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Code generation issue

 
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Pret



Joined: 18 Jul 2006
Posts: 92
Location: Iasi, Romania

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Code generation issue
PostPosted: Wed May 09, 2007 2:19 am     Reply with quote
I have de following declarations:
Code:
struct
{
    int8 Dummy;
    int8 a[20];
} str;

int8 *x;
To get in x a pointer from one element from a, i could do
Code:
void Main()
{
    int8 i = 7;
    x = &str.a[i];
}
which generates:
Code:
...................     int8 i = 7;
0024:  MOVLW  07
0026:  MOVWF  1D
....................     x = &str.a[i];
0028:  CLRF   03
002A:  MOVF   1D,W
002C:  ADDLW  01
002E:  MOVWF  01
0030:  MOVLW  00
0032:  ADDWFC 03,F
0034:  MOVF   01,W
0036:  ADDLW  06
0038:  MOVWF  1B
003A:  MOVLW  00
003C:  ADDWFC 03,W
003E:  MOVWF  1C


or:
Code:
void Main()
{
    int8 i = 7;
    x = str.a + i;
}
which generates
Code:
....................     int8 i = 7;
0024:  MOVLW  07
0026:  MOVWF  1D
....................     x = str.a + i;
0028:  ADDWF  1D,W
002A:  MOVWF  1B
002C:  CLRF   1C
002E:  BTFSC  FD8.0
0030:  INCF   1C,F


Since both C statements are the same, why genereted code is not?
Ttelmah
Guest
PostPosted: Wed May 09, 2007 3:06 am     Reply with quote
This is very common, and fairly 'classic' with compilers.
You will almost certainly find the effects will change if you change the #opt setting.
Basically, in the second case, you say to take the address 'str.a' (which is 'hardwired' at compile time), and add 'i' to this.
In the first you say to access the array element, and then get the address of this. The arithmetic is done in this case, by working out the offset needed for 'i' in the array, then adding this to the array starting point. Just slightly more complex.
The optimiser, will try to improve this sort of behaviour, but it is the price you 'pay' for the convenience of being able to access elements without having to do the arithmetic yourself...

Best Wishes
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