Linkers Lto

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

作者 mohitmishra786bdc58472fa9f無授權條款253 個星標收錄於 2026年10月9日更新於 2026年10月9日儲存庫3 個月前更新

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 about linker flags, -flto, thin LTO, LTCG, --gc-sections, link order errors, weak symbols, or linker scripts.

僅含說明Software Development

AI 產生的概覽

指導連結器選擇、連結器旗標、連結順序問題、LTO 設定以及符號可見性，適用於 GNU ld、gold 和 lld。

功能: 此技能提供在 GNU ld、gold 和 lld 之間選擇的說明，以及常用連結器旗標的套用方式，例如 -Wl,--as-needed、--gc-sections 和 RELRO 選項。它說明封存檔的連結順序規則，包括 --start-group/--end-group，並涵蓋 GCC 與 Clang 的 LTO 啟用方式，包括 ThinLTO 和 CMake 程序間最佳化。它也涉及死碼剝除、透過 -fvisibility=hidden 和版本指令碼控制符號可見性、常見連結錯誤以及對應檔產生。隨附的參考檔案列出連結器和 LTO 旗標。
適用情境: 在為 GNU ld、gold 或 lld 設定連結器旗標時使用，或在診斷連結期問題（例如未定義參考、重複或弱符號、封存檔循環相依）時使用。它也適合在實際專案中安全啟用 LTO、使用 --gc-sections 縮小二進位檔大小，以及理解連結器指令碼或模組間最佳化取捨。
執行需求: 不附指令碼，僅為說明性內容。依範例操作需要帶連結器（GNU ld、gold 或 lld）的 C/C++ 工具鏈、GCC 或 Clang，以及 ar、gcc-ar、gcc-ranlib 等 binutils 工具。不需要憑證或網路存取。

Linkers and LTO

Purpose

Guide agents through linker selection, common linker flags, link-order issues, LTO setup, and symbol-visibility management.

Triggers

"I'm getting undefined reference at link time"
"How do I enable LTO for a real project?"
"Which linker should I use: ld, gold, or lld?"
"How do I reduce binary size with --gc-sections?"
"How do I write or understand a linker script?"
"I have duplicate symbol or weak symbol issues"

Workflow

1. Linker selection

bash

# Use lld with GCC or Clanggcc -fuse-ld=lld -o prog ...clang -fuse-ld=lld -o prog ...
# Check which linker is usedgcc -v -o prog main.c 2>&1 | grep 'Invoking'

# Use lld with GCC or Clanggcc -fuse-ld=lld -o prog ...clang -fuse-ld=lld -o prog ...
# Check which linker is usedgcc -v -o prog main.c 2>&1 | grep 'Invoking'

2. Essential linker flags

bash

# Pass linker flags via compiler driver:# -Wl,flag1,flag2   (comma-separated, no spaces)# -Wl,flag1 -Wl,flag2  (separate -Wl options)
gcc main.c -o prog \  -Wl,-rpath,/opt/mylibs/lib \     # runtime library search path  -Wl,--as-needed \                 # only link libraries that are actually used  -Wl,--gc-sections \               # remove unused sections (requires -ffunction-sections -fdata-sections)  -Wl,-z,relro \                    # mark relocations read-only after startup  -Wl,-z,now \                      # resolve all symbols at startup (full RELRO)  -L/opt/mylibs/lib -lfoo

# Pass linker flags via compiler driver:# -Wl,flag1,flag2   (comma-separated, no spaces)# -Wl,flag1 -Wl,flag2  (separate -Wl options)
gcc main.c -o prog \  -Wl,-rpath,/opt/mylibs/lib \     # runtime library search path  -Wl,--as-needed \                 # only link libraries that are actually used  -Wl,--gc-sections \               # remove unused sections (requires -ffunction-sections -fdata-sections)  -Wl,-z,relro \                    # mark relocations read-only after startup  -Wl,-z,now \                      # resolve all symbols at startup (full RELRO)  -L/opt/mylibs/lib -lfoo

3. Link order matters (GNU ld)

GNU ld processes archives left-to-right. A library must come after the objects that need it.

bash

# Wrong: libfoo provides symbols needed by main.ogcc main.o -lfoo libdep.a -o prog   # can fail if libdep.a needs libfoo
# Correct: dependencies after dependentsgcc main.o -lfoo -ldep -o prog
# If there are circular deps between archives:gcc main.o -Wl,--start-group -lfoo -lbar -Wl,--end-group -o prog# --start-group/--end-group: repeat search until no new symbols resolved

# Wrong: libfoo provides symbols needed by main.ogcc main.o -lfoo libdep.a -o prog   # can fail if libdep.a needs libfoo
# Correct: dependencies after dependentsgcc main.o -lfoo -ldep -o prog
# If there are circular deps between archives:gcc main.o -Wl,--start-group -lfoo -lbar -Wl,--end-group -o prog# --start-group/--end-group: repeat search until no new symbols resolved

