Brief Answer

To inline C functions in ARM assembly for Cortex-M4 interrupt handlers on nRF52840 without branching overhead while keeping assembly code separate, use the __attribute__((always_inline)) directive with inline assembly that directly accesses registers, implement cross-file references with .global and .extern declarations, and optimize your build system with appropriate flags to maintain separate files while eliminating function call overhead.

Contents

• Understanding the Challenge of Inline Assembly in Interrupt Handlers
• Cortex-M4 Architecture and nRF52840 Considerations
• Cross-File Assembly Implementation
• Minimizing Branching Overhead
• Practical Implementation Example
• Build System Configuration
• Optimization Techniques for nRF52840
• Debugging and Validation

Understanding the Challenge of Inline Assembly in Interrupt Handlers

Implementing inline C functions in ARM assembly for interrupt handlers presents unique challenges, particularly when maintaining separate files while eliminating branching overhead. The key issues include:

1. Context Preservation: Interrupt handlers must maintain system state while executing custom code
2. Register Management: Balancing register usage between C calling conventions and assembly optimization
3. Branch Elimination: Removing function call overhead while maintaining modularity
4. File Separation: Keeping assembly code in separate files without performance penalties

The Cortex-M4 processor architecture with its register banking and interrupt handling mechanisms adds specific considerations that differ from other ARM implementations.

Cortex-M4 Architecture and nRF52840 Considerations

The ARM Cortex-M4 processor features that impact interrupt handling optimization:

• Register Banking: Registers r4-r11 are banked for interrupt handlers, reducing save/restore overhead
• Thumb-2 Instruction Set: Mix of 16-bit and 32-bit instructions for optimal balance of code density and performance
• Single-Cycle Operations: Many instructions execute in a single cycle, allowing for highly optimized interrupt handling
• Nested Interrupt Controller (NVIC): Hardware-based interrupt prioritization and nesting

Specific to the nRF52840:

• Maximum CPU frequency of 64 MHz
• Hardware floating-point unit (FPv4-SP) with support for single-precision floating-point operations
• Advanced power management features
• Multiple peripheral interrupt sources

When implementing interrupt handlers, understanding these features allows you to create highly optimized code that leverages the hardware capabilities while maintaining separation between C and assembly code.

Cross-File Assembly Implementation

To keep assembly code in separate files while achieving the performance benefits of inlining:

1. Create Assembly File (e.g., isr_handlers.S):

.global timer0_IRQHandler
.weak timer0_IRQHandler

timer0_IRQHandler:
    push {r0, r1, lr}
    
    // Your optimized assembly code here
    ldr r0, =0x40008000      // TIMER0_BASE address
    ldr r1, [r0, #0x508]     // Load TIMER0_CC[0] value
    adds r1, #1              // Increment value
    str r1, [r0, #0x508]     // Store back
    
    pop {r0, r1, lr}
    bx lr

2. Reference from C Code:

// In your interrupt handler declaration
void timer0_IRQHandler(void) __attribute__((interrupt("IRQ")));

// In your application code
extern void timer0_IRQHandler(void);

3. Linker Considerations:
   • Ensure the interrupt handler is properly placed in the interrupt vector table
   • Use appropriate sections in your linker script
   • Set proper attributes for the interrupt handler function

Minimizing Branching Overhead

To eliminate branching overhead in interrupt handlers:

1. Use Direct Register Operations:

   c

   __asm__ volatile (
       "ldr r0, =0x40000000\n\t"  // Load address directly
       "ldr r1, [r0]\n\t"         // Load value
       "add r1, #1\n\t"           // Increment
       "str r1, [r0]\n\t"         // Store back
   );
   

2. Leverage Conditional Execution:

   c

   __asm__ volatile (
       "cmp r0, #0\n\t"
       "addne r1, r1, #1\n\t"     // Only add if not equal
   );
   

3. Minimize Memory Accesses:

   • Keep variables in registers when possible
   • Use register-to-register operations
   • Implement efficient data structures

4. Optimize Loop Structures:

   • Manually unroll small loops
   • Use conditional execution for loop iterations
   • Implement branchless algorithms where possible

Practical Implementation Example

Here’s a complete example of an optimized timer interrupt handler for nRF52840:

isr_handlers.S:

.global timer0_IRQHandler
.weak timer0_IRQHandler

timer0_IRQHandler:
    // Save registers beyond banking (r0-r3, r12, lr)
    push {r0, r1, lr}
    
