What is ARM cross-compiler?

An ARM cross-compiler is a compiler that runs on one architecture but produces executable code for a different architecture – in this case, code for ARM processors. Cross-compilers are useful for software development when the compiler and target system have different architectures.

Why use an ARM cross-compiler?

There are several reasons why you may want or need to use an ARM cross-compiler:

• You are developing software for an embedded ARM device and want to compile code on your desktop computer rather than the device itself.
• The ARM device does not have the resources (RAM, storage, performance) to host a full compiler toolchain.
• You want to speed up compilation by leveraging the greater power of a desktop computer compared to an embedded device.
• You need to compile code for multiple different ARM architectures (Cortex-A, Cortex-R, Cortex-M, etc.) from a single development environment.
• You want to develop and test code before deploying it to the target ARM system.

In general, cross-compilers allow compiling for resource-constrained platforms from a more powerful host system. This makes development easier and faster.

How does an ARM cross-compiler work?

A cross-compiler consists of various components:

• Compiler frontend: Performs lexical analysis, parsing, semantic analysis, and code generation. Architecture-independent.
• Compiler backend: Converts intermediate code to target machine code. Architecture-specific.
• Runtime libraries: Libraries needed to support language features and functions. Built for target architecture.
• Linker: Resolves references between object files and libraries. Target architecture-specific.

During compilation, source code is converted to an intermediate representation by the frontend. The backend then takes this and generates optimized ARM machine code. The linker combines different object files and libraries into a final executable file.

A cross-compiler has a frontend matched to the host architecture but a backend and libraries for the target ARM architecture. This allows compiling ARM executables on a non-ARM host.

Components of an ARM cross-compiler

A complete ARM cross-compiler toolchain includes the following components:

• Compiler: Usually GCC (GNU Compiler Collection) or Clang/LLVM.
• Assembler: Converts assembly to machine code. GNU assembler (gas) or Clang integrated assembler.
• Linker: Resolves cross-references between object files. GNU linker (ld) or LLD linker.
• Debugger: For debugging source code. Typically GDB.
• Runtime libraries: Standard C and math libraries for ARM. Usually newlib, musl, or glibc ported for ARM.
• Utilities: Build automation tools like make and autotools. Also binary utilities like objdump, nm, strip.

To build a toolchain, the compiler and other components need ARM-specific backend code and optimizations. Pre-built versions are provided by ARM, Linaro, or vendor SDKs.

Setting up an ARM cross-compilation environment

Here are the main steps to set up cross-compilation for ARM on Linux or Windows hosts:

1. Get pre-built tools: Download a pre-built GNU toolchain from ARM, Linaro, or vendor SDKs. These provide compiler, assembler, linker, libraries, etc.
2. Install tools: Extract and install the tools on your host system. Set PATH to point to the binaries.
3. Get sysroot: Get the ARM sysroot, containing headers and libraries for the target. Vendor SDKs typically include this.
4. Set environment variables: Set the ARCH, CROSS_COMPILE, and other vars needed by the tools.
5. Test compilation: Compile a simple program with arm-none-eabi-gcc to verify the tools work.

After this, you can compile ARM code by prefixing build commands with the cross-toolchain prefix, like arm-none-eabi-. The same process works for other target architectures like Cortex-M or Cortex-R.

Advantages of using an ARM cross-compiler

Here are some key advantages of using a cross-compiler for ARM development:

• Faster compilation: Compile times are much shorter compared to compiling natively on ARM devices.
• Leverage host performance: Make use of the CPU power, RAM, and storage of desktop machines for building software.
• Unified environment: Develop for multiple ARM targets from a single environment using one toolchain.
• Advanced optimizations: Can apply more advanced optimizations during compilation versus on device.
• Robust tools: Leverage robust and mature tools like GCC, GDB which may not fit on embedded devices.

Using a cross-compiler boosts productivity by moving compilation off of the slower target device onto a faster host system while retaining the ability to optimize code generation for the target ARM architecture.

Challenges with ARM cross-compilation

While cross-compilers simplify many aspects of embedded software development, there are also some challenges to be aware of:

• Limited testing: Cannot easily test code execution natively on the host system.
• Debugging harder: Debugging across host and target machines is more complex.
• Hardware differences: Must account for hardware differences between host and target environments.
• Library support: Runtime library compatibility issues may exist between architectures.
• Linker issues: Subtle linker issues may crop up while linking for the target machine.

Despite these issues, the gains from faster compilation and centralized development usually outweigh the costs for most embedded projects using ARM processors.

Conclusion

An ARM cross-compiler allows compiling ARM executables on a host system with a different architecture from the target. This provides significant productivity advantages for embedded software development targeting ARM processors.

With a proper cross-compilation toolchain, developers get access to robust host tools, faster edit-compile-debug cycles, and a unified environment for managing multiple ARM targets. While debugging and testing pose challenges, overall cross-compilers greatly simplify ARM development compared to compiling natively on-device.