linkers-lto

Name: linkers-lto
Author: mohitmishra786

319installs19forks

How to install linkers-lto?

mohitmishra786/low-level-dev-skills/linkers-lto

$npx -y skills add mohitmishra786/low-level-dev-skills --skill linkers-lto

Installs into the current project.

Summary

Linker and Link-Time Optimisation (LTO) skill. Use when configuring GNU ld, gold, or lld linker flags, diagnosing link-order issues or undefined symbols at link time, enabling LTO safely in real projects, or understanding inter-module optimisation trade-offs. Activates on queries

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SKILL.md

1	# Linkers and LTO
2
3	## Purpose
4
5	Guide agents through linker selection, common linker flags, link-order issues, LTO setup, and symbol-visibility management.
6
7	## Triggers
8
9	- "I'm getting `undefined reference` at link time"
10	- "How do I enable LTO for a real project?"
11	- "Which linker should I use: ld, gold, or lld?"
12	- "How do I reduce binary size with `--gc-sections`?"
13	- "How do I write or understand a linker script?"
14	- "I have duplicate symbol or weak symbol issues"
15
16	## Workflow
17
18	### 1. Linker selection
19
20	\| Linker \| Invocation \| Strengths \|
21	\|--------\|-----------\|-----------\|
22	\| GNU ld (BFD) \| default on Linux \| Universal, stable \|
23	\| gold \| `-fuse-ld=gold` \| Faster than ld for C++; supports LTO plugins \|
24	\| lld (LLVM) \| `-fuse-ld=lld` \| Fastest, parallel, required for Clang LTO \|
25
26	```bash
27	# Use lld with GCC or Clang
28	gcc -fuse-ld=lld -o prog ...
29	clang -fuse-ld=lld -o prog ...
30
31	# Check which linker is used
32	gcc -v -o prog main.c 2>&1 \| grep 'Invoking'
33	```
34
35	### 2. Essential linker flags
36
37	```bash
38	# Pass linker flags via compiler driver:
39	# -Wl,flag1,flag2 (comma-separated, no spaces)
40	# -Wl,flag1 -Wl,flag2 (separate -Wl options)
41
42	gcc main.c -o prog \
43	-Wl,-rpath,/opt/mylibs/lib \ # runtime library search path
44	-Wl,--as-needed \ # only link libraries that are actually used
45	-Wl,--gc-sections \ # remove unused sections (requires -ffunction-sections -fdata-sections)
46	-Wl,-z,relro \ # mark relocations read-only after startup
47	-Wl,-z,now \ # resolve all symbols at startup (full RELRO)
48	-L/opt/mylibs/lib -lfoo
49	```
50
51	### 3. Link order matters (GNU ld)
52
53	GNU ld processes archives left-to-right. A library must come after the objects that need it.
54
55	```bash
56	# Wrong: libfoo provides symbols needed by main.o
57	gcc main.o -lfoo libdep.a -o prog # can fail if libdep.a needs libfoo
58
59	# Correct: dependencies after dependents
60	gcc main.o -lfoo -ldep -o prog
61
62	# If there are circular deps between archives:
63	gcc main.o -Wl,--start-group -lfoo -lbar -Wl,--end-group -o prog
64	# --start-group/--end-group: repeat search until no new symbols resolved
