Re: Bug with g77 and -mieee on Alpha Linux

[Richard Henderson <rth at cygnus dot com>]

On Thu, Jul 08, 1999 at 03:58:45PM -0000, craig@jcb-sc.com wrote:
> >See "man ieee", and #include <machine/fpu.h>. When the program starts
> >up, make a call to ieee_set_fp_control(IEEE_MAP_UMZ); , which sets the
> >fp control to cause underflow to map to zero.

Underflow to zero always ocurrs when software completion (ie -mieee)
is not requested.  This is still useful for when you need -mieee to
handle Inf and NaN, but don't care about denormals.

> Ah, well, it's good people can do that if they want, though the arguments
> against -mieee being the default included "because it catches bugs in code",
> which, presumably, silently mapping underflows to zero won't.

Actually, most of the bugs are where you _start_ with denormals,
typically because of uninitialized data.  UNZ doesn't stop trapping
when the inputs are non-normal.

> Maybe no machine *today* has that combination of 80-bit computations
> and a "fast" IEEE mode crashing on denormals (which Toon supports as the
> default).  Though it's not hard to imagine Merced or McKinley might.

The user mode documentation for IA-64 is public.  One can see that
it supports an 80 bit mode, but it also has instruction forms that
do proper rounding to single and double precision.  Intermediate
results therefore will be properly rounded for the mode.



r~