Linkers Lto

---
name: linkers-lto
description: 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.
---

# 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

| 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 |

```bash
# Use lld with GCC or Clang
gcc -fuse-ld=lld -o prog ...
clang -fuse-ld=lld -o prog ...

# Check which linker is used
gcc -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
```

### 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.o
gcc main.o -lfoo libdep.a -o prog   # can fail if libdep.a needs libfoo

# Correct: dependencies after dependents
gcc 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
# Compile
gcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.o
gcc -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 ar
gcc-ar rcs libfoo.a foo.o
gcc-ranlib libfoo.a
```

Parallel LTO:

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

### 5. LTO with Clang / lld

```bash
# Full LTO
clang -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 globally
set(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 sections
gcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o

# Link with garbage collection
gcc -Wl,--gc-sections foo.o -o prog

# Verify what was removed
gcc -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 explicitly
gcc -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:

```text
# foo.ver
{
  global: my_public_function; my_other_public;
  local: *;
};
```

```bash
gcc -Wl,--version-script=foo.ver -shared -fPIC -o libfoo.so foo.o
```

### 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.o
less prog.map
```

Useful for debugging binary size and symbol placement.

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

## 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`