4. LTO with GCC

bash

# Compilegcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.ogcc -O2 -flto -ffunction-sections -fdata-sections -c bar.c -o bar.o
# Link (must pass -flto again)gcc -O2 -flto -Wl,--gc-sections foo.o bar.o -o prog
# Archives: must use gcc-ar / gcc-ranlib, not plain argcc-ar rcs libfoo.a foo.ogcc-ranlib libfoo.a

# Compilegcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.ogcc -O2 -flto -ffunction-sections -fdata-sections -c bar.c -o bar.o
# Link (must pass -flto again)gcc -O2 -flto -Wl,--gc-sections foo.o bar.o -o prog
# Archives: must use gcc-ar / gcc-ranlib, not plain argcc-ar rcs libfoo.a foo.ogcc-ranlib libfoo.a

Parallel LTO:

bash

gcc -O2 -flto=auto foo.o bar.o -o prog   # uses jobservergcc -O2 -flto=4   foo.o bar.o -o prog    # 4 parallel jobs

gcc -O2 -flto=auto foo.o bar.o -o prog   # uses jobservergcc -O2 -flto=4   foo.o bar.o -o prog    # 4 parallel jobs

5. LTO with Clang / lld

bash

# Full LTOclang -O2 -flto -fuse-ld=lld foo.c bar.c -o prog
# ThinLTO (faster, nearly same quality)clang -O2 -flto=thin -fuse-ld=lld foo.c bar.c -o prog
# LTO with cmake: set globallyset(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)   # enables -flto

# Full LTOclang -O2 -flto -fuse-ld=lld foo.c bar.c -o prog
# ThinLTO (faster, nearly same quality)clang -O2 -flto=thin -fuse-ld=lld foo.c bar.c -o prog
# LTO with cmake: set globallyset(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)   # enables -flto

ThinLTO caches work: subsequent builds that reuse unchanged modules are faster. Cache location: specify with -Wl,--thinlto-cache-dir=/tmp/thinlto-cache.

6. Dead-code stripping

bash

# Compile with per-function/per-data sectionsgcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o
# Link with garbage collectiongcc -Wl,--gc-sections foo.o -o prog
# Verify what was removedgcc -Wl,--gc-sections -Wl,--print-gc-sections foo.o -o prog 2>&1 | head -20

# Compile with per-function/per-data sectionsgcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o
# Link with garbage collectiongcc -Wl,--gc-sections foo.o -o prog
# Verify what was removedgcc -Wl,--gc-sections -Wl,--print-gc-sections foo.o -o prog 2>&1 | head -20

On macOS, the linker strips dead code by default (-dead_strip).

7. Symbol visibility

Controlling visibility reduces DSO size and enables better LTO:

bash

# Hide all symbols by default, export explicitlygcc -fvisibility=hidden -O2 -shared -fPIC foo.c -o libfoo.so
# In source, mark exports:__attribute__((visibility("default"))) int my_public_function(void);

# Hide all symbols by default, export explicitlygcc -fvisibility=hidden -O2 -shared -fPIC foo.c -o libfoo.so
# In source, mark exports:__attribute__((visibility("default"))) int my_public_function(void);

Or use a version script:

8. Common linker errors

Error	Cause	Fix
`undefined reference to 'foo'`	Missing library or wrong order	Add `-lfoo`; move after the object that needs it
`multiple definition of 'foo'`	Symbol defined in two TUs	Remove duplicate; use `static` or `extern`
`cannot find -lfoo`	Library not in search path	Add `-L/path/to/lib`; install dev package
`relocation truncated`	Address overflow in relocation	Use `-mcmodel=large`; restructure image
`version 'GLIBC_2.33' not found`	Binary needs newer glibc	Link statically or rebuild on older host
`circular reference`	Archives mutually depend	Use `--start-group`/`--end-group`

9. Linker map file

bash

# Generate a map file (shows symbol → section → file)gcc -Wl,-Map=prog.map -o prog foo.o bar.oless prog.map

# Generate a map file (shows symbol → section → file)gcc -Wl,-Map=prog.map -o prog foo.o bar.oless prog.map

Useful for debugging binary size and symbol placement.

For a comprehensive linker and LTO flags reference, see references/flags.md [blocked].

Related skills

Use skills/binaries/elf-inspection for examining the resulting binary
Use skills/compilers/gcc or skills/compilers/clang for compile-phase LTO flags
Use skills/binaries/binutils for ar, strip, objcopy

來源與署名

來源：mohitmishra786/low-level-dev-skills位於skills/binaries/linkers-lto提交bdc5847

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檢舉或申請下架

Linker	Invocation	Strengths
GNU ld (BFD)	default on Linux	Universal, stable
gold	`-fuse-ld=gold`	Faster than ld for C++; supports LTO plugins
lld (LLVM)	`-fuse-ld=lld`	Fastest, parallel, required for Clang LTO

Linkers Lto

Linkers and LTO

Purpose

Triggers

Workflow

1. Linker selection

2. Essential linker flags

3. Link order matters (GNU ld)

4. LTO with GCC

5. LTO with Clang / lld

6. Dead-code stripping

7. Symbol visibility

8. Common linker errors

9. Linker map file

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