clang

Original：🇺🇸 English

Translated

Clang/LLVM compiler skill for C/C++ projects. Use when working with clang or clang++ for diagnostics, sanitizer instrumentation, optimization remarks, static analysis with clang-tidy, LTO via lld, or when migrating from GCC to Clang. Activates on queries about clang flags, clang-tidy, clang-format, better error messages, Apple/FreeBSD toolchains, or LLVM-specific optimizations. Covers flag selection, diagnostic tuning, and integration with LLVM tooling.

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Sourcemohitmishra786/low-level-dev-skills

Added on2026-02-21

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npx skill4agent add mohitmishra786/low-level-dev-skills clang

SKILL.md Content

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Clang

Purpose

Guide agents through Clang-specific features: superior diagnostics, sanitizer integration, optimization remarks, static analysis, and LLVM tooling. Covers divergences from GCC and Apple/FreeBSD specifics.

Triggers

"I want better compiler diagnostics/errors"
"How do I use clang-tidy / clang-format?"
"How do I see what the compiler optimised or didn't?"
"I'm on macOS / FreeBSD using clang"
"clang-cl for MSVC-compatible builds" — see
```
skills/compilers/msvc-cl
```
Sanitizer queries — see
```
skills/runtimes/sanitizers
```

Workflow

1. Build mode flags (identical to GCC)

Clang accepts most GCC flags. Key differences:

Feature	GCC	Clang
Min size	`-Os`	`-Os` or `-Oz` (more aggressive)
Optimise only hot	—	`-fprofile-instr-use` (LLVM PGO)
Thin LTO	`-flto`	`-flto=thin` (faster)
Static analyser	`-fanalyzer`	`clang --analyze` or `clang-tidy`

2. Clang-specific diagnostic flags

bash

# Show fix-it hints inline
clang -Wall -Wextra --show-fixits src.c

# Limit error count
clang -ferror-limit=5 src.c

# Verbose template errors (disable elision)
clang -fno-elide-type src.cpp

# Show tree diff for template mismatch
clang -fdiagnostics-show-template-tree src.cpp

Clang's diagnostics include exact range highlighting and fix-it suggestions that GCC lacks.

3. Optimization remarks

Optimization remarks let you see what Clang did or refused to do:

bash

# Inliner decisions
clang -O2 -Rpass=inline src.c

# Missed vectorisation
clang -O2 -Rpass-missed=loop-vectorize src.c

# Why a loop was not vectorized
clang -O2 -Rpass-analysis=loop-vectorize src.c

# Save all remarks to YAML for post-processing
clang -O2 -fsave-optimization-record src.c
# Produces src.opt.yaml

Interpret remarks:

```
remark: foo inlined into bar
```
— inlining happened; good for hot paths

remark: loop not vectorized: loop control flow is not understood

— restructure the loop

remark: not vectorized: cannot prove it is safe to reorder...

— add

__restrict__

or

#pragma clang loop vectorize(assume_safety)

4. Static analysis

bash

# Built-in analyser (CSA)
clang --analyze -Xanalyzer -analyzer-output=text src.c

# clang-tidy (separate tool, richer checks)
clang-tidy src.c -- -std=c++17 -I/usr/include

# Enable specific check families
clang-tidy -checks='clang-analyzer-*,modernize-*,bugprone-*' src.cpp --

# Apply fixits automatically
clang-tidy -fix src.cpp --

Common

clang-tidy

check families:

```
bugprone-*
```
: real bugs (use-after-move, dangling, etc.)
```
clang-analyzer-*
```
: CSA checks (memory, null deref)
```
modernize-*
```
: C++11/14/17 modernisation
```
performance-*
```
: unnecessary copies, move candidates
```
readability-*
```
: naming, complexity

5. LTO with lld

bash

# Full LTO
clang -O2 -flto -fuse-ld=lld src.c -o prog

# Thin LTO (faster link, nearly same quality)
clang -O2 -flto=thin -fuse-ld=lld src.c -o prog

# Check lld is available
clang -fuse-ld=lld -Wl,--version 2>&1 | head -1

For large projects, ThinLTO is preferred: link times 5-10x faster than full LTO with comparable code quality.

6. PGO (LLVM instrumentation)

bash

# Step 1: instrument
clang -O2 -fprofile-instr-generate prog.c -o prog_inst

# Step 2: run with representative input
./prog_inst < workload.input
# Generates default.profraw

# Step 3: merge profiles
llvm-profdata merge -output=prog.profdata default.profraw

# Step 4: use profile
clang -O2 -fprofile-instr-use=prog.profdata prog.c -o prog

AutoFDO (sampling-based, less intrusive): collect with

perf

, convert with

create_llvm_prof

, use with

-fprofile-sample-use

. See

skills/profilers/linux-perf

.

7. GCC compatibility

Clang is intentionally GCC-compatible for driver flags. Key differences:

Clang does not support all GCC-specific attributes; check with
```
__has_attribute(foo)
```
```
-Weverything
```
enables all Clang warnings (no GCC equivalent); too noisy for production, useful for one-off audits
Some GCC intrinsics need
```
#include <x86intrin.h>
```
on Clang too
```
__int128
```
is supported;
```
__float128
```
requires
```
-lquadmath
```
on some targets

8. macOS specifics

On macOS,

clang

is the system compiler (Apple LLVM). Key points:

```
ld64
```
is the default linker;
```
lld
```
requires explicit
```
-fuse-ld=lld
```
and Homebrew LLVM
Use
```
-mmacosx-version-min=X.Y
```
to set deployment target
Sanitizers on macOS use
```
DYLD_INSERT_LIBRARIES
```
; do not strip the binary
```
xcrun clang
```
resolves to the Xcode toolchain clang

For flag reference, see references/flags.md. For clang-tidy config examples, see references/clang-tidy.md.

Related skills

Use
```
skills/compilers/gcc
```
for GCC-equivalent flag mapping
Use
```
skills/runtimes/sanitizers
```
for
```
-fsanitize=*
```
workflows
Use
```
skills/compilers/llvm
```
for IR-level work (
```
opt
```
,
```
llc
```
,
```
llvm-dis
```
)
Use
```
skills/compilers/msvc-cl
```
for
```
clang-cl
```
on Windows
Use
```
skills/binaries/linkers-lto
```
for linker-level LTO details

clang

NPX Install

Tags

SKILL.md Content

Clang

Purpose

Triggers

Workflow

1. Build mode flags (identical to GCC)

2. Clang-specific diagnostic flags

3. Optimization remarks

4. Static analysis

5. LTO with lld

6. PGO (LLVM instrumentation)

7. GCC compatibility

8. macOS specifics

Related skills