------- Additional Comments From neroden at gcc dot gnu dot org 2005-07-15
05:50 -------
OK, so in X.org, the key area where this is hit is the definition of
MMIO_IN8 in compiler.h. For alpha, sparc, and powerpc, inline volatile
ASM is used instead (much better in some ways).
The vanishing line is:
(void) minb(hwp->IOBase + VGA_IN_STAT_1_OFFSET);
This expands in preprocessing to:
(void) MMIO_IN8(hwp->MMIOBase, (hwp->MMIOOffset + (hwp->IOBase +
VGA_IN_STAT_1_OFFSET)))
which further expands in preprocessing in compiler.h (except for
sparc, alpha and powerpc) to:
(void) *(volatile CARD8 *)( (CARD8*)(hwp->MMIOBase) + hwp->MMIOOffset +
hwp->IOBase + VGA_IN_STAT_1_OFFSET)
The key expansion is this one:
# define MMIO_IN8(base, offset) \
*(volatile CARD8 *)(((CARD8*)(base)) + (offset))
This obviously doesn't work and perhaps it shouldn't work. But what is a
good alternative implementation? Essentially, we're trying to force a read
of an arbitrary memory address. If the "underlying object" has
to be volatile, how do we make it volatile? Can we simply create a new
"underlying object" by throwing in a temporary pointer variable? Is something
like the following sufficient to force an access?
static inline unsigned char myMmioIn8(volatile void* base,
const unsigned long offset)
{
volatile CARD8 * tmp = (CARD8*)base + offset;
return *tmp;
}
# define MMIO_IN8(base, offset) myMmioIn8(base, offset)
If that's not sufficient, I don't see any way to accomplish the required
behavior under gcc 4.0 (without massive rewriting of the X.org code), so I
hope it is.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
