BUG - unsigned long cast to double

[Chris Torek]

>In article <1239500001 at osiris.cso.uiuc.edu> olson at osiris.cso.uiuc.edu writes:
[various compilers convert as if from signed long to double]

In article <2590 at ncr-sd.SanDiego.NCR.COM> matt at ncr-sd.SanDiego.NCR.COM
(Matt Costello) writes:
>The problem has existed in PCC since the beginning,

Probably true (I have not been around long enough to really know; I
believe PCC appeared in phototypesetter V6, which I have never used.)

>as the PDP-11 did not readily support 32 bit integers.

Not a good excuse.  I imagine it was simply never tested.

>Neither the VAX nor the M68881 support conversions to or from unsigned
>values either.

Conversion on a VAX is easy:

	cvtl[fd] src,dst
	bgeq 0f; add[fd]2 $0[fd]4294967296.0,r2; 0:

On a 68000 it would be something longer, but also involve testing
the sign before or during conversion.

>The code sequences to handle the unsigned long to double conversion
>(and vice versa) would have imposed an unacceptable performace penalty,
>so it wasn't done.

Better slow than wrong.  I think it was just an oversight.  One could
make the claim that unsigned division and modulus on a VAX impose an
unacceptable performance penalty (it takes far more than the four
instructions above), yet they were done.

>I don't have my K&R here, but I don't even believe
>that unsigned longs were legal back then.

I believe the One True C Compiler (dmr's '11 compiler) :-) did not have
unsigned <char, short, long> variants.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris