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| == The Original Code ==
| | #REDIRECT [[ist:Code Generation/Exercise 7]] |
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| Consider the following C function:
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| <c>
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| int *traverse(int vec[], int lim) {
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| int *ptr = vec + lim - 1, ix = lim - 2;
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| while (ix >= 0) {
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| ptr = (vec[ix] > *ptr) ? vec + ix : ptr;
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| ix--;
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| }
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| return ptr;
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| }
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| </c>
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| == Postfix Code ==
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| The Postfix code for the above function is as follows:
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| <asm>
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| TEXT
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| ALIGN
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| GLOBL traverse, FUNC
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| LABEL traverse
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| ENTER 8 ; ptr@-4 ix@-8 vec@+8 lim@+12
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| ; init prt (not assignment)
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| LOCV +8 ; vec
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| LOCV +12 ; lim
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| INT 4 ; sizeof int
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| MUL
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| ADD ; vec + lim
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| INT 1
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| INT 4 ; sizeof int
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| MUL
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| SUB ; vec + lim - 1
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| LOCA -4 ; ptr = vec + lim - 1
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| ; init ix (not assignment)
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| LOCV +12 ; lim
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| INT 2
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| SUB
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| LOCA -8 ; ix = lim -2
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| ; while start: test
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| ALIGN
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| LABEL while_test
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| LOCV -8
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| INT 0
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| GE ; ix >= 0
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| JZ while_end
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| ; while block
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| ; ?: operator
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| LOCV +8 ; vec
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| LOCV -8 ; ix
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| INT 4 ; sizeof int
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| MUL
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| ADD ; vec + ix
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| LOAD ; *(vec+ix) --> vec[ix]
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| LOCV -4 ; ptr
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| LOAD ; *ptr
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| GT
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| JZ three_false_part
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| ; ?: true part
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| LOCV +8 ; vec
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| LOCV -8 ; ix
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| INT 4 ; sizeof int
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| MUL
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| ADD ; vec + ix
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| JMP three_end
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| ; ?: false part
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| LOCV -4 ; ptr
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| ; ?: end
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| ALIGN
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| LABEL three_end
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| DUP ; for assignment
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| LOCA -4 ; ptr = ?: ...
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| ; trash expression value (used as instruction)
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| TRASH 4
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| ; decrement ix
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| LOCV -8 ; ix
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| DUP
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| INT 1
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| SUB
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| LOCA -8 ; ix = ix - 1
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| TRASH 4 ; (ix-- used as instruction)
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| ; end while block
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| JMP while_test
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| ; exit while cycle
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| ALIGN
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| LABEL while_end
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| ; return ptr
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| LOCV -4
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| POP
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| LEAVE
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| RET
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| </asm>
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| == Compiling and Running ==
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| To compile the Postfix code directly, [[pf2asm]] can be used:
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| * pf2asm traverse.pf
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| * yasm -felf traverse.asm
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| [[category:Compilers]]
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| [[category:Teaching]]
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