Talk I gave in COSCUP 2021 conference (https://coscup.org/2021)

1. Min-Yih “Min” Hsu @ COSCUP 2021
LLVM Project:
The Good, The Bad, and The Ugly

2. “Min” Hsu
about:me
2

3. “Min” Hsu
• Computer Science PhD Candidate in
University of California, Irvine
• Advised by Prof. Michael Franz
about:me
2

4. “Min” Hsu
• Computer Science PhD Candidate in
University of California, Irvine
• Advised by Prof. Michael Franz
• LLVM contributor since 2015
• Code owner of M68k LLVM backend
• Author of book “LLVM Techniques,
Tips and Best Practices” (2021)
about:me
2

5. “Min” Hsu
• Computer Science PhD Candidate in
University of California, Irvine
• Advised by Prof. Michael Franz
• LLVM contributor since 2015
• Code owner of M68k LLVM backend
• Author of book “LLVM Techniques,
Tips and Best Practices” (2021)
• Motorcycle rider and co
ff
ee junkies
about:me
2

6. *Opinions Are My Own
3

7. What is this talk?
4

8. What is this talk?
4

9. What is this talk?
4

10. An Overview
5

11. An Overview
5
Technical Stu
f

12. An Overview
5
Technical Stu
f
Community

13. Outline
The Good
Design
Some new features
Community
The Bad
The Ugly
6

14. Design

15. 8
Frontend Optimizer Backend
Traditional compiler design

16. 9
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
Traditional compiler design

17. 9
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A Single Compiler Executable
Traditional compiler design

18. 9
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A Single Compiler Executable
Traditional compiler design
Example: GCC

19. Scenario
10
We’re adding C++ auto-complete feature

20. Scenario
10
We’re adding C++ auto-complete feature
Create a new from scratch

21. Scenario
10
We’re adding C++ auto-complete feature
Create a new from scratch
Re-use gcc’s
frontend

22. Scenario
10
We’re adding C++ auto-complete feature
Create a new from scratch
Re-use gcc’s
frontend Gimme a raise

23. Try to re-use gcc’s frontend
11
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A single compiler executable

24. Try to re-use gcc’s frontend
11
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A single compiler executable
Option 1:
Copy / Extract the frontend source code

25. Try to re-use gcc’s frontend
11
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A single compiler executable
Option 1:
Copy / Extract the frontend source code
Drawback: Time-consuming

26. Try to re-use gcc’s frontend
11
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A single compiler executable
Option 1:
Copy / Extract the frontend source code
Option 2:
> gcc -fsyntax-only code_snapshot.c
Drawback: Time-consuming

27. Try to re-use gcc’s frontend
11
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
A single compiler executable
Option 1:
Copy / Extract the frontend source code
Option 2:
> gcc -fsyntax-only code_snapshot.c
Drawback: Time-consuming
Drawbacks:
1. Potentially low performance
2. Interface to the frontend is weak

28. Potential solution: Code sharing
12
Code

29. LLVM Project: A collection of compiler libraries
13

30. LLVM Project: A collection of compiler libraries
13
Clang libraries LLVM libraries
libclangParse libLLVMSupport
libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC
libLLVMX86Info
libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen

31. 14
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
Tools
opt

32. 15
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
clang
Tools
opt

33. 16
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
Tools
My auto-complete tool
clang
opt

34. 17
P…Pa…
Pass plugin is not the only
“modular way” to use LLVM

35. Some (Cool) New Features

36. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

37. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

38. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

39. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

40. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

41. Conventional “performance” measuring on compilers
19
Building Flow
Source Code Executable

42. LLVM’s improvement on building speeding
20
Building Flow
Source Code Executable
CodeGen Linking

43. LLVM’s improvement on building speeding
20
Building Flow
Source Code Executable
CodeGen Linking
GlobalISel

44. Instruction selection in LLVM: Running time
LLVM Passes that have the highest running time
21

45. Instruction selection in LLVM: Running time
LLVM Passes that have the highest running time
21
LLVM Passes
Running Time
0% 10% 20% 30% 40%
DAG->DAG Instruction Selection
Module Veri
fi
er
Live Variable Analysis
Greedy Register Allocator
Live Debug Value Analysis
Prologue / Epilogue Insertion

