=offset —
print=symbols,lines,instructions,scopes <object file>
66

167. Debug Info & Object Files
67
#include <greet.h>
void hello() {…}
#include <greet.h>
void bye() {…}
foo.c bar.c

168. Debug Info & Object Files
67
foo.o
Debug info for:
foo.c + greet.h
#include <greet.h>
void hello() {…}
#include <greet.h>
void bye() {…}
foo.c bar.c

169. Debug Info & Object Files
67
foo.o
Debug info for:
foo.c + greet.h
#include <greet.h>
void hello() {…}
#include <greet.h>
void bye() {…}
foo.c bar.c
bar.o
Debug info for:
bar.c + greet.h

170. Splitting (DWARF) Debug Info
68
#include <greet.h>
void hello() {…}
#include <greet.h>
void bye() {…}
foo.c bar.c
foo.o
foo.c.dwo
bar.o
bar.c.dwo
greet.h.dwo

171. Splitting (DWARF) Debug Info
68
#include <greet.h>
void hello() {…}
#include <greet.h>
void bye() {…}
foo.c bar.c
foo.o
foo.c.dwo
bar.o
bar.c.dwo
greet.h.dwo
Debug info
container

172. Splitting Debug Info
69

173. Splitting Debug Info
• Saving disk spaces
• E.g. Able to de-duplicate type information
69

174. Splitting Debug Info
• Saving disk spaces
• E.g. Able to de-duplicate type information
• Attaching debug info
fi
les (e.g. *.dwo) on release binaries
• E.g. When debugging crashes reported by users
69

175. Splitting Debug Info
• Saving disk spaces
• E.g. Able to de-duplicate type information
• Attaching debug info
fi
les (e.g. *.dwo) on release binaries
• E.g. When debugging crashes reported by users
• Apple platforms are doing this by default (i.e. *.dSYM folders)
69

176. 70
{Struct} 'Point'
{Member} public 'x' -> 'int'
{Location}
{Entry} offset 0
{Member} public 'y' -> 'int'
{Location}
{Entry} offset 4
{Function} extern not_inlined 'foo' -> 'int'
{Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
llvm-dva output DWARF dump

177. 70
0x0000002d: DW_TAG_structure_type
DW_AT_name
("Point")
…
0x0000003a: DW_TAG_member
DW_AT_name
("x")
DW_AT_data_member_location (0x00)
…
0x00000045: DW_TAG_member
DW_AT_name
("y")
DW_AT_data_member_location (0x04)
…
0x00000058: DW_TAG_subprogram
DW_AT_name
("foo")
DW_AT_decl_line
(3)
DW_AT_decl_column
(0x05)
…
0x00000090: DW_TAG_variable
DW_AT_name
("point")
DW_AT_decl_line
(4)
…
DW_AT_location
(DW_OP_fbreg -24)
{Struct} 'Point'
{Member} public 'x' -> 'int'
{Location}
{Entry} offset 0
{Member} public 'y' -> 'int'
{Location}
{Entry} offset 4
{Function} extern not_inlined 'foo' -> 'int'
{Variable} 'point' -> 'Point'
{Location}
{Entry} fbreg -24
llvm-dva output DWARF dump

178. Validating Debug Info in
Optimized Code

179. LLVM Debugify
72
define i32 @add(i32 %0, i32 %1) {
%x = add i32 %0, %1
ret i32 %x
}

180. LLVM Debugify
72
define i32 @add(i32 %0, i32 %1) {
%x = add i32 %0, %1
ret i32 %x
}
define i32 @add(i32 %0, i32 %1) !dbg !7 {
%x = add i32 %0, %1, !dbg, !8
ret i32 %x, !dbg, !9
}
!7 = !DISubprogram(name: "add", line: 1,...)
!8 = !DILocation(line: 2, ...)
!9 = !DILocation(line: 3, ...)

