[WinAVR-users] [ winavr-Bugs-1374531 ] Modulo problem

[SourceForge.net <no...@so...>]

Bugs item #1374531, was opened at 2005-12-06 08:05
Message generated for change (Comment added) made by arcanum
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=520074&aid=1374531&group_id=68108

Please note that this message will contain a full copy of the comment thread,
including the initial issue submission, for this request,
not just the latest update.
Category: None
Group: None
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
>Assigned to: Eric Weddington (arcanum)
Summary: Modulo problem

Initial Comment:
I spot this problem

here for eqivalent codes (not really as some use
constatnt 63 and some 64).
First code is compiled (and work) incorectly

volatile u08 USART_TX_END;

#define USART_BUFFER_LENGTH 64
USART_TX_END=(USART_TX_END+1)%USART_BUFFER_LENGTH;
  001a 8091 0000 		lds r24,Usart1_TxEnd
  001e 9927      		clr r25
  0020 0196      		adiw r24,1
  0022 9C01      		movw r18,r24
  0024 207C      		andi r18,lo8(960) ; I thing this is
WRONG !
  0026 3370      		andi r19,hi8(960); I thing this is
WRONG !
  0028 821B      		sub r24,r18
  002a 930B      		sbc r25,r19
  002c 8093 0000 		sts Usart1_TxEnd,r24
 
#define USART_BUFFER_LENGTH 63 
USART_TX_END=(USART_TX_END+1)%USART_BUFFER_LENGTH;
  001a 8091 0000 		lds r24,Usart1_TxEnd
  001e 9927      		clr r25
  0020 0196      		adiw r24,1
  0022 6FE3      		ldi r22,lo8(63)
  0024 70E0      		ldi r23,hi8(63)
  0026 0E94 0000 		call __divmodhi4
  002a 8093 0000 		sts Usart1_TxEnd,r24

#define USART_BUFFER_LENGTH 63
USART_TX_END++;
USART_TX_END%=USART_BUFFER_LENGTH;
	001a 8091 0000 		lds r24,Usart1_TxEnd
	001e 8F5F      		subi r24,lo8(-(1))
	0020 8093 0000 		sts Usart1_TxEnd,r24
	0024 8091 0000 		lds r24,Usart1_TxEnd
	0028 6FE3      		ldi r22,lo8(63)
	002a 0E94 0000 		call __udivmodqi4
	002e 9093 0000 		sts Usart1_TxEnd,r25


#define USART_BUFFER_LENGTH 64:
  USART_TX_END++;
  USART_TX_END%=USART_BUFFER_LENGTH;
	001a 8091 0000 		lds r24,Usart1_TxEnd
	001e 8F5F      		subi r24,lo8(-(1))
	0020 8093 0000 		sts Usart1_TxEnd,r24
	0024 8091 0000 		lds r24,Usart1_TxEnd
	0028 8F73      		andi r24,lo8(63)
	002a 8093 0000 		sts Usart1_TxEnd,r24


ot...@e-...

----------------------------------------------------------------------

>Comment By: Eric Weddington (arcanum)
Date: 2005-12-11 18:10

Message:
Logged In: YES 
user_id=543419

Please read the WinAVR User Manual. Bugs having to do with
the compiler or avr-libc need to go to those projects and
not here.

This bug is closed as invalid.

Eric

----------------------------------------------------------------------

Comment By: Nobody/Anonymous (nobody)
Date: 2005-12-09 07:55

Message:
Logged In: NO 

I don't see anything wrong with this code.  The index is a
char value.  960 == 0x3C0, The value is incremented, the low
6 bits masked off and subtracted from the original
incremented number.  That effectively masks off the bits >
6.  A bit convoluted, but reasonably efficient optimization
of the MOD operator.

THe only thing wrong is that the internal math is done 16
bits.  That is because you are using the % operator.  A more
traditional way would be to express this like:

end = (end + 1) & (Size -1);

and you would get optimal code (which is why buffers tend to
be in powers of two size).

la...@ba...

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=520074&aid=1374531&group_id=68108