46. Instruction selection in LLVM: Workflow
22
LLVM IR (Linear)
Target Independent

47. Instruction selection in LLVM: Workflow
22
LLVM IR (Linear) SelectionDAG (DAG)
Target Independent

48. Instruction selection in LLVM: Workflow
22
LLVM IR (Linear) SelectionDAG (DAG) MachineInstr (Linear)
Target Independent Target Speci
fi
c

49. Instruction selection in LLVM: Workflow
22
LLVM IR (Linear) SelectionDAG (DAG) MachineInstr (Linear)
Target Independent Target Speci
fi
c

50. Global Instruction Selection (GlobalISel)
23
LLVM IR (Linear) gMIR (Linear) MachineInstr (Linear)
Target Independent Target Speci
fi
c

51. Global Instruction Selection (GlobalISel)
23
LLVM IR (Linear) gMIR (Linear) MachineInstr (Linear)
Target Independent Target Speci
fi
c
Hybrid

52. Global Instruction Selection (GlobalISel)
24
Credit: Ahmad Bougcha et al. @ LLVM Dev Meeting 2017
• Faster compilation

53. Global Instruction Selection (GlobalISel)
24
Credit: Ahmad Bougcha et al. @ LLVM Dev Meeting 2017
• Faster compilation
• Better codegen
quality

54. Global Instruction Selection (GlobalISel)
24
Credit: Ahmad Bougcha et al. @ LLVM Dev Meeting 2017
• Faster compilation
• Better codegen
quality
• Testing GlobalISel
is much easier.

55. 25
Building Flow
Source Code Executable
CodeGen Linking

56. 25
Building Flow
Source Code Executable
CodeGen Linking
LLD / ThinLTO

57. Linker 101
26
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.o B.o C.o
Linker
Executable

58. Linker 101
• Linking speed becomes the
bottleneck while building
large-scale projects.
• We barely can parallelize this
process.
26
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.o B.o C.o
Linker
Executable
Thread Thread Thread

59. LLD: A (much) faster linker
• Gold was the fastest linker, but
now LLD is (mostly) faster than
gold.
27
Credit: Rui Ueyama @ LLVM Dev Meeting 2017

60. LLD: A (much) faster linker
• Gold was the fastest linker, but
now LLD is (mostly) faster than
gold.
• Secret sauce
27
Credit: Rui Ueyama @ LLVM Dev Meeting 2017

61. LLD: A (much) faster linker
• Gold was the fastest linker, but
now LLD is (mostly) faster than
gold.
• Secret sauce
• Laziness
27
Credit: Rui Ueyama @ LLVM Dev Meeting 2017

62. LLD: A (much) faster linker
• Gold was the fastest linker, but
now LLD is (mostly) faster than
gold.
• Secret sauce
• Laziness
• Certain level of parallelization
• Use
fl
ag -fuse-ld=lld on
GCC and Clang to adopt LLD.
27
Credit: Rui Ueyama @ LLVM Dev Meeting 2017

63. Conventional Linker
28
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.o B.o C.o
Linker
Executable

64. WMO / LTO
29
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.ir B.ir C.ir
Linker
Executable
Whole Module / Link Time Optimization

65. WMO / LTO
29
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.ir B.ir C.ir
Linker
Executable
Whole Module / Link Time Optimization
• Perform more aggressive
whole program optimization
during link time.
• Because we can
fi
nally see
the entire program.

