Code Generation/Exercise 7

The Original Code

Consider the following C function:

<c> int *traverse(int vec[], int lim) {

 int *ptr = vec + lim - 1, ix = lim - 2;
 while (ix >= 0) {
   ptr = (vec[ix] > *ptr) ? vec + ix : ptr;
   ix--;
 }
 return ptr;

} </c>

Postfix Code

The Postfix code for the above function is as follows:

<asm> TEXT ALIGN GLOBL traverse, FUNC LABEL traverse ENTER 8  ; ptr@-4 ix@-8 vec@+8 lim@+12

init prt (not assignment)

LOCAL +8 LOAD  ; vec LOCAL +12 LOAD ; lim INT 4  ; sizeof int MUL ADD  ; vec + lim INT 1 INT 4  ; sizeof int MUL SUB  ; vec + lim - 1 LOCAL -4 STORE  ; ptr = vec + lim - 1

init ix (not assignment)

LOCAL +12 LOAD ; lim INT 2 SUB LOCAL -8 STORE  ; ix = lim -2

while start

test

ALIGN LABEL while_test LOCAL -8 LOAD INT 0 GE  ; ix >= 0 JZ while_end

while block

?

operator

LOCAL +8 LOAD  ; vec LOCAL -8 LOAD  ; ix INT 4  ; sizeof int MUL ADD  ; vec + ix LOAD  ; *(vec+ix) --> vec[ix]

LOCAL -4 LOAD  ; ptr LOAD  ; *ptr

GT JZ three_false_part

?

true part

LOCAL +8 LOAD  ; vec LOCAL -8 LOAD  ; ix INT 4  ; sizeof int MUL ADD  ; vec + ix

JMP three_end

?

false part

ALIGN LABEL three_false_part

LOCAL -4 LOAD  ; ptr

?

end

ALIGN LABEL three_end

DUP  ; for assignment LOCAL -4 STORE  ; ptr = ?: ...

trash expression value (used as instruction)

TRASH 4

decrement ix

LOCAL -8 LOAD  ; ix DUP INT 1 SUB LOCAL -8 STORE  ; ix = ix - 1 TRASH 4  ; (ix-- used as instruction)

end while block

JMP while_test

exit while cycle

ALIGN LABEL while_end

return ptr

LOCAL -4 LOAD POP LEAVE RET </asm>

Compiling and Running

To compile the Postfix code directly, pf2asm can be used:

pf2asm traverse.pf
yasm -felf traverse.asm