    // Direct register access pattern for minimal branching
    ldr r0, =0x40008000          // TIMER0_BASE
    ldr r1, [r0, #0x508]        // TIMER0_CC[0] register
    
    // Optimized counter increment
    adds r1, #1                  // Add with update of status flags
    str r1, [r0, #0x508]        // Store back
    
    // Clear interrupt event
    ldr r1, [r0, #0x50C]        // TIMER0_EVENTS_COMPARE[0]
    
    // Restore registers and return
    pop {r0, r1, lr}
    bx lr

main.c:

#include <stdint.h>
#include "nrf.h"

// Function declarations
void timer0_IRQHandler(void) __attribute__((interrupt("IRQ")));

// Timer initialization function
void timer_init(void) {
    // Configure timer hardware
    NRF_TIMER0->MODE = TIMER_MODE_MODE_Timer;
    NRF_TIMER0->PRESCALER = 4;     // 16MHz/2^5 = 500kHz
    NRF_TIMER0->CC[0] = 50000;    // 100ms period (500kHz/5000)
    NRF_TIMER0->INTENSET = TIMER_INTENSET_COMPARE0_Msk;
    NRF_TIMER0->TASKS_START = 1;
    
    // Enable timer interrupt
    NVIC_EnableIRQ(TIMER0_IRQn);
    NVIC_SetPriority(TIMER0_IRQn, 3);
}

int main(void) {
    timer_init();
    
    while(1) {
        // Main application loop
    }
    
    return 0;
}

Build System Configuration

To maintain separate files while achieving optimal performance:

1. GCC Compiler Flags:

   makefile

   CFLAGS += -O3 -fno-inline-functions-called-once
   CFLAGS += -ffunction-sections -fdata-sections
   CFLAGS += -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16
   

2. Assembler Flags:

   makefile

   ASFLAGS += -Wa,-mimplicit-it=thumb
   ASFLAGS += -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16
   

3. Linker Configuration:

   makefile

   LDFLAGS += -Wl,--gc-sections
   LDFLAGS += -Wl,--undefined=g_pfnVectors
   LDFLAGS += -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16
   

4. Makefile Rule for Assembly Files:

   makefile

   %.o: %.S
           $(CC) $(CFLAGS) $(ASFLAGS) -c $< -o $@
   

Optimization Techniques for nRF52840

1. Leverage Cortex-M4 Instructions:

   • Use IT (If-Then) blocks for conditional execution
   • Implement DSP instructions for mathematical operations
   • Utilize PLD (preload) instructions for memory access optimization

2. Memory Access Optimization:

   • Use LDRD/STRD for paired register operations
   • Implement cache-friendly access patterns
   • Consider the nRF52840’s memory acceleration features

3. Interrupt Latency Reduction:

   • Set appropriate NVIC priorities
   • Use priority grouping to optimize nested interrupt handling
   • Minimize the number of interrupts in critical sections

4. Power Management:

   • Use WFI (Wait For Interrupt) instruction in idle loops
   • Implement clock gating for unused peripherals
   • Take advantage of the nRF52840’s low-power modes

Debugging and Validation

When optimizing interrupt handlers with assembly code:

1. Register Verification:

   • Use debugger register view to confirm proper register preservation
   • Verify that banking registers (r4-r11) are correctly managed

2. Interrupt Latency Measurement:

   c

   // GPIO toggle method for measuring interrupt latency
   #define LATENCY_MEAS_GPIO_PIN 18
   
   void latency_test_init(void) {
       NRF_GPIO->DIRSET = (1 << LATENCY_MEAS_GPIO_PIN);
       NRF_GPIO->OUTCLR = (1 << LATENCY_MEAS_GPIO_PIN);
   }
   
   void __attribute__((interrupt("IRQ"))) TIMER0_IRQHandler(void) {
       NRF_GPIO->OUTSET = (1 << LATENCY_MEAS_GPIO_PIN);
       // ... rest of handler
   }
   

3. Stack Usage Analysis:

   • Monitor stack pointer to ensure no overflow
   • Use linker-generated stack usage reports

4. Performance Profiling:

   • Utilize cycle-accurate measurement tools
   • Compare performance before and after optimizations

By following these techniques, you can create highly optimized interrupt handlers for the nRF52840 that maintain the benefits of separate assembly files while eliminating branching overhead through careful register management, direct memory access patterns, and optimized build configurations.