[sdcc-devel] [ sdcc-Bugs-1838000 ] Bad code for a bitfield in triple operator

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Bugs item #1838000, was opened at 2007-11-25 10:32
Message generated for change (Comment added) made by rlar
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Category: msc51(8051) target
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Bad code for a bitfield in triple operator

Initial Comment:
SDCC 2.7.0 produces erroneous code for a statement like 

  *bp++ = sp->b ? 'A' : 'B';

The compiled code will crash when executed. More details in the enclosed file.

Submitted by: j (dot) kangas (at) luukku (dot) com



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

Comment By: rlar (rlar)
Date: 2008-02-26 21:37

Message:
Logged In: YES 
user_id=1840151
Originator: NO

Problem Analysis:

  within genPointerGet ()
    sometimes a getFreePtr() is called (and emits a push) to free a
register
    then sometimes genUnpackBits()/genIfxJump() is called
      and emits a conditional jump
    and after that freeAsmop() is called and emits a pop

this results in something like:
  push ..
  ...
  conditional jump to label
  pop

  next-icode
  ...

label:
  ...


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

call trace:

gen51Code()

  processing the icode:
     bug-20-a.c(l13:s7:k8:d0:s0)		iTemp5 [k10 lr7:8 so:0]{ ia0 a2p0 re0
rm0 nos0 ru0 dp0}{unsigned-bitfield {0,1}} = @[iTemp4 [k9 lr6:7 so:0]{ ia1
a2p0 re0 rm0 nos0 ru0 dp0}{unsigned-bitfield {0,1} near* }[r2 ]]

calls:

  genPointerGet () calls
   genNearPointerGet() calls
     getFreePtr() 
       emits a "push ..."

  then 

  genPointerGet () calls
    genUnpackBits (operand * result, char *rname, int ptype, iCode *ifx)
calls 
      genIfxJump (iCode * ic, char *jval, operand *left, operand *right,
operand *result)
        emits a conditional jump

  then

  genNearPointerGet()

    /* now some housekeeping stuff */
    if (aop)       /* we had to allocate for this iCode */
      {
        ...
        freeAsmop (NULL, aop, ic, RESULTONSTACK (ic) ? FALSE : TRUE);

     and freeAsmop() emits a "pop"


this results in "emit"-age of 

# Breakpoint 13, emitcode (inst=0x81249d9 "push", fmt=0x81249e2 "%s") at
gen.c:158
# Breakpoint 13, emitcode (inst=0x81249d0 "mov", fmt=0x8124a96 "%s,%s") at
gen.c:158
# Breakpoint 13, emitcode (inst=0x81249d0 "mov", fmt=0x8124bb0 "a,@%s") at
gen.c:158
# Breakpoint 13, emitcode (inst=0x81255bd "jb", fmt=0x812532d "%s,%05d$")
at gen.c:158
# Breakpoint 13, emitcode (inst=0x8125289 "ljmp", fmt=0x81252ce "%05d$")
at gen.c:158
# Breakpoint 13, emitcode (inst=0x81249bf "", fmt=0x81249b8 "%05d$:") at
gen.c:158
# Breakpoint 4, freeAsmop (op=0x0, aaop=0x81e6600, ic=0x81dac10, pop=1) at
gen.c:1076
# Breakpoint 13, emitcode (inst=0x81249de "pop", fmt=0x8124b1a "ar0") at
gen.c:158

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

Robert Larice

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

Comment By: rlar (rlar)
Date: 2008-02-26 21:09

Message:
Logged In: YES 
user_id=1840151
Originator: NO

for debugging purpose the offending code can be reduced to:

struct { char b:1; } data * glbl;

void func(char data *p) {
  *p++ = glbl->b ? 'A' : 'B';
  *p   = 'C';
}

Robert Larice


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

Comment By: Nobody/Anonymous (nobody)
Date: 2007-11-25 10:38

Message:
Logged In: NO 

System sis not accept the enclosed file, I'll enclose it here:

/* Demonstrates a problem in SDCC 2.7.0

   This program will crash when run in an 8051. The error is
   on the line marked below. The compiler produces PUSH AR0,
   but the corresponding POP AR0 is misplaced and is not
   executed when s.b is 0. This leaves stack corrupted and 
   executing RET in the end of func() crashes the program.
   
   The assembly for the marked statement:
   
    ; triple.c:33: *bp++ = sp->b ? 'A' : 'B';
    mov   ar1,r0
    inc   r0
    push  ar0
    mov   r0,#_s
    movx  a,@r0
    jnb   acc.0,00103$
    pop   ar0
    mov   r2,#0x41
    sjmp  00104$
00103$:
    mov   r2,#0x42
00104$:
    mov   @r1,ar2

   
   Compiler command line:
     sdcc --xram-size 256 triple.c
   
   Compiler version:
   SDCC : mcs51/gbz80/z80/avr/ds390/pic16/pic14/TININative/xa51/ds400/hc08
2.7.0 #4818 (May 31 2007) (MINGW32)
   
   Submitted by: 
   j dot kangas at luukku dot com

*/

typedef struct st
{
    unsigned char b:1;
} ST;

__pdata ST s;
__idata unsigned char buf[5];

void func( void )
{
    __pdata ST *sp;
    __idata unsigned char *bp;
    
    sp = &s;
    bp = buf;
    
    *bp++ = sp->b ? 'A' : 'B';   // statement producing bad code
    *bp++ = 'C';
}

void main( void )
{
    s.b = 0;

    func();
    
    while (1);
}


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

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