ARM Cross-Compilation Tips

Cross-compiling for ARM can seem daunting at first, but with the right tools and techniques, it can be straightforward and efficient. Here are some tips to help you get started with ARM cross-compilation.

Choose the Right Toolchain

The first step is choosing the right toolchain for your needs. Popular options include:

• GNU Arm Embedded Toolchain – Provided by ARM, includes GCC, GDB, and other tools.
• Linaro – Optimized versions of GCC and other tools.
• Android NDK – For compiling code targeting Android on ARM.

Consider factors like licensing, optimization level, and support when selecting a toolchain.

Set up the Cross-Compilation Environment

Once you’ve chosen a toolchain, set up your development environment for cross-compilation. This usually involves:

1. Downloading and installing the toolchain.
2. Configuring environment variables like PATH and CC to point to the toolchain binaries.
3. Installing ARM headers and libraries to match your target.

Setting up a dedicated cross-compilation environment keeps your host and target builds separate.

Select the Right Compiler Flags

Compiler flags are key for optimizing code generation and utilizing hardware capabilities. Common flags include:

• -mcpu – Target a specific ARM CPU architecture.
• -mfpu – Use hardware FPU if present.
• -mfloat-abi – Calling convention for floating point code.
• -march – Optimize for a CPU architecture.
• -mtune – Schedule code for a specific CPU.

Refer to documentation on supported architectures and options when choosing flags.

Use Compiler Built-ins

Compiler built-ins allow generating optimized ARM-specific instructions like SIMD intrinsics. For example: #include float32x4_t vaddq_f32(float32x4_t a, float32x4_t b) { return vaddq_f32(a, b); }

This uses NEON SIMD instead of plain C code. Check documentation for supported built-ins.

Enable Link-time Optimization

Link-time optimization (LTO) allows the compiler to optimize code across translation units. This can significantly improve performance. Enable LTO with flags like: -flto -O3

The tradeoff is increased compile time and memory usage.

Profile Guided Optimization

Profile guided optimization (PGO) uses runtime profiling to guide optimization decisions. This can provide significant speedups but requires running instrumented binaries on-device to capture profiling data. The general process is:

1. Compile with -fprofile-generate.
2. Run instrumented binary on device to generate profile data.
3. Compile with -fprofile-use using profile data.

Use Position Independent Code

Position independent code (PIC) allows generating shared/dynamic libraries and code that can be loaded at any address. PIC is often required for security on ARM. Compile with -fPIC.

Analyze Assembly Output

Examining the generated assembly with -S can help validate that code is efficient and uses the expected instructions. Look for things like:

• Efficient looping and addressing modes.
• SIMD instructions when expected.
• Inlined functions.
• Tail call optimization.

Tweaking compiler flags and source based on assembly analysis can optimize hot code paths.

Use the Right ABI

The ABI (application binary interface) determines things like:

• Function calling conventions.
• Register usage.
• Stack and alignment behavior.

Common ARM ABIs include AAPCS and EABI. Match the ABI used by your libraries and kernel.

Verify Code Generation

Validating that the compiled code runs properly on your target hardware is critical. Some options for verification include:

• Basic unit tests on target hardware.
• Runtime asserts to check assumptions.
• Tracing and profiling using tools like perf.
• Testing corner cases and error handling.

Having a test device helps ensure quality code generation.

Use Compiler Hints

Compiler hints allow providing additional information to guide optimization. For example: __attribute__((hot)) // Optimize this function for frequent calls void foo() { // … }

Read compiler documentation to see available attributes and pragmas.

Build Assembly Files Directly

For time-critical low-level code, writing assembly directly allows meticulous control over generated instructions. Key tips:

• Use .syntax unified assembly syntax.
• Understand ARM instruction encoding.
• Use conditional execution for branchless logic.
• Optimize register usage carefully.

Prefer C when possible, but assembly allows optimizing hot paths.

Use Linker Scripts Wisely

The linker script controls how code and data are mapped into memory. Tips for linker scripts:

• Separate code and data sections.
• Adjust alignments based on use.
• Place time-critical code in fast memory.
• Map memory sections efficiently.

Linker scripts can help optimize memory usage.

Debug with Hardware Tracing

Hardware tracing modules like ETM and PTM provide low overhead tracing of program execution without halting the processor. This allows non-invasive debugging. Useful for:

• Analyzing real-time behavior.
• Profiling code execution.
• Understanding outlier events.

Hardware tracing is invaluable for analyzing ARM system issues.

Use the Right Optimization Level

Higher optimization levels like -O3 enable more compiler optimizations but increase compile time and code size. The right level depends on requirements like:

• Speed vs size tradeoffs.
• Debugging needs.
• Performance bottlenecks.

Benchmark and experiment to select appropriate optimization levels.

Profile on Target Hardware

Different hardware characteristics and workloads can drastically alter optimization priorities. Profile on real hardware under representative workloads. Useful techniques:

• Measure with CPU performance counters.
• Profile cache miss rates.
• Add tracepoints and log key data.
• Use perf for comprehensive profiling.

Target profiling guides practical optimization tradeoffs.

Use Existing Libraries

Leveraging existing optimized ARM libraries avoids reinventing the wheel and reduces bugs. For example:

• Math libraries like BLAS, LAPACK.
• Multimedia libraries like ffmpeg, OpenCV.
• Compression libraries like zlib, lzma.

Evaluate licensing and target support when using libraries.

Optimize Algorithms and Data Structures

Efficient algorithms and data structures provide the largest performance gains. Focus on:

• Reducing asymptotic complexity.
• Optimizing inner loops.
• Minimizing memory usage.
• Streamlining I/O and memory access.

Clean code optimizes better than micro-optimizations.

Use Both C and C++ Appropriately

C is useful for low-level code requiring careful control of data representation, memory layout, and predictable ABIs for interfaces. C++ provides features like templates, exceptions, and classes useful for higher level application logic and abstraction. Consider:

• Performance critical routines in C.
• Higher level orchestration in C++.
• Clearly defined boundaries and APIs between them.

A pragmatic combination leverages strengths of both languages.

Enable Link Time Optimization for Libraries

Enabling LTO when building libraries allows the compiler to optimize across the library boundary when linking executables and shared objects. This can improve performance but increases library build time. Use when:

• Building reusable static or shared libraries.
• Library performance is critical.
• Executable is frequently rebuilt.

Library LTO maximizes optimization potential.

Use Inline Assembly Judiciously

Inline assembly allows embedding ARM assembly within C/C++ code. This is sometimes necessary for things like hardware MMIO. But it has drawbacks:

• Reduces portability.
• Can inhibit compiler optimization.
• Increases complexity.

Limit inline assembly to small time-critical sections when needed.

Conclusion

ARM cross-compilation opens up an exciting world of embedded development. Following cross-compilation best practices helps harness the full power of the ARM architecture efficiently. With the right techniques, you can produce highly optimized binaries tailored precisely for your target device.

The key takeaways are:

• Choose an appropriate modern toolchain.
• Use compiler flags to target your hardware.
• Enable optimizations like LTO and PGO.
• Verify code generation thoroughly.
• Profile on real hardware under load.
• Leverage existing libraries when possible.
• Focus on efficient algorithms and data structures.

ARM CPUs provide an awesome platform for everything from low-power IoT devices to blazing fast mobile appliances. With diligent cross-compilation, you can unleash the full potential of the ARM architecture efficiently.