MIPS Case Study: Leaf Procedure Example (Lecture 12)

Leaf Procedure Example  C code: int leaf_example(int g, h, i, j) { int f; f = (g + h) – (i + j) return f; }  Arguments g, …, j in $a0, …, $a3  f in $s0 (hence, need to save $s0 on stack)  Result in $v0...

Leaf Procedure Example  C code: int leaf_example(int g, h, i, j) { int f; f = (g + h) – (i + j) return f; }  Arguments g, …, j in $a0, …, $a3  f in $s0 (hence, need to save $s0 on stack)  Result in $v0 1 Leaf Procedure Example (cont’d) Stack “grows” from  Compiled MIPS code: high address to low address leaf_example: addi $sp, $sp, -4 sw $s0, 0($sp) Save $s0 on stack add $t0, $a0, $a1 add $t1, $a2, $a3 sub $s0, $t0, $t1 Procedure body add $v0, $s0, $zero Result lw $s0, 0($sp) addi $sp, $sp, 4 Restore $s0 jr $ra Return 2 Stack and $sp in Leaf Procedure Example Contents of Register $s0 3 Non-leaf Procedures  Procedures that call other procedures  For nested call, caller needs to save on the stack  Its return address  Any arguments and temporaries needed after the call  Restore from the stack after call  Example C code: int fact (int n) Argument n in $a0 { Result in $v0 if (n < 1) return 1; else return n * fact(n-1); } 4 Non-leaf Procedures (cont’d) int fact (int n) { if (n < 1) return 1; else return n * fact(n-1);  Compiled MIPS code } fact: addi $sp, $sp, -8 // adjust stack for 2 items sw $ra, 4($sp) // save return address sw $a0, 0($sp) // save argument slti $t0, $a0, 1 // test for n < 1 beq $t0, $zero, L1 addi $v0, $zero, 1 // if so, result is 1 addi $sp, $sp, 8 // pop 2 items from stack jr $ra // and return L1: addi $a0, $a0, -1 // else decrement n jal fact // recursive call lw $a0, 0($sp) // restore original n lw $ra, 4($sp) // and return address addi $sp, $sp, 8 // pop 2 items from stack mul $v0, $a0, $v0 // multiply to get result jr $ra // and return 5 Local Data on the Stack  Stack is also used to store variables that are local to procedure but do not fit in registers  E.g., local arrays or structures  The segment of the stack containing a procedure’s saved registers and local variables is its procedure frame (aka activation record)  The frame pointer ($fp) points to the first word of the frame of a procedure – providing a stable “base” register for the procedure  $fp is initialized using $sp on a call and $sp is restored using $fp on a return 6 Memory Layout  Text: program code  Static data: global variables  e.g., static variables in C, constant arrays and strings  $gp initialized to address allowing ±offsets into this segment  Dynamic data: heap  e.g., malloc in C, new in Java  Stack: automatic storage 7 Example C Program 8 From Program to Process 9 Arrays vs. Pointers  Array indexing involves  Multiplying index by element size  Adding to array base address  Pointers correspond directly to memory addresses  Can avoid indexing complexity clear1(int array[], int size) { int i;  Example : Clearing an array for (i = 0; i < size; i += 1) array[i] = 0; } move $t0,$zero # i = 0 loop1: sll $t1,$t0,2 # $t1 = i * 4 add $t2,$a0,$t1 # $t2 = &array[i] sw $zero, 0($t2) # array[i] = 0 addi $t0,$t0,1 # i = i + 1 slt $t3,$t0,$a1 # $t3 = (i < size) bne $t3,$zero,loop1 # if (…) # goto loop1 10 Reading Recommended  Chapter 2  2.2 Operations of the Computer Hardware  2.3 Operand of the Computer Hardware  2.5 Representing Instructions in the Computer  2.7 Instructions for Making Decisions  2.8 Supporting Procedures in Computer Hardware  Appendix A-2  A.2 Assemblers  A.3 Linkers  A.4 Loading  A.5 Memory Usage  A.6 Procedure Call Convention 11