Re: [avr-gcc-list] Why does this code not work?

[Ian Caddy]

Hi,

I would say your problem revolves around the fact that you are using 
unsigned numbers.  In this case, they are not valid below 0 and so it 
only needs to check if the numbers are the same == 0 to fail the test.

The simple way to solve the issue, would be to either make your numbers 
in the structure be signed, or change the first test to something like:


  if (Sonar.LeftFront > Sonar.LeftBack )

I hope this helps.

Ian Caddy


wbounce wrote:
> avr-gcc (GCC) 3.4.1
> With no optimization
>
> typedef struct ranges_t
> {
> uint16_t          Front;   
> uint16_t          Back;   
> uint16_t          LeftFront;   
> uint16_t          LeftBack;   
> uint16_t          RightFront;   
> uint16_t          RightBack;   
> } RANGES;
>
> RANGES Sonar;
> .....
> if (Sonar.LeftFront - Sonar.LeftBack > 0 ) 
> {
>  if (Sonar.LeftFront - Sonar.LeftBack > 100)
>   {
>   BearLeft();
>   }
>  else
>   {
>   Forward();
>   }
> }       
> else
> {
>  if (Sonar.LeftBack - Sonar.LeftFront > 100)
>   {
>   BearRight();
>   }
>  else
>   {
>   Forward();
>   }
> }
>
> In my simulation Sonar.LeftBack = 1010 and Sonar.LeftFront = 900. The
> 1st if should fail but it does not. When I look at the disassembly it
> looks like this
> 140:                            if (Sonar.LeftFront - Sonar.LeftBack > 0
> ) 
> +00000D89:   912001DB    LDS     R18,0x01DB       Load direct from data
> space
> +00000D8B:   913001DC    LDS     R19,0x01DC       Load direct from data
> space
> +00000D8D:   918001DD    LDS     R24,0x01DD       Load direct from data
> space
> +00000D8F:   919001DE    LDS     R25,0x01DE       Load direct from data
> space
> +00000D91:   1728        CP      R18,R24          Compare
> +00000D92:   0739        CPC     R19,R25          Compare with carry
> +00000D93:   F099        BREQ    PC+0x14          Branch if equal
> ---- 
> 142:      				if (Sonar.LeftFront -
> Sonar.LeftBack > 100)
> +00000D94:   918001DB    LDS     R24,0x01DB       Load direct from data
> space
> ---- No Source 
> +00000D96:   919001DC    LDS     R25,0x01DC       Load direct from data
> space
> +00000D98:   912001DD    LDS     R18,0x01DD       Load direct from data
> space
> +00000D9A:   913001DE    LDS     R19,0x01DE       Load direct from data
> space
> +00000D9C:   1B82        SUB     R24,R18          Subtract without carry
> +00000D9D:   0B93        SBC     R25,R19          Subtract with carry
> +00000D9E:   3685        CPI     R24,0x65         Compare with immediate
> +00000D9F:   0591        CPC     R25,R1           Compare with carry
> +00000DA0:   F018        BRCS    PC+0x04          Branch if carry set
>
> It is doing a compare to 0 in the 1st if instead of a subtraction and
> then a compare as it does in the 2nd if. What am I doing wrong?
>
>
>
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