C stack, Arrays

• C examples
• Assembly equivalent
• Code Generation
• m4 macros
• One-Dimensional Arrays
• Optimizations

m4

• begin_main starts main function (including the save)
• end_main calls exit
• align_d(number, alignment) reduces the given number to match the required alignment
• local_var starts declaration of stack (local) variables
• var(name, size) allocates aligned space for a variable of the given size
• var(name, alignment, size) allocates aligned space for a variable of the given size and alignment

C Example 1

int a, b;
char c1;
int c, d;
register int x, y, z;
x = 17; y = -5;
for (z = 1; z < x + y; z++)
  for (a = z; a >= z * y; a -= 10) {
    d = a + z;
    c1 = d * b;
    c = a + y / z;
  }
}

Notes:

Example Characterization

• Local variables (on stack)
• Integer/char variables
• Register variables
• Nested for loops
• Tests involving computation

Variable Declarations

int a, b;
char c1;
int c, d;
register int x, y, z;
        localvar
        var(as, 4)
        var(bs, 4)
        var(c1s, 1)
        var(cs, 4)
        var(ds, 4)
        define(xr, l0)  ! `xr l0'
        define(yr, l1)  ! `yr l1'
        define(zr, l2)  ! `zr l2'
        beginmain

Nested for

for (z = 1; ; z++)
  for (a = z; ; a -= 10)
    ...
        ba outertest
        mov 1, %zr
outer:  ba innertest
        st %zr, [%fp + as]  ! a = z
inner:  ...
ininc:  ld [%fp + as], %o0
        sub %o0, 10, %o0
        st %o0, [%fp + as]
intest: 
        bge inner
        nop
outinc: add %zr, 1, %zr
outtest:
        bl outer
        nop

Complex tests

for (z = 1; z <= x + y; z++)
  for (a = z; a >= z * y; a -= 10)
innertest:
        mov %zr, %o0
        call mul
        ld [%fp + as, %o1]
        cmp %o1, %o0
        bge inner
        nop
outerinc:
        add %zr, 1, %zr
outertest:
        add %xr, %yr, %o0
        cmp %zr, %o0
        bl outer
        nop

1-D arrays

• An array is a block of memory
• If array has n elements, each of size s bytes, the total array size is n * s bytes

int a;
char c1;
int ary[5];
        localvar
        var(as, 4)
        var(c1s, 1)
        var(arys, 4, 4 * 5)

        as = -4
        c1s = -5
        arys = -28

1-D array access

• load ary[i] into %o0, with i being %i_r:

          sll %ir, 2, %o0  ! 4*i->o0
          add %fp, %o0, %o0 ! fp+4i->o0
          ld [%o0 + arys], %o0
  

More m4

• ifelse(t1, t2, then, else) returns the then if t1 = t2, the else otherwise
• initialize(pos, v1, v2, ...) initializes nums_s with the given values

C Example 2

int nums[100] = {1, 45, -16, 23,
                 38, 45, 17}
int n = 7;        /* elements in nums */
register int i;   /* index */
register int max; /* max found so far */
max = nums[0];
for (i = 1; i < n; i++) {
  if (nums[i] > max)
    max = nums[i];
}

Example Characterization

• 1-D array declaration
• 1-D array access
• Array access optimizations

Array Declaration

int nums[100] = {1, 45, -16, 23,
                 38, 45, 17}
int n = 7;        /* elements in nums */
register int i;   /* index */
register int max; /* max found so far */
        localvar
        var(numss, 4, 100 * 4)
        var(ns, 4)
        define(ir, l0)  ! `ir l0'
        define(maxr, l1)! `maxr l1'
        beginmain
define(initialize, `   mov   2, %o0
        st %o0, [%fp+numss+1]
ifelse(3,,,`initialize(
eval(1+4),3,4, 5,6,7,8,9)')')
        initialize(0, 1, 45, -16, 23,
                   38, 45, 17)
        mov 7, %o0
        st %o0, [%fp + ns]

Array Access

max = nums[0];
for (i = 1; i < n; i++)
  if (nums[i] > max)
    max = nums[i];
        ba fortest
        mov 1, %ir
for:    sll %ir, 2, %o0 ! i*4
        add %fp, %o0, %o0
        ld [%o0+nums], %o0
        cmp %o0,  %maxr
        ble keep
        nop
keep:   add %ir, 1, %ir
fortest:ld  [%fp+ns], %o0
        cmp %ir, %o0
        bl for
        nop

Optimizing the loop

• fill delay slots
• replace subscript computation with pointer arithmetic
• count down to zero (set condition codes to eliminate test)
• reorder instructions to load several instructions before use

Notes

• PC accounts -- change password ASAP
• /home/1/esb/312/macro_defs.m (please report any errors)
• return quiz
• project status