66. WMO / LTO
29
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
A.ir B.ir C.ir
Linker
Executable
Whole Module / Link Time Optimization
• Perform more aggressive
whole program optimization
during link time.
• Because we can
fi
nally see
the entire program.
• (Traditionally) It’s still hard to
parallelize WMO / LTO.
Thread Thread Thread

67. ThinLTO
30
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
Linker
Executable
Thread Thread Thread
A.o B.o C.o
Optimizer Optimizer Optimizer

68. ThinLTO
30
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
Linker
Executable
• Perform inter-procedural
optimizations by exchanging
function summary.
• “Seeing” other functions via
function summary.
Thread Thread Thread
A.o B.o C.o
Optimizer Optimizer Optimizer

69. ThinLTO
30
A.cpp B.cpp C.cpp
Compiler Compiler Compiler
Linker
Executable
• Perform inter-procedural
optimizations by exchanging
function summary.
• “Seeing” other functions via
function summary.
• ~3x speed up*†
• Using ~10x less memory*†
Thread Thread Thread
A.o B.o C.o
Optimizer Optimizer Optimizer
* Teresa Johnson et al. @ LLVM Dev Metting 2016
† Compare to conventional LTO

70. Community

71. A federated community
32

72. A federated community
32

73. A federated community
33
Code Owner
Code Owner
Code Owner

74. A federated community
33
Code Owner
Code Owner
Code Owner
M68k Backend

75. A federated community
33
Code Owner
Code Owner
Code Owner
M68k Backend
(LLVM Foundation) Board Members

76. “Linux style” contribution process
34

77. “Linux style” contribution process
34
Maintainer Maintainer

78. “Linux style” contribution process
34
Individual Branch
(e.g.
fi
lesystem)
Maintainer
Individual Branch
(e.g. network)
Maintainer

79. “Linux style” contribution process
34
Individual Branch
(e.g.
fi
lesystem)
Maintainer
Individual Branch
(e.g. network)
Maintainer
Patch Patch

80. “Linux style” contribution process
34
Individual Branch
(e.g.
fi
lesystem)
Maintainer
Individual Branch
(e.g. network)
Maintainer
Primary Tree
Patch Patch

81. “Linux style” contribution process
34
Individual Branch
(e.g.
fi
lesystem)
Maintainer
Individual Branch
(e.g. network)
Maintainer
Primary Tree
Pull Request
Pull Request
Patch Patch

82. LLVM’s contribution process
35
The main
branch

83. LLVM’s contribution process
35
The main
branch
Tip of Tree (ToT)

84. LLVM’s contribution process
35
The main
branch
Tip of Tree (ToT)
Author
Reviewer Reviewer
Author

85. LLVM’s contribution process
36
The main
branch
Tip of Tree (ToT)
Author
Reviewer Reviewer
Author

86. LLVM’s contribution process
36
The main
branch
Tip of Tree (ToT)
Author
Reviewer Reviewer
Author
Commit Commit

87. Outline
The Good
The Bad (things that LLVM probably should fix)
Code Review
Learning Resources for Beginners
The Ugly
37

88. Recap: A federated community
38

89. Code review with community members
39
Code Review

90. Code review with community members
40
Code Review
New Contributor

91. Code review with community members
40
Code Review
New Contributor
?

92. Code review with community members
41
Code Review
New Contributor
?
Why?

93. Code review with community members
41
Code Review
New Contributor
?
• New member does not have
enough “review currency” yet.
Why?

94. Code review with community members
41
Code Review
New Contributor
?
• New member does not have
enough “review currency” yet.
• Patches are less visible.
Why?

95. Code review with community members
41
Code Review
New Contributor
?
• New member does not have
enough “review currency” yet.
• Patches are less visible.
• Most of the core contributors are
busy.
Why?

96. Code review with community members
41
Code Review
New Contributor
?
• New member does not have
enough “review currency” yet.
• Patches are less visible.
• Most of the core contributors are
busy.
• Nearly all of them are working on
LLVM as their day jobs!
Why?

97. Code review with community members
41
Code Review
New Contributor
?
• New member does not have
enough “review currency” yet.
• Patches are less visible.
• Most of the core contributors are
busy.
• Nearly all of them are working on
LLVM as their day jobs!
• Code owners are obligate to review
the patch…but they’re also busy.
Why?

