Can C default to float? Are there float regs?

[Douglas J Freyburger]

The discussion has been about single vs double precision
floating point as well as keeping them in registers.  It
was mentioned that there w{{uld be a big speed difference
on the VAX.

At site "cithep", the Caltech High Enerrgy Physics
Department, they made a C compiler that used single
precision for "float".  At first, they got ALMOST NO SPEED
IMPROVEMENT.  After adding floating point immediate values
to the assembler produced (instead of storing constants
like strings and then referring to them by name), they got
a pretty good improvement.  10-20%.  I don't know if they
were running a 750 or 780.  Still, for calculations that
only involve variables and no constants, the difference is
much smaller than you'd think.

The difference is smaller than you would think, but then,
has anyone ever seen an instruction timing diagram for any
VAX model published by DEC?  No.  The large number of
models is one reason.  The complexity of cache-hit vs
cache-miss vs page-fault out of the physical memory pool vs
page-fault out of disk is another reason.  Still, I think
they should be able to publish the timings for cache-hits
and cache-misses on a per-model basis.  They DO have the
microcode sources available to count from.  I am guessing
that they put a lot more effort into optimizing the double
precision floating point, and they are hesitant to show
that in public.

Ever look at the assembler produced by BLISS-32 on a VMS
machine?  That BLISS compiler obviously knows some stuff
that the rest of us don't.  It uses indexed address
modes instead of ADDs, in-line multiplies instead of
POLYs, and other tricks that I don't even recognize that
LOOK like they should be slower than the obvious way.  It
looks like instructions that people think are faster are
even sometimes slower.
-- 

Doug Freyburger		DOUG at JPL-VLSI, DOUG at JPL-ROBOTICS,
JPL 171-235		...escher!doug, doug at aerospace,
Pasadena, CA 91109	etc.

Disclaimer: The opinions expressed above are far too
ridiculous to be associated with my employer.

Unix is a trademark of Bell Labs, VMS is a trade mark of
DEC, and there are others that I'm probably forgeting to
mention.