Revision as of 09:30, 3 March 2015

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.

<c> 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;

} </c>

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".

Variables

<asm>

variable "a"

DATA ALIGN LABEL a CONST 10

variable "v"

DATA  ; "static" variables are always initialized ALIGN LABEL v CONST 0  ; this is a null pointer </asm>

Declaration of external function

<asm> EXTERN malloc </asm>

Function "mkvec"

<asm>

start of BB1

TEXT ALIGN LABEL mkvec ENTER 8  ; s@-4 v@-8 LOCV 8 INT 1 LT JZ ifend1

end of BB1

start of BB2

INT 0 POP LEAVE RET

end of BB2

start of BB3

ALIGN LABEL ifend1 INT 8  ; sizeof(double) LOCA -4 LOCV +8 LOCV -4 MUL CALL malloc

end of BB3

start of BB4

TRASH 4 PUSH LOCA -8 LOCV -8 POP LEAVE RET

end of BB4

</asm>

Function "compute"

In the following, BB# means basic block #.

<asm>

start of BB1

TEXT ALIGN GLOBAL compute, FUNC LABEL compute ENTER 4  ; i@-4 ADDRV a INT 4 MUL CALL mkvec

end of BB1

start of BB2

TRASH 4 PUSH DUP ADDRA v TRASH 4 INT 1 LOCA -4

end of BB2

start of BB3

ALIGN LABEL fortest LOCV -4 ADDRV a LT JZ forend

end of BB3

start of BB4

ADDRV v LOCV -4 INT 8 MUL ADD DLOAD ADDRV v INT 0 INT 8 MUL ADD DLOAD DCMP INT 0 GT JZ ifend2

end of BB4

start of BB5

INT 3 LOCV -4 MUL INT 1 SUB I2D DDUP ADDRV v LOCV -4 INT 8 MUL ADD DSTORE TRASH 8

end of BB5

start of BB6

ALIGN LABEL ifend2 ALIGN LABEL forincr LOCV -4 DUP INT 1 ADD LOCA -4 TRASH 4 JMP fortest

end of BB6

start of BB7

ALIGN LABEL forend ADDRV v POP LEAVE RET

end of BB7

</asm>

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