Code Generation/Exercise 9

The Original Code

Consider the following C code and assume that the size of pointers, int, and unsigned long is 32 bits and that the size of double is 64 bits.

static unsigned long a = 10;
static double *v;
extern void *malloc(unsigned long);

static double *mkvec(unsigned long n) {
  if (n < 1) return (double *)0;
  unsigned long s = sizeof(double);
  double *v = (double *)malloc(n * s);
  return v;
}

double *compute() {
  v = mkvec(a * 4);
  for (unsigned long i = 1; i < a; i++)
    if (v[i] > v[0]) v[i] = 3 * i - 1;
  return v;
}

Postfix Code

The Postfix code for the above code is as follows: (code has not been thoroughly checked: bug reports are welcome)

In the following code sections, BB# means "basic block number".

Static (global) variables
;; variable "a" DATA ALIGN LABEL a SINT 10 ;; variable "v" DATA ; "static" variables are always initialized ALIGN LABEL v SINT 0 ; this is a null pointer

Declaration of external function
EXTERN malloc

Function "mkvec"
;; start of BB1 TEXT ALIGN LABEL mkvec ENTER 8 ; s@-4 v@-8 LOCAL 8 LDINT INT 1 LT JZ ifend1 ;; end of BB1 ;; start of BB2 INT 0 STFVAL32 LEAVE RET ;; end of BB2 ;; start of BB3 ALIGN LABEL ifend1 INT 8 ; sizeof(double) LOCAL -4 STINT LOCAL +8 LDINT LOCAL -4 LDINT MUL CALL malloc ;; end of BB3 ;; start of BB4 TRASH 4 LDFVAL32 LOCAL -8 STINT LOCAL -8 LDINT STFVAL32 LEAVE RET ;; end of BB4

Function "compute"

;; start of BB1 TEXT ALIGN GLOBAL compute, FUNC LABEL compute ENTER 4 ; i@-4 ADDR a LDINT INT 4 MUL CALL mkvec ;; end of BB1 ;; start of BB2 TRASH 4 LDFVAL32 DUP32 ADDR v STINT TRASH 4 INT 1 LOCAL -4 STINT ;; end of BB2 ;; start of BB3 ALIGN LABEL fortest LOCAL -4 LDINT ADDR a LDINT LT JZ forend ;; end of BB3 ;; start of BB4 ADDR v LDINT LOCAL -4 LDINT INT 8 MUL ADD LDDOUBLE ADDR v LDINT INT 0 INT 8 MUL ADD LDDOUBLE DCMP INT 0 GT JZ ifend2 ;; end of BB4 ;; start of BB5 INT 3 LOCAL -4 LDINT MUL INT 1 SUB I2D DUP64 ADDR v LDINT LOCAL -4 LDINT INT 8 MUL ADD STDOUBLE TRASH 8 ;; end of BB5 ;; start of BB6 ALIGN LABEL ifend2 ALIGN LABEL forincr LOCAL -4 LDINT DUP32 INT 1 ADD LOCAL -4 STINT TRASH 4 JMP fortest ;; end of BB6 ;; start of BB7 ALIGN LABEL forend ADDR v LDINT STFVAL32 LEAVE RET ;; end of BB7

Compiling and Running

To compile the Postfix code directly, pf2asm can be used (assuming a 32-bit architecture):

pf2asm compute.pf
yasm -felf compute.asm