Converting C++ code to ARM assembly can significantly improve performance for specific, computationally intensive tasks. While rewriting entire applications in assembly is generally impractical, strategically incorporating assembly code into performance-critical sections can yield substantial speedups. This guide explores various techniques for achieving this, focusing on practicality and best practices.
Table of Contents
- Generating ARM Assembly with GCC
- Using External Assembly Functions
- Generating ARM Assembly with armclang
- Inline Assembly (with Cautions)
- Best Practices for C++ to ARM Assembly Conversion
Generating ARM Assembly with GCC
The GNU Compiler Collection (GCC) offers robust cross-compilation capabilities. To generate ARM assembly code from your C++ source, utilize the -S flag along with the appropriate ARM cross-compiler. The optimization level significantly impacts the generated assembly; higher levels (e.g., -O2, -O3) often result in more complex but potentially faster code.
arm-linux-gnueabi-gcc -S -O2 myprogram.cpp -o myprogram.s
Remember to substitute arm-linux-gnueabi-gcc with the correct cross-compiler for your target architecture (e.g., for 64-bit ARM you might use aarch64-linux-gnu-gcc). The output file, myprogram.s, will contain the equivalent ARM assembly instructions.
Using External Assembly Functions
For more complex assembly routines, it’s often cleaner to write separate assembly files (typically with a .s extension). This allows for better organization and reusability. Here’s an example of a modulus function implemented in assembly:
// C++ code (main.cpp)
#include <iostream>
extern "C" int mod_asm(int a, int b);
int main() {
int result = mod_asm(10, 3);
std::cout << "Result: " << result << std::endl;
return 0;
}
// Assembly code (mod_asm.s)
.global mod_asm
mod_asm:
udiv r0, r0, r1 @ Divide a (r0) by b (r1)
mls r0, r1, r0, r0 @ Multiply r1 and the quotient (r0), subtract from a (r0) - this gives the remainder
bx lr @ Return
Compilation and linking would then involve separate steps:
arm-linux-gnueabi-gcc -c mod_asm.s -o mod_asm.o
arm-linux-gnueabi-gcc main.cpp mod_asm.o -o myprogram
Generating ARM Assembly with armclang
The ARM Compiler’s armclang provides an alternative to GCC. Its usage is similar, employing the -S flag for assembly generation:
armclang -S -O2 myprogram.cpp -o myprogram.s
armclang often produces different assembly code compared to GCC, sometimes with varying optimization effectiveness. Experimentation might be necessary to determine which compiler yields better results for your specific needs.
Inline Assembly (with Cautions)
Inline assembly, using compiler-specific keywords (e.g., __asm in GCC/Clang), allows embedding short assembly snippets directly within your C++ code. However, this approach is significantly less portable and more prone to errors. It’s generally best reserved for very small, highly optimized sections where portability isn’t a major concern. The syntax is compiler-dependent, requiring careful consultation of the compiler’s documentation.
Best Practices for C++ to ARM Assembly Conversion
When converting C++ to ARM assembly, consider these best practices:
- Profile first: Identify performance bottlenecks before optimizing. Don’t guess where the slow parts are; use profiling tools.
- Start small: Begin with small, critical sections of code. Incremental changes are easier to manage and debug.
- Test thoroughly: Rigorous testing is crucial to ensure correctness and performance gains.
- Maintainability: Prioritize readability and maintainability of your assembly code. Use comments liberally.
- Understand the architecture: A solid grasp of ARM architecture (registers, instruction set, memory model) is essential for effective assembly programming.