clang

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

# 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:

# 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)