[sdcc-devel] [ sdcc-Bugs-3486135 ] Masking bits generates non-atomic code

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Bugs item #3486135, was opened at 2012-02-09 02:05
Message generated for change (Comment added) made by patryks
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Category: mcs51(8051) target
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Phil ()
Assigned to: Nobody/Anonymous (nobody)
Summary: Masking bits generates non-atomic code

Initial Comment:
An expression to clear a bit (or bits) in a byte in the DATA area (or an SFR) should be atomic.

// sdcc -c testcase.c

#define MASK    0x0f
unsigned char   reg;

void testcase(void)
{
        reg &= ~MASK;
}

should generate:

        anl _reg,#0xFe

but actually generates:

        mov r7,_reg
        anl ar7,#0xF0
        mov _reg,r7

Using SDCC : mcs51 3.1.0 #7066 (Nov 28 2011) (Mac OS X x86_64)

Suggestion: Could this be fixed by a peeophole rule?

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Comment By: Patryk (patryks)
Date: 2012-06-14 13:00

Message:
>> Also, are SFR's implitly considered DATA or IDATA?
SFR's can be accessed only directly, i.e. as DATA.

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Comment By: Phil ()
Date: 2012-06-12 02:27

Message:
See
https://sourceforge.net/projects/sdcc/forums/forum/1865/topic/5343383/index/page/1
for discussion of workaround using a peephole rule.

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

Comment By: Phil ()
Date: 2012-02-14 01:07

Message:
Note also that by using an inverse macro, avoiding the need for the ~
operator, the correct code is generated with 3.1.0:

#define IMASK 0xf0
unsigned char reg;

void testcase(void)
{
	reg &= IMASK;
}


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Comment By: Gabriele Gorla (gorlik)
Date: 2012-02-14 00:58

Message:
sdcc 3.0 compiles it as anl _reg,#0xF0 without the cast.


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Comment By: Thomas Sailer (tsailer)
Date: 2012-02-09 04:55

Message:
If you change reg &= ~MASK to reg &= (unsigned char)~MASK, then it will
generate what you want, i.e. anl _reg,#0xF0

And yes, that cast shouldn't be necessary...

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Comment By: Phil ()
Date: 2012-02-09 02:32

Message:
On checking the instruction set reference
(http://www.win.tue.nl/~aeb/comp/8051/set8051.html#51anl), I may have this
wrong as anl only suppords an indirect address
as the first operand. However, I would then expect SDCC to generate an
atomic operation if 'reg' is declared as IDATA, viz:

unsigned char __idata reg;

Instead, it generates the following:

	mov	r0,#_reg
	mov	ar7,@r0
	anl	ar7,#0xF0
	mov	r0,#_reg
	mov	@r0,ar7

Also, are SFR's implitly considered DATA or IDATA?

Apologies if I'm being daft here :)

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

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