Wierd bug with volatile pointer expressions

[Eric Doenges <doenges at lpr dot e-technik dot tu-muenchen dot de>]

Hi all,

I found the following wierd behaviour (which I think is a bug) of
gcc-2.95.2. Consider the following code:

-----------------------------------------------------------------
#define WIERD ({\
  *((volatile short *)1000) |= 5;\
  while (*((volatile short *)1000) != 5);\
})

#define OK {\
  *((volatile short *)1000) |= 5;\
  while (*((volatile short *)1000) != 5);\
}


void test1(void)
{
  WIERD;
}

void test2(void)
{
  OK;
}


'test1' is compiled (m68k-coff-gcc -S -mcpu32 -O3  -Wall -fno-builtin
-gcoff test.c) to:

[ ... ]
        move.w #1000,%a0
        move.w (%a0),%d0
        or.w #5,%d0
        move.w %d0,(%a0)
        move.w (%a0),%d0
        .even
.L5:
        move.w (%a0),%d0
        cmp.w #5,%d0
        jbne .L5
[ ... ]


Note the superfluous move.w (%a0),%d0 before the .even - d0 is loaded with
the exact same value in the next statement.

'test2' looks like this:

[ ... ]
        move.w #1000,%a0
        move.w (%a0),%d0
        or.w #5,%d0
        move.w %d0,(%a0)
        .even
.L9:
        move.w (%a0),%d0
        cmp.w #5,%d0
        jbne .L9
[ ... ]

Here, there is no unecessary move.
-----------------------------------------------------------------

The only difference in the code is that 'WIERD' is defined with round
brackets. So why is this a bug ? Suppose that reading memory address 1000
triggers some hardware function. In this case, having unwanted references
to memory is clearly a problem.

Without the 'volatile' keyword, the superflous move is not emitted.

The error occurs on all Version of gcc-2.95.2 availible to me, i.e. native
Compilers on i386-linux, alpha-dec-osf4.0f and sparc-sun-solaris2.8, as
well as the m68k-coff cross compiler on i386-linux.
-- 
________________________________________________________________________
Dipl.-Ing. Eric Doenges                         http://www.rcs.ei.tum.de
Institute for Real-Time Computer Systems (RCS)      fon +49-89-289-23590
Technische Universitaet Muenchen, D-80290 Muenchen  fax +49-89-289-23555