Lecture 6

Subroutines. Call stack. Calling conventions.

Lecture

Slides (PDF, PPTX).

Outline

• Functions, caller, callee
• Jump-and-link instructions
• Register conventions
• Stack pointer
• Stack frame
• Caller-saved and callee-saved registers

Examples

• macrolib.s
• leaf.s
• fact.s
• fp.s
• fp_square.s
• euclid.s
• min_max.s
• min_max2.s
• swap.s
• ackermann.s

Workshop

Outline

• Demonstrate using the RISC-V toolchain in Cloud Ubuntu VM (optional)
• Demonstrate using Godbolt, show both GCC and Clang (optional)
• Practice programs that allocate variables and arrays on stack
• Practice writing programs that use functions
• Practice using caller-saved and callee-saved registers
• Practice writing nested and recursive functions

How to call a function?

Saving registers:

• The caller does not know what registers are used by the callee.
• The callee does not know what registers are used by the caller.
• Therefore, the callee is likely to overwrite registers that are important for the caller.
• To avoid this situation, a programmer must follow the calling conventions.

Register conventions:

1. Callee-saved registers: sp, s0-s11
2. Caller-saved registers: ra, a0-a7, t0-t6

What is done by the caller?

1. Save all caller-saved registers, which must be preserved, to the stack.
2. Put the arguments into registers a0-a7.
3. Call the function.
4. When the function returns, read the return values from a0 and a1.
5. Restore all the previously saved caller-saved registers from the stack.

What is done by the callee?

1. Save all callee-saved registers, which will be modified, to the stack.
2. Perform some operations (if the callee wants to call a function, it becomes the caller for this callee function).
3. Save the result to registers a0 and a1.
4. Restore all the previously saved callee-saved registers from the stack.
5. Return back to the caller.

Tasks

1. Translate the following C code into the RISC-V assembly language:

   int f(int x, int y) {
       return 2 * x + y;
   }
   
   int g(int x, int y) {
       return 3 * y - x);
   }
   
   int main() {
       int x = read_int();
       int y = read_int();
       int z = f(x, y) + x + g(x, y) - y;
       print_int(z);
   }
   

2. Translate the following C code into the RISC-V assembly language:

   int f(int x, int y) {
       return 2 * x + y;
   }
   
   int g(int a, int b, int c, int d) {
       return f(a, c) - f(b, d);
   }
   
   int main() {
       int a = read_int();
       int b = read_int();
       int c = read_int();
       int d = read_int();
       int x = g(a, b, c, d);
       print_int(x);
   }
   

3. Write program divide.s that inputs two positive integer values N and D, finds their quotient (Q) and remainder (R) using the algorithm below, and prints the result. The algorithm must be implemented as a function (the code from the previous seminar can be reused).

   function divide_unsigned(N, D)
       Q := 0; R := N
       while R ≥ D do
           Q := Q + 1
           R := R − D
       end
       return (Q, R)
   end
   

4. Write program gcd.s that inputs two positive integer values a and b, finds their greatest common divisor using the algorithm below, and prints the result. The algorithm must be implemented as a recursive function.

   function gcd(a, b)
       if b = 0
           return a
       else
           return gcd(b, a mod b)
   

5. Write program fib.s that inputs integer value n, computes n-th Fibonacci number using the algorithm below, and prints the result. The algorithm must be implemented as a recursive function.

   int fib(int n) {
       if (n < 2)
           return n;
       else
           return fib(n-1) + fib(n-2);
   }
   

6. Write program sum.s that first inputs integer value n, after that inputs n integer elements and stores them in the stack, then calls function sum adding all the elements, and, finally, prints the sum.

Homework

Solve the following tasks and submit then into Ejudge:

1. ASCIIGrid
2. CheckTriangles
3. FuncSort
4. KeySort
5. BinarySearch

References

• Functions in RISC-V. Section 2.8 in [CODR].
• RISC-V Calling Conventions (MIT slides)
• Call stack (Wikipedia).
• Calling convention (Wikipedia).