Re: [Mspgcc-users] Could this be a bug with the logical inversion operator

[dimmy <di...@ei...>]

Colin,

Yes, you've discovered a bug in shift right operation...

I'll fix it shortly.

I dunno, why it generates such a long code, but I'll check.

gcc actually thinks, that 
if(a) a= 0 ;
else a = 1;

can be done better with shifts.

~d





On Wednesday 06 March 2002 18:39, Colin Domoney wrote:
> Hi
>
> I have the following code segment whose purpose is to toggle an LED every
> time the watchdog timer interrupt occurs:
>
> interrupt(WDT_VECTOR) WatchdogHandler(void)
> {
>   static char st = 0;
>
>   IFG1 &= ~WDTIFG;
>
>   if (st)
>   {
>     LED1_ON();
>   }
>   else
>   {
>     LED1_OFF();
>   }
>
>   st = !st;
>
>   IE1 = (IE1 | WDTIE);
> }
>
> The problem I am seeing is that the static 'st' variable is not being
> inverted correctly. The assembler code produced with -01 is:
>
>   71:watchdog.c    **** 	st = !st;
>  140               		.stabn 68,0,71,.LM11-WatchdogHandler
>  141               	.LM11:
>  142 0028 5C42 0000 		mov.b	&st.0, r12
>  143 002c 4C93      		tst.b	r12
>  144 002e 0234      		jge	.Leaq33
>  145 0030 7CE3      		inv.b	r12
>  146 0032 5C53      		inc.b	r12
>  147               	.Leaq33:
>  148 0034 7C53      		add.b	#llo(-1), r12
>  149 0036 4C11      		rra.b	r12
>  150 0038 4C43      		clr.b	r12
>  151 003a 4C10      		rrc.b	r12
>  152 003c C24C 0000 		mov.b	r12, &st.0
>
> If I watch the memory location where 'st' is stored (it is 0x200 on my app,
> I checked by disassembling the output ELF file) I can see that it gets
> inverted correctly from 0x00 to 0x80 the first time around the loop but
> then is always left at 0x80 and never inverted again to 0x00. If I write
> the above explicitly ie.
>   if (st)
>     st = 0;
>   else
>     st = 1 (or 0x80, whatever as long as it isn't 0x00)
> my handler behaves as expected ie. the LED is toggled.
>
> If I build the same code with -O0 (optimisations off) I get this:
>  149               		.stabn 68,0,71,.LM11-WatchdogHandler
>  150               	.LM11:
>  151 0036 C143 0200 		mov.b	#llo(0), 2(r1)
>  152 003a C293 0000 		tst.b	&st.0
>  153 003e 0220      		jne	.L5
>  154 0040 D143 0200 		mov.b	#llo(1), 2(r1)
>  155               	.L5:
>  156 0044 D241 0200 		mov.b	2(r1), &st.0
> which is a lot more compact and works correctly !
>
> I am using a MSP430F149 and a fairly (a week or two old) build of the
> MSP430GCC. Any comments ? What command line options do people typically use
> with msp430-gcc ie. in terms of optimisations, etc. I guess for now I will
> use -O0
>
> Regards.
>
> Colin
>
> ---------------------------------------------------------------------------
>------ Colin Domoney
> Digital Hardware Design Engineer
> NCipher, Jupiter House, Station Road, Cambridge, CB1 2JD
> e-mail: cdo...@nc...
> Telephone: +44-1223-723630
>
> _______________________________________________
> Mspgcc-users mailing list
> Msp...@li...
> https://lists.sourceforge.net/lists/listinfo/mspgcc-users

-- 
*********************************************************************
   ("`-''-/").___..--''"`-._     (\       Dimmy the Wild      UA1ACZ
    `6_ 6  )   `-.  (     ).`-.__.`)      Enterprise Information Sys 
    (_Y_.)'  ._   )  `._ `. ``-..-'       Nevsky prospekt,   20 / 44
  _..`--'_..-_/  /--'_.' ,'               Saint Petersburg,   Russia
 (il),-''  (li),'  ((!.-'                 +7 (812) 314-8860, 5585314
*********************************************************************