Difference between revisions of "Code Generation/Exercise 9"

Revision as of 08:35, 9 May 2017

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".

[Expand] Static (global) 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>

[Expand] Declaration of external function
<asm> EXTERN malloc </asm>

[Expand] Function "mkvec"
<asm> start of BB1 TEXT ALIGN LABEL mkvec ENTER 8 ; s@-4 v@-8 LOCAL 8 LOAD 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) LOCAL -4 STORE LOCAL +8 LOAD LOCAL -4 LOAD MUL CALL malloc end of BB3 start of BB4 TRASH 4 PUSH LOCAL -8 STORE LOCAL -8 LOAD POP LEAVE RET end of BB4 </asm>

[Expand] Function "compute"
<asm> start of BB1 TEXT ALIGN GLOBAL compute, FUNC LABEL compute ENTER 4 ; i@-4 ADDR a LOAD INT 4 MUL CALL mkvec end of BB1 start of BB2 TRASH 4 PUSH DUP ADDR v STORE TRASH 4 INT 1 LOCAL -4 STORE end of BB2 start of BB3 ALIGN LABEL fortest LOCAL -4 LOAD ADDR a LOAD LT JZ forend end of BB3 start of BB4 ADDR v LOAD LOCAL -4 LOAD INT 8 MUL ADD DLOAD ADDR v LOAD INT 0 INT 8 MUL ADD DLOAD DCMP INT 0 GT JZ ifend2 end of BB4 start of BB5 INT 3 LOCAL -4 LOAD MUL INT 1 SUB I2D DDUP ADDR v LOAD LOCAL -4 LOAD INT 8 MUL ADD DSTORE TRASH 8 end of BB5 start of BB6 ALIGN LABEL ifend2 ALIGN LABEL forincr LOCAL -4 LOAD DUP INT 1 ADD LOCAL -4 STORE TRASH 4 JMP fortest end of BB6 start of BB7 ALIGN LABEL forend ADDR v LOAD 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

@@ Line 30: / Line 30: @@
 In the following code sections, '''BB#''' means "[[Optimization Topics|basic block]] number".
-{{CollapsedCode|Variables|
+{{CollapsedCode|Static (global) variables|
 <asm>
 ;; variable "a"