181. LLVM Debugify
• Adding arti
fi
cially-created (fake) debug info metadata to every instructions
72
define i32 @add(i32 %0, i32 %1) {
%x = add i32 %0, %1
ret i32 %x
}
define i32 @add(i32 %0, i32 %1) !dbg !7 {
%x = add i32 %0, %1, !dbg, !8
ret i32 %x, !dbg, !9
}
!7 = !DISubprogram(name: "add", line: 1,...)
!8 = !DILocation(line: 2, ...)
!9 = !DILocation(line: 3, ...)

182. LLVM Debugify
• Adding arti
fi
cially-created (fake) debug info metadata to every instructions
• Useful to test if a compiler transformation preserves debug info as expected
72
define i32 @add(i32 %0, i32 %1) {
%x = add i32 %0, %1
ret i32 %x
}
define i32 @add(i32 %0, i32 %1) !dbg !7 {
%x = add i32 %0, %1, !dbg, !8
ret i32 %x, !dbg, !9
}
!7 = !DISubprogram(name: "add", line: 1,...)
!8 = !DILocation(line: 2, ...)
!9 = !DILocation(line: 3, ...)

183. LLVM Debugify
• Adding arti
fi
cially-created (fake) debug info metadata to every instructions
• Useful to test if a compiler transformation preserves debug info as expected
• Don’t need a compiler frontend to generate debug info
72
define i32 @add(i32 %0, i32 %1) {
%x = add i32 %0, %1
ret i32 %x
}
define i32 @add(i32 %0, i32 %1) !dbg !7 {
%x = add i32 %0, %1, !dbg, !8
ret i32 %x, !dbg, !9
}
!7 = !DISubprogram(name: "add", line: 1,...)
!8 = !DILocation(line: 2, ...)
!9 = !DILocation(line: 3, ...)

184. DExTer: End-to-End Debug Info Tests
73
void bar(int *test) {}
int main() {
int test;
test = 23;
bar(&test); // DexLabel('before_bar')
return test; // DexLabel('after_bar')
}
// DexExpectWatchValue('test', '23', on_line=ref('before_bar'))
// DexExpectWatchValue('test', '23', on_line=ref('after_bar'))
foo.c

185. DExTer: End-to-End Debug Info Tests
73
void bar(int *test) {}
int main() {
int test;
test = 23;
bar(&test); // DexLabel('before_bar')
return test; // DexLabel('after_bar')
}
// DexExpectWatchValue('test', '23', on_line=ref('before_bar'))
// DexExpectWatchValue('test', '23', on_line=ref('after_bar'))
Expectations / Assertions
foo.c

186. DExTer: End-to-End Debug Info Tests
73
void bar(int *test) {}
int main() {
int test;
test = 23;
bar(&test); // DexLabel('before_bar')
return test; // DexLabel('after_bar')
}
// DexExpectWatchValue('test', '23', on_line=ref('before_bar'))
// DexExpectWatchValue('test', '23', on_line=ref('after_bar'))
Expectations / Assertions
foo.o
foo.c
Debugger

187. DExTer: End-to-End Debug Info Tests
73
void bar(int *test) {}
int main() {
int test;
test = 23;
bar(&test); // DexLabel('before_bar')
return test; // DexLabel('after_bar')
}
// DexExpectWatchValue('test', '23', on_line=ref('before_bar'))
// DexExpectWatchValue('test', '23', on_line=ref('after_bar'))
Expectations / Assertions
foo.o
foo.c
Debugger
DExTer

188. DExTer: End-to-End Debug Info Tests
73
void bar(int *test) {}
int main() {
int test;
test = 23;
bar(&test); // DexLabel('before_bar')
return test; // DexLabel('after_bar')
}
// DexExpectWatchValue('test', '23', on_line=ref('before_bar'))
// DexExpectWatchValue('test', '23', on_line=ref('after_bar'))
Expectations / Assertions
foo.o
foo.c
Debugger
DExTer
Validation

189. Example: Merging Debug Locations
74
store i32 9, i32* %1
...
store i32 9, i32* %1
...
store i32 9, i32* %1
...