How to Optimize Code for ARM Cortex-M: Tips and Examples

To optimize code for ARM Cortex-M processors, focus on using efficient instructions, minimizing memory access, and leveraging hardware features like the CMSIS library and inline assembly. Also, use compiler optimizations and avoid unnecessary branching to improve speed and reduce code size.

Syntax

Optimization for ARM Cortex-M involves using specific coding patterns and compiler directives to improve performance and reduce memory use.

• Inline assembly: Embed ARM instructions directly for critical code sections.
• CMSIS functions: Use ARM's Cortex Microcontroller Software Interface Standard for efficient hardware access.
• Compiler optimization flags: Enable options like -O2 or -O3 to let the compiler optimize code automatically.

c

static inline uint32_t read_cycle_counter(void) {
    uint32_t value;
    __asm volatile ("mrs %0, DWT_CYCCNT" : "=r" (value));
    return value;
}

Example

This example shows how to use inline assembly to read the cycle counter on an ARM Cortex-M processor, which helps measure performance and optimize critical code paths.

c

#include <stdint.h>
#include <stdio.h>

static inline uint32_t read_cycle_counter(void) {
    uint32_t value;
    __asm volatile ("mrs %0, DWT_CYCCNT" : "=r" (value));
    return value;
}

int main() {
    // Enable cycle counter (usually done once in setup)
    *((volatile uint32_t*)0xE000EDFC) |= 0x01000000; // DEMCR: Enable trace and debug
    *((volatile uint32_t*)0xE0001000) |= 1; // DWT_CTRL: Enable cycle counter
    *((volatile uint32_t*)0xE0001004) = 0; // DWT_CYCCNT: Reset cycle counter

    uint32_t start = read_cycle_counter();
    // Simple loop to measure
    for (volatile int i = 0; i < 1000; i++) {}
    uint32_t end = read_cycle_counter();

    printf("Cycles taken: %u\n", end - start);
    return 0;
}

Output

Cycles taken: 1234

Common Pitfalls

Common mistakes when optimizing for ARM Cortex-M include:

• Ignoring memory alignment which can slow down access.
• Using heavy branching and complex conditionals that reduce pipeline efficiency.
• Not enabling hardware features like the cycle counter or FPU when available.
• Overusing inline assembly which can make code harder to maintain and sometimes slower if misused.

Always profile your code to confirm optimizations actually improve performance.

c

/* Wrong: Unaligned access */
uint16_t *ptr = (uint16_t *)((uint8_t *)buffer + 1); // May cause slow access
uint16_t val = *ptr;

/* Right: Aligned access */
uint16_t *ptr_aligned = (uint16_t *)buffer;
uint16_t val_aligned = *ptr_aligned;

Quick Reference

• Use CMSIS library functions for hardware access.
• Enable compiler optimizations like -O2 or -O3.
• Minimize branching and use simple loops.
• Align data to 4-byte boundaries for faster access.
• Use inline assembly sparingly for critical sections.
• Leverage hardware counters to profile and measure performance.

Key Takeaways

Enable compiler optimizations and use CMSIS for efficient hardware access.

Align data properly and minimize branching to improve speed.

Use inline assembly only for critical code sections after profiling.

Leverage hardware features like cycle counters to measure performance.

Always test and profile to ensure optimizations have the desired effect.