Re: [avr-gcc-list] Serial Communication

[Dave Hansen]

From: Neil Johnson <address@hidden>
[...]
> Just tried a quick experiment with GCC 3.2.2 for Redhat Linux to see if
> there is a possibility of GCC spotting this.
>
> Code:
>
> ------------------------------------------------------------------
>
> #define USART_TX_BUFFER_SIZE    ( 64 )
> #define USART_TX_BUFFER_MASK    ( USART_TX_BUFFER_SIZE - 1 )
>
> int withmodop(int buffer[], unsigned int i)
> {
>         int x = buffer[i % USART_TX_BUFFER_SIZE];
>
>         return x;
> }
>
> int withandop(int buffer[], unsigned int i)
> {
>         int x = buffer[i & USART_TX_BUFFER_MASK];
>
>         return x;
> }
>
> ------------------------------------------------------------------
>
> GCC generates the exact same code for both functions.  But only if "i" is
> unsigned int.  If it is declared as int it generates code to noodle around
> with i before using it to index into buffer.
>
> It would be worth repeating this for the AVR target (I'm at work where I
> don't have AVRGCC installed).

OK, I tested it with avr-gcc, and the resulting functions were identical.

Just to play around a little, I changed the type of the second parameter (i) 
in each function to unsigned char.  The code using the mod (%) operator was 
actually two bytes shorter (42 vs. 44 bytes): the code using & converted i 
to a two byte value before performing the AND.  But the AND was performed in 
all cases.

Regards,
  -=Dave

_________________________________________________________________
Let the new MSN Premium Internet Software make the most of your high-speed 
experience. http://join.msn.com/?pgmarket=en-us&page=byoa/prem&ST=1