98. Some attempts…
42
http://llvmweekly.org/reviewcorner

99. Some attempts…
43
Subscribe to (patch) topics that you’re interested in

100. Learning Resources
For Beginners

101. 45
LLVM
is hard

102. LLVM is Flexible
46

103. LLVM is Flexible
47
“Where should I start?”

104. 48
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
clang
Tools
opt My auto-complete tool

105. 48
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
clang
Tools
opt My auto-complete tool

106. 48
Clang libraries LLVM libraries
libclangParse libLLVMSupport libLLVMCore
libLLVMTransformsUtils libLLVMScalarOpts
libLLVMLLParser
libLLVMMC libLLVMX86Info libLLVMX86CodeGen
libclangAST
libclangSema
libclangCodeGen
clang
Tools
opt My auto-complete tool

107. 49

108. 49

109. What do you think?
50

110. What do you think?
50
• What’s your story?
• Project-oriented tutorial
v.s.
A broad overview
• Online forum / mailing list
experiences (for newbie questions)?
• Wiki(-style) book?

111. Outline
The Good
The Bad
The Ugly (things that are difficult to solve)
API & ABI stability
51

112. API & ABI Stability

113. API stability in a nutshell
53
lib.h
int foo(int x, int y);
MySrc.cpp
int main() {
return foo(94, 87);
}
Version 0.1

114. API stability in a nutshell
54
lib.h
int foo(Point point);
MySrc.cpp
int main() {
return foo(94, 87);
}
Version 0.5

115. API stability in a nutshell
54
lib.h
int foo(Point point);
MySrc.cpp
int main() {
return foo(94, 87);
}
Version 0.5
Compile Error

116. LLVM API stability for downstream users
Some common LLVM downstream use cases
55

117. LLVM API stability for downstream users
Some common LLVM downstream use cases
55
My C++ auto-complete plugin
libclangSema libclangParse

118. LLVM API stability for downstream users
Some common LLVM downstream use cases
55
My C++ auto-complete plugin
libclangSema libclangParse
My Toy compiler
LLVM IR
Optimizer
LLVM X86
Backend

119. LLVM API stability for downstream users
56
LLVM
C++ API
Stability
Policy

120. LLVM API stability for downstream users
56
LLVM
C++ API
Stability
Policy
NONE

121. LLVM API stability for downstream users
56
LLVM
C++ API
Stability
Policy
NONE
• A pain in the a** for downstream users.
• Behaviors of APIs might also
change!

122. LLVM API stability for downstream users
56
LLVM
C++ API
Stability
Policy
NONE
• A pain in the a** for downstream users.
• Behaviors of APIs might also
change!
• The C API is (mostly) stable…
• …But the provided features are lag
(way) behind.

123. LLVM API stability for downstream users
56
LLVM
C++ API
Stability
Policy
NONE
• A pain in the a** for downstream users.
• Behaviors of APIs might also
change!
• The C API is (mostly) stable…
• …But the provided features are lag
(way) behind.
• A design decision made since day 1.
• Trade stability for faster feature
delivery.

124. ABI stability in a nutshell
57
MyExecutable
int foo(Point point);
int main() {
return foo(Point{94, 87});
}
libFoo.so
Version 0.1

125. ABI stability in a nutshell
57
MyExecutable
int foo(Point point);
int main() {
return foo(Point{94, 87});
}
libFoo.so
Version 0.1
Shared Library

126. ABI stability in a nutshell
57
MyExecutable
int foo(Point point);
int main() {
return foo(Point{94, 87});
}
libFoo.so
Version 0.1
Shared Library
Plugin

127. ABI stability in a nutshell
58
MyExecutable
int foo(Point point);
int main() {
return foo(Point{94, 87});
}
libFoo.so
Version 0.5
?
Shared Library
Plugin

128. ABI stability in a nutshell
Name mangling
59
MyExecutable
int foo(Point point); libFoo.so
Version 0.5
?
Old: _Z3foo5Point

129. ABI stability in a nutshell
Name mangling
59
MyExecutable
int foo(Point point); libFoo.so
Version 0.5
?
Old: _Z3foo5Point
New: ?foo@@YAHUPoint@@@Z

130. ABI stability in a nutshell
Type layout
60
MyExecutable
int foo(Point point) {
return point.X + point.Y;
}
libFoo.so
Version 0.5
?
struct Point {
int X;
int Y;
};
Old

