gcc

Original：🇺🇸 English

Translated

GCC compiler skill for C/C++ projects. Use when selecting optimization levels, warning flags, debug builds, LTO, sanitizer instrumentation, or diagnosing compilation errors with GCC. Covers flag selection for debug vs release, ABI concerns, preprocessor macros, profile-guided optimization, and integration with build systems. Activates on queries about gcc flags, compilation errors, performance tuning, warning suppression, or cross-standard compilation.

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Sourcemohitmishra786/low-level-dev-skills

Added on2026-02-21

NPX Install

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

SKILL.md Content

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GCC

Purpose

Guide agents through GCC invocation: flag selection, build modes, warning triage, PGO, LTO, and common error patterns. Assume the project uses GNU Make, CMake, or a shell script.

Triggers

"What flags should I use for a release build?"
"GCC is giving me a warning/error I don't understand"
"How do I enable LTO / PGO with GCC?"
"How do I compile with
```
-fsanitize
```
?"
"My binary is too large / too slow"
Undefined reference errors, ABI mismatch, missing symbols

Workflow

1. Choose a build mode

Goal	Recommended flags
Debug	`-g -O0 -Wall -Wextra`
Debug + debuggable optimisation	`-g -Og -Wall -Wextra`
Release	`-O2 -DNDEBUG -Wall`
Release (max perf, native only)	`-O3 -march=native -DNDEBUG`
Release (min size)	`-Os -DNDEBUG`
Sanitizer (dev)	`-g -O1 -fsanitize=address,undefined`

Always pass

-std=c11

/

-std=c++17

(or the required standard) explicitly. Never rely on the implicit default.

2. Warning discipline

Start with

-Wall -Wextra

. For stricter standards compliance add

-Wpedantic

. To treat all warnings as errors in CI:

-Werror

.

Suppress a specific warning only in a narrow scope:

c

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
// ...
#pragma GCC diagnostic pop

Do not pass

-w

(silences everything) except as a last resort for third-party headers.

3. Debug information

```
-g
```
— DWARF debug info, default level 2
```
-g3
```
— includes macro definitions (useful with GDB
```
macro expand
```
)
```
-ggdb
```
— DWARF extensions optimal for GDB
```
-gsplit-dwarf
```
— splits
```
.dwo
```
files; reduces link time, needed for
```
debuginfod
```

Pair

-g

with

-Og

(not

-O0

) when you need readable optimised code in GDB.

4. Optimisation decision tree

text

Need max throughput on a fixed machine?
  yes -> -O3 -march=native -flto
  no  -> profiling available?
           yes -> -O2 -fprofile-use
           no  -> -O2
Size-constrained (embedded, shared lib)?
  yes -> -Os  (or -Oz with clang)

-O3

vs

-O2

:

-O3

adds aggressive loop transformations (

-funswitch-loops

,

-fpeel-loops

,

-floop-interchange

) and more aggressive inlining. Use

-O3

only after benchmarking; it occasionally regresses due to i-cache pressure.

-Ofast

: enables

-ffast-math

which breaks IEEE 754 semantics (NaN handling, associativity). Avoid unless the numerical domain explicitly permits it.

5. Link-time optimisation (LTO)

bash

# Compile
gcc -O2 -flto -c foo.c -o foo.o
gcc -O2 -flto -c bar.c -o bar.o
# Link (must pass -flto again)
gcc -O2 -flto foo.o bar.o -o prog

Use

gcc-ar

/

gcc-ranlib

instead of

ar

/

ranlib

when archiving LTO objects into static libs.

For parallel LTO:

-flto=auto

(uses

make

-style jobserver) or

-flto=N

.

See references/flags.md for full flag reference. See

skills/binaries/linkers-lto

for linker-level LTO configuration.

6. Profile-guided optimisation (PGO)

bash

# Step 1: instrument
gcc -O2 -fprofile-generate prog.c -o prog_inst
# Step 2: run with representative workload
./prog_inst < workload.input
# Step 3: optimise with profile
gcc -O2 -fprofile-use -fprofile-correction prog.c -o prog

-fprofile-correction

handles profile data inconsistencies from multi-threaded runs.

7. Preprocessor and standards

Inspect macro expansion:
```
gcc -E file.c | less
```
Dump predefined macros:
```
gcc -dM -E - < /dev/null
```
Force strict standard:
```
-std=c11 -pedantic-errors
```
Disable GNU extensions:
```
-std=c11
```
(not
```
-std=gnu11
```
)

8. Common error triage

Symptom	Likely cause	Fix
`undefined reference to 'foo'`	Missing `-lfoo` or wrong link order	Add `-lfoo` ; move `-l` flags after object files
`multiple definition of 'x'`	Variable defined (not just declared) in a header	Add `extern` in header, define in one `.c`
`implicit declaration of function`	Missing `#include`	Add the header
`warning: incompatible pointer types`	Wrong cast or missing prototype	Fix the type; check headers
ABI errors with C++	Mixed `-std=` or different `libstdc++`	Unify `-std=` across all TUs
`relocation truncated`	Overflow on a 32-bit relative relocation	Use `-mcmodel=large` or restructure code

For sanitizer reports, use

skills/runtimes/sanitizers

.

9. Useful one-liners

bash

# Show all flags enabled at -O2
gcc -Q --help=optimizers -O2 | grep enabled

# Preprocess only (check includes/macros)
gcc -E -dD src.c -o src.i

# Assembly output (Intel syntax)
gcc -S -masm=intel -O2 foo.c -o foo.s

# Show include search path
gcc -v -E - < /dev/null 2>&1 | grep -A20 '#include <...>'

# Check if a flag is supported
gcc -Q --help=target | grep march

For a complete flag cheatsheet, see references/flags.md. For common error patterns and examples, see references/examples.md.

Related skills

Use
```
skills/runtimes/sanitizers
```
to add
```
-fsanitize=*
```
builds
Use
```
skills/compilers/clang
```
when switching to clang/LLVM
Use
```
skills/binaries/linkers-lto
```
for advanced LTO linker flags
Use
```
skills/debuggers/gdb
```
for debugging GCC-built binaries

gcc

NPX Install

Tags

SKILL.md Content

GCC

Purpose

Triggers

Workflow

1. Choose a build mode

2. Warning discipline

3. Debug information

4. Optimisation decision tree

5. Link-time optimisation (LTO)

6. Profile-guided optimisation (PGO)

7. Preprocessor and standards

8. Common error triage

9. Useful one-liners

Related skills