Puzzle on unsigned promotions

[Chris Torek]

In article <736 at vsi.UUCP> friedl at vsi.UUCP (Stephen J. Friedl) writes:
>... unsigned- vs value-perserving rules ...

[and the following code, reproduced without > to fool inews; slightly
compressed]
	main()
	{
		unsigned short	sval;
		long		lval1, lval2;

		sval = 0xffff;

		lval1 = -sval;
		lval2 = -(unsigned short)0xffff;

		printf("lval #1 = %ld    #2 = %ld\n", lval1, lval2);
	}

Well, first, no matter what, you should get the same value in lval1
and lval2.  A cast is equivalent to assignment to an unnamed temporary,
so the second assignment is like writing

		{ unsigned short tmp; tmp = 0xffff; lval2 = -tmp; }

which is semantically identical to `lval1 = -sval'.

Now, to answer `what should I get', you need to know the relative
sizes of short, int, and long (and their unsigned variants).

    sval = 0xffff; sizeof(short) = 2; sizeof(int) = sizeof(long) = 4:

	lval = -sval:

	Old rule: unsigned stays unsigned
		First turn sval into an rvalue: (u_short)0xffff is
		extended to (u_int), result is (u_int)0x0000ffff
	
		Next negate: result is (u_int)0xffff0001

		Next convert to (signed long) by `bits-as-is' due
		to assignment: (long)0xffff0001 or -65535.

	New rule: unsigned stays unsigned if the new type does not
	have more bits, otherwise unsigned becomes signed:
		First turn sval into an rvalue: (u_short)0xffff is
		extended to (signed int), result is (int)0x0000ffff.

[note that the bit pattern produced by both rules is always identical;
the difference is in the interpretation of that bit pattern---signed vs
unsigned]

		Next negate: result is (int)0xffff0001

		Convert to long by `bits-as-is': (long)0xffff0001 or
		-65535.

>The normal answer I get is:
>
>	lval #1 = -65535    #2 = -65535

which is correct.

>but on the HP9000 I see:
>
>	lval #1 = 1    #2 = -65535

Your HP9000 compiler is broken.

>Please email, I'll summarize and post.

Oops :-)
-- 
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