131. ABI stability in a nutshell
Type layout
60
MyExecutable
int foo(Point point) {
return point.X + point.Y;
}
libFoo.so
Version 0.5
?
struct Point {
int X;
int Y;
};
Old
struct Point {
int X;
int LMAO;
int Y;
};
New

132. LLVM ABI stability for plugins
All available plugin options in Clang & LLVM
61
Clang LLVM
Preprocessor: Pragma plugin
Sema: Attribute plugin
AST: AST plugin
LLVM IR: Pass plugin (old / new)

133. LLVM ABI stability for plugins
If LLVM Pass plugin has a stable ABI…
62
opt -my-pass
LLVM version X.Y
MyPassPlugin.so

134. LLVM ABI stability for plugins
If LLVM Pass plugin has a stable ABI…
62
opt -my-pass
LLVM version X.Y
MyPassPlugin.so

135. LLVM ABI stability for plugins
If LLVM Pass plugin has a stable ABI…
62
opt -my-pass
LLVM version X.Y
opt -my-pass
LLVM version Y.Z
MyPassPlugin.so

136. LLVM ABI stability for plugins
63
LLVM Plugin
LLVM Plugin
“You have stable
ABI, right?”
“…right?…”

137. LLVM ABI stability for plugins
63
LLVM Plugin
LLVM Plugin
“You have stable
ABI, right?”
“…right?…”
• Again, trading stability for
fl
exibility
• LLVM plugins are pretty powerful.

138. LLVM ABI stability for plugins
63
LLVM Plugin
LLVM Plugin
“You have stable
ABI, right?”
“…right?…”
• Again, trading stability for
fl
exibility
• LLVM plugins are pretty powerful.
• Real world use case: Chromium

139. LLVM ABI stability for plugins
63
LLVM Plugin
LLVM Plugin
“You have stable
ABI, right?”
“…right?…”
• Again, trading stability for
fl
exibility
• LLVM plugins are pretty powerful.
• Real world use case: Chromium
• Using custom clang plugin to enforce
coding styles inside Chromium’s code
base.
• E.g. Catching bad C++ dtor

140. LLVM ABI stability for plugins
63
LLVM Plugin
LLVM Plugin
“You have stable
ABI, right?”
“…right?…”
• Again, trading stability for
fl
exibility
• LLVM plugins are pretty powerful.
• Real world use case: Chromium
• Using custom clang plugin to enforce
coding styles inside Chromium’s code
base.
• E.g. Catching bad C++ dtor
• “Don’t write a clang plugin” @ Chromium
Docs - https://bit.ly/3rIUg0f

141. Thank You!

142. Questions?
65
Email: minyihh@uci.edu
Github: mshockwave

143. Appendix

144. 67

145. 67
> ./greeting -say “World”
Hello, World

146. 67
#include “llvm/Support/CommandLine.h”
#include <iostream>
#include <string>
using namespace llvm;
static cl::opt<std::string>
GreetingText(“say”, cl::desc(“What to say?”));
int main(int argc, char **argv) {
cl::ParseCommandLineOptions(argc, argv);
std::cout << “Hello, ” << GreetingText << “n”
return 0;
}
> ./greeting -say “World”
Hello, World

147. 68
#include “llvm/Support/CommandLine.h”
#include <iostream>
#include <string>
using namespace llvm;
static cl::opt<std::string>
GreetingText(“say”, cl::desc(“What to say?”));
int main(int argc, char **argv) {
cl::ParseCommandLineOptions(argc, argv);
std::cout << “Hello, ” << GreetingText << “n”
return 0;
}
> c++ file.cc … -I<LLVM include dir>
-L<LLVM library path>
-o greeting -lLLVMSupport

148. Scenario: Adding a new target (backend)
69
…
Assembly syntax
Instruction encoding
…

149. 70
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
Compiler
Assembler
Assembler
Source Files

150. 70
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
Compiler
Assembler
Assembler
Source Files
Know how to
print assembly

151. 70
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
Compiler
Assembler
Assembler
Source Files
Know how to
print assembly
Know how to
parse assembly

152. 70
Frontend
Source Files
Optimizer
Source Files
Backend
Source Files
Compiler
Assembler
Assembler
Source Files
Know how to
print assembly
Know how to
parse assembly
Di
ff
erent Codebases!

