Re: [avr-gcc-list] Tablejumps - needless run time conversion to byteaddress

[Andrew Hutchinson]

Thanks!

This will work just fine. Seems all the attempts I made were too optimal!

Andy


Wouter van Gulik wrote:
> I can make GCC use a jumptable using this code:
>
> test.c
> ===========================================
> volatile int x;
> volatile int y;
>
> void foo  (void) {
>     x++;
> }
>
> void main(void)
> {       
>         switch(y)
>         {
>                 case 0  : foo();        
>                 case 1  : foo();        
>                 case 2  : foo();        
>                 case 3  : foo();        
>                 case 4  : foo();        
>                 case 5  : foo();        
>                 case 6  : foo();        
>                 case 7  : foo();        
>                 case 8  : foo();        
>                 case 9  : foo();        
>                 case 10 : foo();        
>                 case 11 : foo();        
>                 case 12 : foo();        
>                 case 13 : foo();        
>                 case 14 : foo();        
>                 case 15 : foo();        
>                 case 16 : foo();        
>         }               
>         
> }
> ===========================================
> Compiling using:
>
> avr-gcc -g -Os -Wall -mmcu=atmega16 -fno-inline test.c
> (Using no inline to keep disassembly small)
>
> gcc version  4.2.2
>
> Gives:
> =======================================================================
> main:
> .LFB3:
> .LM3:
> /* prologue: frame size=0 */
> /* prologue end (size=0) */
> .LM4:
>         lds r30,y
>         lds r31,(y)+1
>         cpi r30,17
>         cpc r31,__zero_reg__
>         brsh .L23
> .LM5:
>         subi r30,lo8(-(gs(.L22)))
>         sbci r31,hi8(-(gs(.L22)))
>         lsl r30
>         rol r31
>         lpm __tmp_reg__,Z+
>         lpm r31,Z
>         mov r30,__tmp_reg__
>         ijmp
>         .data
>         .section .progmem.gcc_sw_table, "a", @progbits
>         .p2align 1
> .L22:
>         .data
>         .section .progmem.gcc_sw_table, "a", @progbits
>         .p2align 1
>         .word gs(.L5)
>         .word gs(.L6)
>         .word gs(.L7)
>         .word gs(.L8)
>         .word gs(.L9)
>         .word gs(.L10)
>         .word gs(.L11)
>         .word gs(.L12)
>         .word gs(.L13)
>         .word gs(.L14)
>         .word gs(.L15)
>         .word gs(.L16)
>         .word gs(.L17)
>         .word gs(.L18)
>         .word gs(.L19)
>         .word gs(.L20)
>         .word gs(.L21)
>         .text
> .L5:
> .LM6:
>         call foo
> <snip>
> etc...
> ==========================================================================
>
> Some interesting notes:
> It works only from 17 cases and up.
>
> For smaller devices (e.g. atmega8)
> It works from already from 3 cases. But then an rjmp table is used.
>
> Why is GCC not using this rjmp scheme for the atmega16? Is it too difficult
> to predict it will not pass 4k boundary?
>
> HTH,
>
> Wouter
>
>   
> > Hi
> >
> > Does anyone have some code that creates tablejump in Avr-gcc? This is
> > where gcc will create table instead of long line of if-then-else tests
> >
> > I cant seem to create enough switch cases to force one!
> >
> > I have been looking at compilation patterns and noticed that gcc address
> > is multiplied by 2 to form address for LPM (table being in ROM). LPM
> > needs byte address and gcc has word address.
> >
> >  "lsl r30
> >    rol r31
> >    lpm __tmp_reg__,Z+
> >    lpm r31,Z
> >    mov r30,__tmp_reg__
> >    ijmp"
> >
> > Asm Pattern currently expects value to be in R30. However, it would
> > appear that this would be better with a symbol rather than value in
> > register - thus providing a means to multiply that value by 2 at compile
> > time. (and I cant see any reason it would be called with other than
> > constant address in ROM)
> >
> > Obviously, I'd like to test it.
> >
> > Andy
> >
> >
> >
> >
> > _______________________________________________
> > AVR-GCC-list mailing list
> > address@hidden
> > http://lists.nongnu.org/mailman/listinfo/avr-gcc-list
> >     
>
>
>