🚀 go-pugleaf

We've had some trouble with Epoch on HP9000s800's, and I tracked it down to the
following defines in m-hp9000s800.h, which are identical to the ones in GNU
Emacs 18.55:

#define XMARKBIT(a) ((a) < 0)
#define XSETMARKBIT(a,b) ((a) = ((b) ? (a)|MARKBIT : (a) & ~MARKBIT))
#define XUNMARK(a) ((a) = (((a) << INTBITS-GCTYPEBITS-VALBITS) >> \
                           INTBITS-GCTYPEBITS-VALBITS))


In addition, GCTYPEBITS is set to 5. What happens is that under Epoch, the
top bit of the TYPE field can be 1 (because Epoch has more types than GNU
Emacs). This bit gets left shifted to the top bit position, and then the
right shift leaves the top bit (the GC mark bit) set, so that XUNMARK()
does not actually unmark objects with type > 15.

What I'd like to know is, why are the macros defined this way? The comments
claim that these are faster, but I find it difficult to believe that 2 shifts
are faster than 1 mask on any architecture. Would getting rid of these
defines and just using the defaults be a problem?
-- 
Alan M. Carroll                "It's psychosomatic. You need a lobotomy,
Epoch Development Team          I'll get a saw."
CS Grad / U of Ill @ Urbana    ...{ucbvax,pur-ee,convex}!cs.uiuc.edu!carroll