153. 71
Frontend
Optimizer
Backend
Assembler
LLVM Solution: Sharing target description library

154. 71
Frontend
Optimizer
Backend
Assembler
Target Description Library
(e.g. libLLVMX86Desc)
LLVM Solution: Sharing target description library

155. 71
Frontend
Optimizer
Backend
Assembler
Target Description Library
(e.g. libLLVMX86Desc)
Assembly Printer
LLVM Solution: Sharing target description library

156. 71
Frontend
Optimizer
Backend
Assembler
Target Description Library
(e.g. libLLVMX86Desc)
Assembly Printer
Assembly Parser
LLVM Solution: Sharing target description library

157. LLVM’s binary tooling ecosystem
72
Target Description Library

158. LLVM’s binary tooling ecosystem
72
Target Description Library
MC Layer
Binary Format Library

159. LLVM’s binary tooling ecosystem
72
Target Description Library
MC Layer
Assembler
Disassembler
Binary Format Library
} llvm-mc

160. LLVM’s binary tooling ecosystem
72
Target Description Library
MC Layer
Assembler
Disassembler
Binary Format Library
llvm-objdump
llvm-nm
} llvm-mc

161. LLVM’s binary tooling ecosystem
72
Target Description Library
MC Layer
Assembler
Disassembler
Binary Format Library
llvm-objdump
llvm-nm
LLD (Linker)
} llvm-mc

162. LLVM’s binary tooling ecosystem
73
Target Description Library
MC Layer
Assembler
Disassembler
llvm-objdump
Binary Format Library
llvm-nm
LLD (Linker)
Why?

163. LLVM’s binary tooling ecosystem
73
Target Description Library
MC Layer
Assembler
Disassembler
llvm-objdump
Binary Format Library
llvm-nm
LLD (Linker)
Why?
• Binary tools (e.g. binutils) are
essential for many compilation
fl
ows.

164. LLVM’s binary tooling ecosystem
73
Target Description Library
MC Layer
Assembler
Disassembler
llvm-objdump
Binary Format Library
llvm-nm
LLD (Linker)
Why?
• Binary tools (e.g. binutils) are
essential for many compilation
fl
ows.
• Faster feature updates.

165. LLVM’s binary tooling ecosystem
73
Target Description Library
MC Layer
Assembler
Disassembler
llvm-objdump
Binary Format Library
llvm-nm
LLD (Linker)
Why?
• Binary tools (e.g. binutils) are
essential for many compilation
fl
ows.
• Faster feature updates.
• Avoid version mismatch.

166. LLVM’s binary tooling ecosystem
74
Target Description Library
MC Layer
Assembler
Disassembler
llvm-objdump
Binary Format Library
llvm-nm
LLD (Linker)

167. 75
Frontend
Optimizer
Backend
Assembler
Target Description Library
(e.g. libLLVMX86Desc)
Recap: Sharing target information

168. 76
Frontend
Optimizer
Backend
Compiler Process
Assembler Process
Assembler
Linker Process
Linker
Recap: Traditional compilation pipeline

169. 76
Frontend
Optimizer
Backend
Compiler Process
Assembler Process
Assembler
Linker Process
Linker
Recap: Traditional compilation pipeline
.s
fi
le .o
fi
le

170. 77
Frontend
Optimizer
Backend
Compiler + Assembler Process
Assembler
Linker Process
Linker
?

171. 77
Frontend
Optimizer
Backend
Compiler + Assembler Process
Assembler
Linker Process
Linker
?
• An assembly
fi
le always comes
from a single source
fi
le

172. 77
Frontend
Optimizer
Backend
Compiler + Assembler Process
Assembler
Linker Process
Linker
?
• An assembly
fi
le always comes
from a single source
fi
le
• Avoid round-trip (i.e. read/write
assembly
fi
le) to disk

173. 78
Frontend
Optimizer
Backend
A clang that uses integrated assembler
Assembler
Clang’s integrated assembler
In-memory LLVM IR
In-memory LLVM IR
Textual assembly code
In-memory MCInst objects