What is GNU GCC compiler for ARM?

The GNU Compiler Collection (GCC) is a free and open source compiler system produced by the GNU Project. GCC supports compiling code written in various programming languages like C, C++, Objective-C, Fortran, Ada, Go and more for many different processor architectures including ARM.

GCC can be used to compile code specifically for ARM processors. The ARM architecture refers to the set of Reduced Instruction Set Computing (RISC) architectures developed by ARM Holdings for use in mobile devices and other embedded systems.

Overview of GCC for ARM

GCC contains different front ends for compiling code written in various languages and machine-dependent back ends for generating optimized code for different architectures. For ARM, GCC contains the arm-none-eabi and arm-linux-gnueabihf back ends to target bare metal ARM systems and ARM Linux systems respectively.

The key components of GCC for ARM include:

• Front ends like gcc, g++, gfortran etc. to compile code written in C, C++, Fortran etc.
• Back ends like arm-none-eabi and arm-linux-gnueabihf to generate optimized ARM machine code.
• Optimizations specifically for the ARM architecture to generate efficient code.
• Support for different ARM architectures like ARMv6, ARMv7, ARMv8 etc.
• Support for generating code for 32-bit ARM (AArch32) and 64-bit ARM (AArch64).
• Support for various ARM instruction set extensions like NEON, VFP, Thumb-2 etc.
• Libraries and headers optimized for ARM Linux and bare metal environments.

Benefits of using GCC for ARM

Here are some of the major benefits of using GCC for compiling code targeting ARM processors:

• Free and open source – GCC is free to download, use and modify without any licensing costs.
• Cross platform – GCC can be used on Linux, macOS and Windows to compile code for ARM.
• Highly optimized code generation – GCC generates efficient ARM machine code using architecture-specific optimizations.
• Supports multiple languages – C, C++, Objective-C, Fortran, Ada etc. can be compiled for ARM using GCC.
• Supports latest ARM architectures – Optimized support for compiling to the latest ARMv6, ARMv7, ARMv8 architectures.
• Active development – GCC is under active development with frequent updates and bug fixes.
• Mature and robust – GCC is stable, mature and has been used commercially for decades.
• Customizable – Engineers can fine tune compilation settings to generate tailored ARM code.
• IDE integration – Integrates well with IDEs like Eclipse, NetBeans etc.

How to install GCC for ARM

Here are the general steps to install GCC for compiling code targeting ARM processors:

1. Get GCC source code – Download the latest source release of GCC from https://gcc.gnu.org or clone the Git repository.
2. Install prerequisites – Install required tools like binutils, GMP, MPFR etc. GCC also needs a supported C/C++ compiler to bootstrap.
3. Configure for ARM – Run the configure script with –target=arm-none-eabi or –target=arm-linux-gnueabihf options.
4. Build – Run make which will build GCC and components like binutils targeted for ARM.
5. Install – Finally run make install to install the ARM targeted GCC toolchain on your system.

For ARM Linux targets, pre-built toolchain binaries are also available from various sources.

How to use GCC for compiling ARM code

Once GCC is installed for ARM, it can be used to build ARM projects with commands like: # Compile C source for ARM arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -o main.o -c main.c # Link object files into ARM ELF executable arm-none-eabi-gcc -o main.elf main.o # Compile C++ source for ARM arm-linux-gnueabihf-g++ -march=armv8-a -o main main.cpp

The target triple prefixes like arm-none-eabi or arm-linux-gnueabihf specify which ARM toolchain to use. Flags like -mcpu, -mthumb etc. control chip architecture, instruction sets etc.

For bare metal ARM development, the newlib C library is typically used. For Linux ARM development, GCC links against the glibc C library by default.

GCC ARM code generation

GCC performs many optimizations during code generation to produce efficient machine code for ARM processors including:

• Register allocation – Optimally using the 16 ARM registers like r0-r15.
• Instruction scheduling – Scheduling instructions to avoid pipeline stalls.
• Loop optimizations – Loop inversion, rotation, unrolling etc.
• Branch optimizations – Reducing costly branch instructions.
• Tail call optimizations – Using jump instead of call for tail recursion.
• Function inlining – Inlining small functions reduces call overhead.
• Constant propagation – Replace variable reads with constant values when possible.
• Dead code elimination – Remove unreachable code segments.
• Thumb-2 optimization – Utilize efficient Thumb-2 instruction set extensions.

Higher levels of optimization can be enabled in GCC using flags like -O3, -Ofast etc. Profile guided optimization can further enhance performance.

GCC ARM floating point support

For generating floating point code, GCC supports multiple floating point ABIs for interfacing between code and ARM FPUs including:

• VFP – For VFP coprocessor in older ARM cores.
• VFPv3-D16 – For VFP in Cortex-M4 etc.
• VFPv3 – For VFPv3 in Cortex-A cores.
• VFPv3-D32 – For VFPv3-D32 in Cortex-A cores.
• VFPv4-D16 – For VFPv4-D16 in Cortex-R cores.
• VFPv4 – For VFPv4 in Cortex-A cores.
• NEON – For NEON SIMD in Cortex-A cores.

The appropriate floating point ABI can be selected in GCC using -mfloat-abi flag during compilation.

Conclusion

GCC is a mature, flexible and robust compiler system capable of generating highly optimized code for the ARM architecture. Both commercial enterprises and open-source projects use GCC extensively for ARM development. With its ARM optimization capabilities and support for multiple languages, GCC simplifies the process of building high quality software for ARM Linux devices and bare-metal ARM systems.