[sdcc-devel] [ sdcc-Bugs-1739475 ] pointer double-dereference doesn't work

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Bugs item #1739475, was opened at 2007-06-18 21:29
Message generated for change (Comment added) made by nobody
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Category: Icode generator
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Stas Sergeev (stsp)
Assigned to: Nobody/Anonymous (nobody)
Summary: pointer double-dereference doesn't work

Initial Comment:
Hi.

The following (also attached) code miscompiles:
---
idata volatile unsigned char t = 3;
idata volatile unsigned char a = (unsigned char)&t;
idata volatile unsigned char b = 0;
idata volatile unsigned char c = (unsigned char)&b;

unsigned char main()
{
    idata unsigned char * idata unsigned char *p =
            (idata unsigned char * idata unsigned char *)(c - 1);
#define BUG 1
#if BUG
    return **p;
#else
    idata volatile unsigned char *tmp = *p;
    return *tmp;
#endif
}
---

The asm looks like this:
---
;       ddptr.c:8: idata unsigned char * idata unsigned char *p =
        mov     r0,#_c
        mov     ar2,@r0
        mov     r3,#0x00
        dec     r2
        cjne    r2,#0xff,00103$
        dec     r3
00103$:
;       ddptr.c:12: return **p;
----------------- BUG! ------------------
        mov     ar0,@r0
        mov     ar2,@r0
        mov     dpl,r2
        ret
---

Where I put "BUG!", obviously "mov r0, r2"
is missing.
The test-case should return 3, but returns 0.
Define BUG to 0 to get the correct result.

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Comment By: Nobody/Anonymous (nobody)
Date: 2007-07-07 03:23

Message:
Logged In: NO 

the actual problem seems to be peephole rule 177.c
the unoptimized code contains the sequence:

 mov	ar0,r2
 mov	ar0,@r0

and rule 177.c will match:

replace {
	mov	%1,%2
	mov	%1,%3
} by {
	;	Peephole 177.c	removed redundant mov
	mov	%1,%3
} if notVolatile %1 %2

this rule doesn't check for %3 beeing independent of %1
here %1 is r0, and %3 is @r0

Robert Larice

larice 0x40 vidisys 0x2e de


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Comment By: Nobody/Anonymous (nobody)
Date: 2007-07-07 02:20

Message:
Logged In: NO 

for debuging purpose the offending code can be reduced to

char foo(unsigned char c)
{
  return ** ((idata  char * idata *) c);
}


compilation with sdcc --no-peep --fverbose-asm -c -mmcs51
generates correct code:

;	genReceive
	mov	r2,dpl
;	bug-4a.c:3: return ** ((idata  char * idata *) c);
;	genCast
	mov	ar0,r2
;	genPointerGet
;	genNearPointerGet
	mov	ar0,@r0
;	genPointerGet
;	genNearPointerGet
	mov	ar2,@r0
;	genRet
	mov	dpl,r2
00101$:
	ret

but without no-peep, incorrect code is generated

;	genReceive
;	Peephole 301	mov r2,dpl removed
;	bug-4a.c:3: return ** ((idata  char * idata *) c);
;	genCast
;	genPointerGet
;	genNearPointerGet
;	Peephole 177.c	removed redundant mov
	mov	ar0,@r0
;	genPointerGet
;	genNearPointerGet
	mov	ar2,@r0
;	genRet
	mov	dpl,r2
;	Peephole 500	removed redundant label 00101$
	ret


the peephole comment
  ;	Peephole 301	mov r2,dpl removed
is quite intresting.



greetings,
Robert Larice

larice 0x40 vidisys 0x2e de


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

Comment By: Nobody/Anonymous (nobody)
Date: 2007-07-06 12:34

Message:
Logged In: NO 

sorry, drop that last comment of mine

greetings,
Robert Larice

larice 0x40 vidisys 0x2e de


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

Comment By: Nobody/Anonymous (nobody)
Date: 2007-07-06 11:53

Message:
Logged In: NO 

with #4870 the asm looks ok.

	mov	r0,#_c
	mov	ar2,@r0
	mov	r3,#0x00
	dec	r2
	cjne	r2,#0xff,00103$
	dec	r3
00103$:
	mov	ar0,@r0
	mov	ar2,@r0
	mov	dpl,r2
	ret

greetings,
Robert Larice

larice 0x40 vidisys 0x2e de


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

Comment By: Nobody/Anonymous (nobody)
Date: 2007-07-06 11:41

Message:
Logged In: NO 

with #4870 the asm looks ok.

	mov	r0,#_c
	mov	ar2,@r0
	mov	r3,#0x00
	dec	r2
	cjne	r2,#0xff,00103$
	dec	r3
00103$:
	mov	ar0,@r0
	mov	ar2,@r0
	mov	dpl,r2
	ret

greetings,
Robert Larice

larice 0x40 vidisys 0x2e de


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

Comment By: Patryk (patryks)
Date: 2007-06-19 01:54

Message:
Logged In: YES 
user_id=1788180
Originator: NO

Reproduced with SDCC #4847.
Shouldn't your 'p' definition look like below?
idata unsigned char * idata * p =
(idata unsigned char * idata *)(c - 1);
Anyway it doesn't change the result, but is at least free of syntax errors
missed by SDCC.


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

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=100599&aid=1739475&group_id=599