Re: [avr-gcc-list] switch/case optimisation in TWI ISR

[James Washer]

I guess I should have said "an array of labels", sorry. also.. I should have 
included ellipses to represent cases 2 through 30

On Fri, 23 Dec 2005 11:35:22 -0800
James Washer <address@hidden> wrote:

> In GCC, it's legal to have an array of gotos, so this should work... and save 
> a lot of space (and execution too, I would think)
> 
> 
> void *jumptable[]={ &&label0, &&label1, &&label31};
> 
> goto *jumptable[TWSR>>3];
> 
> label0:
>         //code for state 0
>       return;
> label1:
>         //code for state 1
>       return;
> label31:
>         //code for state31
>       return;
> 
> 
> 
> On Fri, 23 Dec 2005 17:49:35 +0100
> Vincent Trouilliez <address@hidden> wrote:
> 
> > On Fri, 2005-12-23 at 17:17 +0100, David Bourgeois wrote:
> > > I have a TWI interrupt routine inspired from the proycon avrlib and which 
> > >  
> > > handles multi-master transmissions in an atmega48.
> > > It's basically a sequence of cases for every value of the twi status.
> > > 
> > > Right now, my twi code is 780 bytes long but 292 are for the SWITCH 
> > > alone,  
> > > while only 266 are for what I'm actually doing in the cases.
> > > 
> > > Is there a better way to write this? I've been told that a if/else if  
> > > sequence could be more efficient but can't really understand why. I guess 
> > >  
> > > this also depends on the compiler. What about AVR-GCC in such a case?
> > > And even if I have to write all this ISR in assembler, could I be able to 
> > >  
> > > reduce this code significantly?
> > 
> > 
> > If you are willing to write assembly, then there is room for significant
> > improvement I think:
> > 
> > 
> > PUSH TWI_STATUS_REG
> > RETURN
> > 
> > Two instructions, hard to beat ;-)
> > That was for an Intel 8051 but surely it can be adapted for an AVR ???
> > 
> > In the 8051 the I2C status register's lower 3 bits are unused, if in the
> > above code, so you have 8 bytes to deal with each status case.
> > 
> > Basically it's a look-up table with the status byte as the index. Stated
> > like this, you can now translate it into C statements: write separate
> > functions for all the various cases, put there address in a table of
> > pointers, and use the TWI status byte as the table index to retrieve the
> > matching function pointer, then jump to the required function. Done.
> > Obviously, the more cases you have to process, the more
> > efficient/interesting the look-up table becomes. Doing it for only two
> > cases would be a tad overkill ;-P
> > 
> > Sounds complex but it should still occupy much less than 292 I would
> > think !!
> > 
> > 
> > HTH,
> > 
> > 
> > --
> > Vince
> > 
> > 
> > 
> > _______________________________________________
> > AVR-GCC-list mailing list
> > address@hidden
> > http://lists.nongnu.org/mailman/listinfo/avr-gcc-list
> > 
> 
> 
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