[sdcc-devel] [ sdcc-Bugs-777768 ] compiler makes buggy code for 8051

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Bugs item #777768, was opened at 2003-07-25 14:45
Message generated for change (Comment added) made by epetrich
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Category: msc51(8051) target
>Group: fixed
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Josef Pavlik (jetset)
>Assigned to: Erik Petrich (epetrich)
Summary: compiler makes buggy code for 8051

Initial Comment:
Hello, I found that the compiler makes bad code in some situation. The following piece of asm result describes the situation.
You can found entire source code in the attachment.


$sdcc --version
SDCC : mcs51/gbz80/z80/avr/ds390/pic14/pic16/TININative/xa51/ds400 2.3.5 (Jul 25 2003) (UNIX) 
$sdcc bug_xor.c -c

bug_xor.asm:

;bug_xor.c:145: *(data unsigned int*)(&msg[2])^=x;
;     genAssign
;     genXor
	mov	a,0x0001 + _msg
	xrl	a,0x0002 + _msg
	mov	r2,a
	mov	a,(0x0001 + _msg + 1)
	xrl	a,(0x0002 + _msg + 1)
	mov	r3,a
;     genPointerSet
;     genNearPointerSet
;     genDataPointerSet
	mov	(_msg + 0x0002),r2
	mov	((_msg + 0x0002) + 1),r3



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>Comment By: Erik Petrich (epetrich)
Date: 2004-04-23 00:52

Message:
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Fixed; see ChangeLog 1.711 for the list of updated files


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Comment By: Bernhard Held (bernhardheld)
Date: 2003-07-25 17:07

Message:
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Thanks for the short report :-> These 11 lines ar enough to expose the 
problem: 
 
char msg[3]; 
 
int 
foo (void) 
{ 
  int x; 
 
  x=*(data int *)(&msg[1]); 
  *(data int *)(&msg[0])^=x; 
  return x; 
} 
 
I'm not sure, if SDCC is guilty with all these typecasts from (char *) to 
(int *), but I'll think about it. 
 
> This occures when msg is declared as volatile too. 
Here SDCC's behaviour is correct. Define x as volatile, and it'll work as 
you expect. 

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Comment By: Josef Pavlik (jetset)
Date: 2003-07-25 15:32

Message:
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in the meantime I found the origin of the problem. The compiler tries to optimize the following sequence

x=msg[1];
//some code in the middle
msg[1]^=x;

to

msg[1]^=msg[1];

the problem is that the code in the middle changes the array msg
This occures when msg is declared as volatile too.


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