65	```
66
67	### 4. LTO with GCC
68
69	```bash
70	# Compile
71	gcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.o
72	gcc -O2 -flto -ffunction-sections -fdata-sections -c bar.c -o bar.o
73
74	# Link (must pass -flto again)
75	gcc -O2 -flto -Wl,--gc-sections foo.o bar.o -o prog
76
77	# Archives: must use gcc-ar / gcc-ranlib, not plain ar
78	gcc-ar rcs libfoo.a foo.o
79	gcc-ranlib libfoo.a
80	```
81
82	Parallel LTO:
83
84	```bash
85	gcc -O2 -flto=auto foo.o bar.o -o prog # uses jobserver
86	gcc -O2 -flto=4 foo.o bar.o -o prog # 4 parallel jobs
87	```
88
89	### 5. LTO with Clang / lld
90
91	```bash
92	# Full LTO
93	clang -O2 -flto -fuse-ld=lld foo.c bar.c -o prog
94
95	# ThinLTO (faster, nearly same quality)
96	clang -O2 -flto=thin -fuse-ld=lld foo.c bar.c -o prog
97
98	# LTO with cmake: set globally
99	set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON) # enables -flto
100	```
101
102	ThinLTO caches work: subsequent builds that reuse unchanged modules are faster.
103	Cache location: specify with `-Wl,--thinlto-cache-dir=/tmp/thinlto-cache`.
104
105	### 6. Dead-code stripping
106
107	```bash
108	# Compile with per-function/per-data sections
109	gcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o
110
111	# Link with garbage collection
112	gcc -Wl,--gc-sections foo.o -o prog
113
114	# Verify what was removed
115	gcc -Wl,--gc-sections -Wl,--print-gc-sections foo.o -o prog 2>&1 \| head -20
116	```
117
118	On macOS, the linker strips dead code by default (`-dead_strip`).
119
120	### 7. Symbol visibility
121
122	Controlling visibility reduces DSO size and enables better LTO:
123
124	```bash
125	# Hide all symbols by default, export explicitly
126	gcc -fvisibility=hidden -O2 -shared -fPIC foo.c -o libfoo.so
127
128	# In source, mark exports:
129	__attribute__((visibility("default"))) int my_public_function(void);
130	```
131
132	Or use a version script:
133
134	```text
135	# foo.ver
136	{
137	global: my_public_function; my_other_public;
138	local: *;
139	};
140	```
141
142	```bash
143	gcc -Wl,--version-script=foo.ver -shared -fPIC -o libfoo.so foo.o
144	```
145
146	### 8. Common linker errors
147
148	\| Error \| Cause \| Fix \|
149	\|-------\|-------\|-----\|
150	\| `undefined reference to 'foo'` \| Missing library or wrong order \| Add `-lfoo`; move after the object that needs it \|
151	\| `multiple definition of 'foo'` \| Symbol defined in two TUs \| Remove duplicate; use `static` or `extern` \|
152	\| `cannot find -lfoo` \| Library not in search path \| Add `-L/path/to/lib`; install dev package \|
153	\| `relocation truncated` \| Address overflow in relocation \| Use `-mcmodel=large`; restructure image \|
154	\| `version 'GLIBC_2.33' not found` \| Binary needs newer glibc \| Link statically or rebuild o

