🚀 go-pugleaf

In article <1991May21.191034.25980@murdoch.acc.Virginia.EDU>, Clark
Coleman writes:

> As a compiler writer in this RISCy age, I am inclined to agree with this
> statement. If we compiler writers, in our infinite brilliance, cannot
> figure out how to use an instruction, it doesn't belong in the instruction
> set. Before this becomes a mantra that is chanted mindlessly, let's look
> at an example that might make us think a little harder.

While I generally agree with his sentiment and his example, I am
inclined to believe in at least one major exception.  Not to mindlessly
encourage H. L. Rubin, but population count and leading zero count
on a word value are quite useful, yet (though I'm not a compiler writer)
I can't think of easy methods for a (non-vector) compiler to use these
instructions often.

Yet I think they are undeniably useful.  Many simple operations can
be built easily and quickly in straight-line code with these
instructions, and simple things like dispatching on a bit mask and
converting octal mask bit constants to shift counts, etc. are much
simplified with them.  Examples of functions easily built using
population_count(x) and leading_zeroes(x) include signum(x), abs(x),
!a, a == b, a < b, a > b, ... trailing_zeroes(x), ceil(log2(x)), and
more.  At the same time, these operations are not that common in
most codes, so I wouldn't expect large performance gains, except
in programs that make heavy use of either assembly codes, compiler
intrinsics, or library routines written in assembler or with
intrinsics.

Perhaps I am mistaken here, and you compiler writers can use these
instructions to significant advantage.  If so, I'd be curious to
know how much these instructions typically save.  Also, in this one
case, I can sympathise with Mr. Rubin, because there isn't any elegant
and portable way to express, for instance, leading_zeroes(x) in a
language other than (say, in C) to make it a function or #define
and define it according to the capabilities of the hardware/compiler
at hand.  Not terribly difficult, but it does seem to lack elegance,
and not all compilers for machines that have these functions have such
intrinsics or deal with them well.  Still, IMHO that is something to
take up with the compiler writers, rather than the language designers.

I will note however, that I am very happy to see that many micros
are beginning to include these instructions...

				Robert Herndon
-- 
Robert Herndon		    --	not speaking officially for Cray Computer.
Cray Computer Corporation			719/540-4240
1110 Bayfield Dr.				rh@craycos.com
Colorado Springs, CO   80906		   Ignore these three words.