GCC Toolchain

GCC version selection, configuration options, optimization flags, and hardening settings in OpenWrt.

OpenWrt builds GCC from source as part of its toolchain. The compiler is configured specifically for the target architecture and C library, ensuring optimal code generation for embedded systems.

Supported GCC Versions

The following GCC versions are available, selectable via menuconfig:

Version	Config Symbol	Notes
GCC 12.x	GCC_USE_VERSION_12	Older stable branch
GCC 13.x	GCC_USE_VERSION_13	Previous stable branch
GCC 14.x	GCC_USE_VERSION_14	Default
GCC 15.x	GCC_USE_VERSION_15	Latest branch

The default is GCC 14 (version 14.3.0). To change the GCC version:

make menuconfig
# Navigate to: Advanced configuration options -> Toolchain Options
#   -> GCC compiler Version

GCC Configuration Options

These options are available under Toolchain Options in menuconfig (requires TOOLCHAINOPTS or DEVEL mode):

Option	Config Symbol	Description
PIE by default	GCC_DEFAULT_PIE	Builds executables with Position Independent Execution enabled (-fPIE -pie)
SSP by default	GCC_DEFAULT_SSP	Enables Stack-Smashing Protection by default (-fstack-protector-strong)
Graphite	GCC_USE_GRAPHITE	Enables the Graphite loop optimization framework
SJLJ exceptions	SJLJ_EXCEPTIONS	Use setjmp/longjmp for C++ exception handling (increases code size)
Fortran compiler	INSTALL_GFORTRAN	Build and install gfortran in the toolchain
Go compiler	INSTALL_GCCGO	Build and install gccgo (glibc only)
Extra config options	EXTRA_GCC_CONFIG_OPTIONS	Arbitrary additional ./configure flags

Optimization Flags

Target optimization flags are configured per-target in target/linux/<target>/Makefile via the TARGET_OPTIMIZATION variable, which defaults to DEFAULT_TARGET_OPTIMIZATION from the toolchain config.

Typical embedded target flags:

-Os           # Optimize for size (most common for flash-constrained devices)
-O2           # Optimize for speed
-pipe         # Use pipes instead of temp files between compilation stages
-fno-caller-saves  # Avoid saving registers for call-saved register allocation

For MIPS targets:

-march=24kc   # Target MIPS 24K core
-mtune=24kc

For ARM targets:

-march=armv7-a
-mtune=cortex-a9
-mfpu=vfpv3-d16
-mfloat-abi=hard

LTO (Link Time Optimization)

LTO allows GCC to optimize across translation unit boundaries, often reducing binary size and improving performance.

Global LTO can be enabled via:

make menuconfig
# Advanced configuration options -> Toolchain Options -> Enable LTO
# Config symbol: CONFIG_USE_LTO

Per-package LTO control uses PKG_BUILD_FLAGS:

# Enable LTO for this package
PKG_BUILD_FLAGS:=lto

# Disable LTO for this package even when globally enabled
PKG_BUILD_FLAGS:=no-lto

When LTO is active, the following flags are added (from include/package.mk):

TARGET_CFLAGS  += -flto=auto -fno-fat-lto-objects
TARGET_CXXFLAGS += -flto=auto -fno-fat-lto-objects
TARGET_LDFLAGS += -flto=auto -fuse-linker-plugin

Hardening Flags

Security hardening flags are applied from include/hardening.mk. These are automatically included for all package builds.

Common hardening options controlled via menuconfig:

Config Symbol	Flag Applied	Effect
CONFIG_PKG_CC_STACKPROTECTOR_REGULAR	-fstack-protector	Basic stack canary protection
CONFIG_PKG_CC_STACKPROTECTOR_STRONG	-fstack-protector-strong	Stronger stack protection
CONFIG_PKG_RELRO_FULL	-Wl,-z,now -Wl,-z,relro	Full RELRO (read-only after relocation)
CONFIG_PKG_RELRO_PARTIAL	-Wl,-z,relro	Partial RELRO
CONFIG_PKG_ASLR_PIE_ALL	-fPIE -pie	PIE for all packages

Wrapper Scripts

OpenWrt wraps the cross-compilers in shell scripts located in toolchain/wrapper/. These wrappers automatically inject the required flags (-fhonour-copts, sysroot paths, etc.) so that package build systems do not need to know all of the OpenWrt-specific flags.

The wrapper is responsible for:

• Enforcing GCC_HONOUR_COPTS to detect packages that incorrectly override compiler flags
• Adding the correct --sysroot path
• Injecting target-specific flags from the build environment

GCC Build Stages

GCC is built in three stages to bootstrap the toolchain:

Stage	Directory	Purpose
Minimal	toolchain/gcc/minimal/	Bare-minimum compiler to build the C library
Initial	toolchain/gcc/initial/	Compiler using the newly built C library headers
Final	toolchain/gcc/final/	Full compiler with complete C library support