Tail Recursion and Extern Functions in Arm64 #raspberrypi #assembly #optimization

Introduction to Tail Recursion and Low-Level Optimization

Overview of the Video Content

• The video discusses tail recursion, focusing on low-level optimization techniques and the use of external functions for better organization. It begins with connecting to a Raspberry Pi via SSH using user credentials.

Implementing Tail Recursion in Assembly

Example 42: Factorial Calculation

• The file 42_facts.s is edited using nano, where the global directive is defined with the function name. The parameter number is received in register x0, which is not preserved; hence it’s moved to x9.

• A neutral value for multiplication (1) is set in x10. The tail recursion will be translated into a loop that checks if the received number (x9) equals zero, jumping to end_rack if true.

Loop Translation of Tail Recursion

• Multiplication occurs between x10 (neutral value) and x9 (the received number). As iterations proceed, x9 decrements while accumulating results.

• When x9 reaches zero, control jumps to endrec, concluding the factorial calculation.

Returning Results from Factorial Function

• The final result of the factorial computation remains in register x10 and is moved back to x0 for returning to the caller.

• After saving changes in nano, assembly commands are executed to create an object file necessary for later reuse.

Input/Output Handling: ITO Function

Implementation Steps

• To implement ITO functionality, start with parameters: received number, empty buffer, and base 10 definition. If the number equals zero, return a buffer containing zero followed by a newline.

• For non-zero numbers, calculate length and write digits backward into the buffer while iterating until all digits are processed.

Initialization Process for ITO Function

• Initialize by moving arguments into registers: argument number goes into x9 and buffer address into x10. Base 10 is defined in X11.

• Special handling for zero cases involves assigning ASCII character '0' to register X9 before storing it at position zero of the buffer.

Counting Digits in Non-Zero Case

• A loop performs successive divisions by base 10 to count digits until reaching zero. This count determines how many digits will be written backward into the buffer.

Writing Digits Backward

• After counting digits, adjust pointers within the buffer as needed before writing each digit from right-to-left based on calculated values from divisions by base 10.

Finalizing Buffer Output

• Since ARM64 lacks modulo operations directly, remainders must be calculated manually. Each extracted digit gets converted into its ASCII representation before being stored back into the buffer.

Setting Up External Functions

Example 44: Using Extern Directives

• In 44_extern.s, extern directives are added alongside defining an 11-byte buffer suitable for receiving output from IOA functions after invoking them correctly.

Invoking Functions

• Assign test values and invoke factorial function using BL instruction; results return through register X0. Addressing buffers correctly ensures data flows properly between functions.

Printing Results

• Prepare system calls for printing results stored in buffers by setting appropriate registers before executing print commands via svc0 system call.

Linking Object Files

• Link three object files (42_fact.o, 43_it.o, 44_extern) together to create an executable named 44_extern. Execution confirms successful output displaying "3,628,800".

Conclusion

• The video wraps up with encouragement for viewers to engage through likes or subscriptions while summarizing key outcomes achieved during programming exercises related to tail recursion and assembly language optimizations.