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linkers-lto

319installs19forks

View on GitHub

How to install linkers-lto?

mohitmishra786/low-level-dev-skills/linkers-lto

$npx -y skills add mohitmishra786/low-level-dev-skills --skill linkers-lto

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Linkers and LTO
2
3	## Purpose
4
5	Guide agents through linker selection, common linker flags, link-order issues, LTO setup, and symbol-visibility management.
6
7	## Triggers
8
9	- "I'm getting `undefined reference` at link time"
10	- "How do I enable LTO for a real project?"
11	- "Which linker should I use: ld, gold, or lld?"
12	- "How do I reduce binary size with `--gc-sections`?"
13	- "How do I write or understand a linker script?"
14	- "I have duplicate symbol or weak symbol issues"
15
16	## Workflow
17
18	### 1. Linker selection
19
20	\| Linker \| Invocation \| Strengths \|
21	\|--------\|-----------\|-----------\|
22	\| GNU ld (BFD) \| default on Linux \| Universal, stable \|
23	\| gold \| `-fuse-ld=gold` \| Faster than ld for C++; supports LTO plugins \|
24	\| lld (LLVM) \| `-fuse-ld=lld` \| Fastest, parallel, required for Clang LTO \|
25
26	```bash
27	# Use lld with GCC or Clang
28	gcc -fuse-ld=lld -o prog ...
29	clang -fuse-ld=lld -o prog ...
30
31	# Check which linker is used
32	gcc -v -o prog main.c 2>&1 \| grep 'Invoking'
33	```
34
35	### 2. Essential linker flags
36
37	```bash
38	# Pass linker flags via compiler driver:
39	# -Wl,flag1,flag2 (comma-separated, no spaces)
40	# -Wl,flag1 -Wl,flag2 (separate -Wl options)
41
42	gcc main.c -o prog \
43	-Wl,-rpath,/opt/mylibs/lib \ # runtime library search path
44	-Wl,--as-needed \ # only link libraries that are actually used
45	-Wl,--gc-sections \ # remove unused sections (requires -ffunction-sections -fdata-sections)
46	-Wl,-z,relro \ # mark relocations read-only after startup
47	-Wl,-z,now \ # resolve all symbols at startup (full RELRO)
48	-L/opt/mylibs/lib -lfoo
49	```
50
51	### 3. Link order matters (GNU ld)
52
53	GNU ld processes archives left-to-right. A library must come after the objects that need it.
54
55	```bash
56	# Wrong: libfoo provides symbols needed by main.o
57	gcc main.o -lfoo libdep.a -o prog # can fail if libdep.a needs libfoo
58
59	# Correct: dependencies after dependents
60	gcc main.o -lfoo -ldep -o prog
61
62	# If there are circular deps between archives:
63	gcc main.o -Wl,--start-group -lfoo -lbar -Wl,--end-group -o prog
64	# --start-group/--end-group: repeat search until no new symbols resolved
65	```
66
67	### 4. LTO with GCC
68
69	```bash
70	# Compile
71	gcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.o
72	gcc -O2 -flto -ffunction-sections -fdata-sections -c bar.c -o bar.o
73
74	# Link (must pass -flto again)
75	gcc -O2 -flto -Wl,--gc-sections foo.o bar.o -o prog
76
77	# Archives: must use gcc-ar / gcc-ranlib, not plain ar
78	gcc-ar rcs libfoo.a foo.o
79	gcc-ranlib libfoo.a
80	```
81
82	Parallel LTO:
83
84	```bash
85	gcc -O2 -flto=auto foo.o bar.o -o prog # uses jobserver
86	gcc -O2 -flto=4 foo.o bar.o -o prog # 4 parallel jobs
87	```
88
89	### 5. LTO with Clang / lld
90
91	```bash
92	# Full LTO
93	clang -O2 -flto -fuse-ld=lld foo.c bar.c -o prog
94
95	# ThinLTO (faster, nearly same quality)
96	clang -O2 -flto=thin -fuse-ld=lld foo.c bar.c -o prog
97
98	# LTO with cmake: set globally
99	set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON) # enables -flto
100	```
101
102	ThinLTO caches work: subsequent builds that reuse unchanged modules are faster.
103	Cache location: specify with `-Wl,--thinlto-cache-dir=/tmp/thinlto-cache`.
104
105	### 6. Dead-code stripping
106
107	```bash
108	# Compile with per-function/per-data sections
109	gcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o
110
111	# Link with garbage collection
112	gcc -Wl,--gc-sections foo.o -o prog
113
114	# Verify what was removed
115	gcc -Wl,--gc-sections -Wl,--print-gc-sections foo.o -o prog 2>&1 \| head -20
116	```
117
118	On macOS, the linker strips dead code by default (`-dead_strip`).
119
120	### 7. Symbol visibility
121
122	Controlling visibility reduces DSO size and enables better LTO:
123
124	```bash
125	# Hide all symbols by default, export explicitly
126	gcc -fvisibility=hidden -O2 -shared -fPIC foo.c -o libfoo.so
127
128	# In source, mark exports:
129	__attribute__((visibility("default"))) int my_public_function(void);
130	```
131
132	Or use a version script:
133
134	```text
135	# foo.ver
136	{
137	global: my_public_function; my_other_public;
138	local: *;
139	};
140	```
141
142	```bash
143	gcc -Wl,--version-script=foo.ver -shared -fPIC -o libfoo.so foo.o
144	```
145
146	### 8. Common linker errors
147
148	\| Error \| Cause \| Fix \|
149	\|-------\|-------\|-----\|
150	\| `undefined reference to 'foo'` \| Missing library or wrong order \| Add `-lfoo`; move after the object that needs it \|
151	\| `multiple definition of 'foo'` \| Symbol defined in two TUs \| Remove duplicate; use `static` or `extern` \|
152	\| `cannot find -lfoo` \| Library not in search path \| Add `-L/path/to/lib`; install dev package \|
153	\| `relocation truncated` \| Address overflow in relocation \| Use `-mcmodel=large`; restructure image \|
154	\| `version 'GLIBC_2.33' not found` \| Binary needs newer glibc \| Link statically or rebuild o

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