An Example Language Program
D
- The following code shows an example C language program; the second example code shows the corresponding assembly code generated by SPARCompiler C 3.0.2 that runs on the Solaris 2.x operating environment. Comments have been added to the asembly code to show correspondence to the C code.
- The following C Program computes the first n Fibonacci numbers:
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Figure D-1 C Program Example Source
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/* a simple program computing the first n Fibonacci numbers */
extern unsigned * fibonacci();
#define MAX_FIB_REPRESENTABLE 49
/* compute the first n Fibonacci numbers */
unsigned * fibonacci(n)
int n;
{
static unsigned fib_array[MAX_FIB_REPRESENTABLE] = {0,1};
unsigned prev_number = 0;
unsigned curr_number = 1;
int i;
if (n >= MAX_FIB_REPRESENTABLE) {
printf("Fibonacci(%d) cannot be represented in a 32 bit word\n", n);
exit(1);
}
for (i = 2; i < n; i++) {
fib_array[i] = prev_number + curr_number;
prev_number = curr_number;
curr_number = fib_array[i];
}
return(fib_array);
}
main()
{
int n, i;
unsigned * result;
printf("Fibonacci(n):, please enter n:\n");
scanf("%d", &n);
result = fibonacci(n);
for (i = 1; i <= n; i++)
printf("Fibonacci (%d) is %u\n", i, *result++);
}
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- The C SPARCompiler generates the following assembler output for the Fibonacci number C source. Annotation has been added to help you understand the code.
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Figure D-2 Assembler Output From C Source
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!
! a simple program computing the first n Fibonacci numbers,
! showing various pseudo-operations, sparc instructions, synthetic instructions
!
! pseudo-operations: .align, .ascii, .file, .global, .ident, .proc, .section,
! .size, .skip, .type, .word
! sparc instructions: add, bg, bge, bl, ble, ld, or, restore, save, sethi, st
! synthetic instructions: call, cmp, inc, mov, ret
!
.file "fibonacci.c" ! the original source file name
.section ".text" ! text section (executable instructions)
.proc 79 ! subroutine fibonacci, it's return
! value will be in %i0
.global fibonacci ! fibonacci() can be referenced
! outside this file
.align 4 ! align the beginning of this section
! to word boundary
fibonacci:
save %sp,-96,%sp ! create new stack frame and register
! window for this subroutine
/* if (n >= MAX_FIB_REPRESENTABLE) { */
! note, C style comment strings are
! also permitted
cmp %i0,49 ! n >= MAX_FIB_REPRESENTABLE ?
! note, n, the 1st parameter to
! fibonacci(), is stored in %i0 upon
! entry
bl .L77003
mov 0,%i2 ! initialization of variable
! prev_number is executed in the
! delay slot
/* printf("Fibonacci(%d) cannot be represented in a 32 bits word\n", n); */
sethi %hi(.L20),%o0 ! if branch not taken, call printf(),
or %o0,%lo(.L20),%o0 ! set up 1st, 2nd argument in %o0, %o1;
call printf,2 ! the ",2" means there are 2 out
mov %i0,%o1 ! registers used as arguments
/* exit(1); */
call exit,1
mov 1,%o0
.L77003: ! initialize variables before the loop
/* for (i = 2; i < n; i++) { */
mov 1,%i4 ! curr_number = 1
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mov 2,%i3 ! i = 2
cmp %i3,%i0 ! i <= n?
bge .L77006 ! if not, return
sethi %hi(.L16+8),%o0 ! use %i5 to store &fib_array[i]
add %o0,%lo(.L16+8),%i5
.LY1: ! loop body
/* fib_array[i] = prev_number + curr_number; */
add %i2,%i4,%i2 ! fib_array[i] = prev_number+curr_number
st %i2,[%i5]
/* prev_number = curr_number; */
mov %i4,%i2 ! prev_number = curr_number
/* curr_number = fib_array[i]; */
ld [%i5],%i4 ! curr_number = fib_array[i]
inc %i3 ! i++
cmp %i3,%i0 ! i <= n?
bl .LY1 ! if yes, repeat loop
inc 4,%i5 ! increment ptr to fib_array[]
.L77006:
/* return(fib_array); */
sethi %hi(.L16),%o0 ! return fib_array in %i0
add %o0,%lo(.L16),%i0
ret
restore ! destroy stack frame and register
! window
.type fibonacci,#function ! fibonacci() is of type function
.size fibonacci,(.-fibonacci) ! size of function:
! current location counter minus
! beginning definition of function
.proc 18 ! main program
.global main
.align 4
main:
save %sp,-104,%sp ! create stack frame for main()
/* printf("Fibonacci(n):, please input n:\n"); */
sethi %hi(.L31),%o0 ! call printf, with 1st arg in %o0
call printf,1
or %o0,%lo(.L31),%o0
/* scanf("%d", &n); */
sethi %hi(.L33),%o0 ! call scanf, with 1st arg, in %o0
or %o0,%lo(.L33),%o0 ! move 2nd arg. to %o1, in delay slot
call scanf,2
add %fp,-4,%o1
/* result = fibonacci(n); */
call fibonacci,1
ld [%fp-4],%o0
! some initializations before the for-
! loop, put the variables in registers
/* for (i = 1; i <= n; i++) */
mov 1,%i5 ! %i5 <-- i
mov %o0,%i4 ! %i4 <-- result
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sethi %hi(.L38),%o0 ! %i2 <-- format string for printf
add %o0,%lo(.L38),%i2
ld [%fp-4],%o0 ! test if (i <= n) ?
cmp %i5,%o0 ! note, n is stored in [%fp-4]
bg .LE27
nop
.LY2: ! loop body
/* printf("Fibonacci (%d) is %u\n", i, *result++); */
ld [%i4],%o2 ! call printf, with (*result) in %o2,
mov %i5,%o1 ! i in %o1, format string in %o0
call printf,3
mov %i2,%o0
inc %i5 ! i++
ld [%fp-4],%o0 ! i <= n?
cmp %i5,%o0
ble .LY2
inc 4,%i4 ! result++
.LE27:
ret
restore
.type main,#function ! type and size of main
.size main,(.-main)
.section ".data" ! switch to data section
! (contains initialized data)
.align 4
.L16:
/* static unsigned fib_array[MAX_FIB_REPRESENTABLE] = {0,1}; */
.align 4 ! initialization of first 2 elements
.word 0 ! of fib_array[]
.align 4
.word 1
.skip 188
.type .L16,#object ! storage allocation for the rest of
! fib_array[]
.section ".data1" ! the ascii string data are entered
! into the .data1 section;
! #alloc: memory would be allocated
! for this section during run time
! #write: the section contains data
! that is writeable during process
! execution
.align 4
.L20: ! ascii strings used in the printf stmts
.ascii "Fibonacci(%d) cannot be represented in a 32 bit w"
.ascii "ord\n\0"
.align 4 ! align the next ascii string to word
! boundary
.L31:
.ascii "Fibonacci(n):, please enter n:\n\0"
.align 4
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.L33:
.ascii "%d\0"
.align 4
.L38:
.ascii "Fibonacci (%d) is %u\n\0"
.ident "acomp: (CDS) SPARCompilers 2.0 05 Jun 1991"
! an idenitfication string produced
! by the compiler to be entered into
! the .comment section
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