Re: Red-faced professor gets bitten in search for portability

[milton!mfeldman@beaver.cs.washington.edu (Michael Feldman)]

In article <11919@spim.mips.COM> murphy@mips.com (Mike Murphy) writes:
>
>You are right about your first version not being portable, but
>your second version seems a bit convulated to me.  Why not just
>check whether you indeed rounded down after doing the integer conversion?
>For example:
>	FUNCTION Trunc (X: Float) RETURN Integer IS
>		itrunc : integer = integer(x - 0.5);
>	BEGIN
>		if x - float(itrunc) >= 1.0 then
>			-- rounded down, so add back a 1
>			return itrunc+1;
>		else
>			return itrunc;
>		end if;
>	END Trunc;
Sure - this seems equivalent. I kinda like the halving and doubling,
because these are pretty fast to implement (as shifts) given a
good optimizer. Your method has the advantage, I suppose, of being immune
to a possible overflow from the doubling.

>p.s. btw, some machines, e.g. MIPS, can either round down, or round up,
>or round to the even number (the default setting).
Right. My argument about Trunc is that it's such a common (and easy to
implement) operation that it's better done as an intrinsic, so that
the compiler writer can use whatever hardware trickery is available.

Mike