View source for 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)
LOCV +8  ; vec
LOCV +12 ; lim
INT 4    ; sizeof int
MUL
ADD      ; vec + lim
INT 1
INT 4    ; sizeof int
MUL
SUB      ; vec + lim - 1
LOCA -4  ; ptr = vec + lim - 1

; init ix (not assignment)
LOCV +12 ; lim
INT 2
SUB
LOCA -8  ; ix = lim -2

; while start: test
ALIGN
LABEL while_test
LOCV -8
INT 0
GE       ; ix >= 0
JZ while_end

; while block

; ?: operator
LOCV +8  ; vec
LOCV -8  ; ix
INT 4    ; sizeof int
MUL
ADD      ; vec + ix
LOAD     ; *(vec+ix) --> vec[ix]

LOCV -4  ; ptr
LOAD     ; *ptr

GT
JZ three_false_part

; ?: true part
LOCV +8  ; vec
LOCV -8  ; ix
INT 4    ; sizeof int
MUL
ADD      ; vec + ix

JMP three_end

; ?: false part
ALIGN
LABEL three_false_part

LOCV -4  ; ptr

; ?: end
ALIGN
LABEL three_end

DUP      ; for assignment
LOCA -4  ; ptr = ?: ...

; trash expression value (used as instruction)
TRASH 4

; decrement ix
LOCV -8  ; ix
DUP
INT 1
SUB
LOCA -8  ; ix = ix - 1
TRASH 4  ; (ix-- used as instruction)

; end while block
JMP while_test

; exit while cycle
ALIGN
LABEL while_end

; return ptr
LOCV -4
POP
LEAVE
RET
</asm>

== Compiling and Running ==

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

* pf2asm traverse.pf
* yasm -felf traverse.asm

[[category:Compilers]]
[[category:Teaching]]