🚀 go-pugleaf

In article <4cpv1n$apm@der.twinsun.com>,
Paul Eggert <eggert@twinsun.com> wrote:
>thads@csn.net (Thad Smith) writes:
>
>> it is possible to rewrite the function so that it doesn't
>> invoke undefined behavior
>
>Yes.  But it slows things down measurably for an interpreter to use
>such a clumsy method for integer overflow detection.  The (x + y < x)
>!= (y < 0) method requires a comparison, an exclusive OR of the
>comparison result bit with a sign bit, and a conditional branch.
>The method you suggested requires a comparison, a conditional branch, a
>subtraction, another comparison, and another conditional branch.
>
>Since this is comp.std.c, I'll mention that the (x + y < x) != (y < 0) method
>and Daniel Wood's example both port to any C implementation
>claiming conformance to LIA-1 (ISO/IEC 10967-1:1994(E),
>`Information technology -- Language independent arithmetic -- Part 1:
>Integer floating point arithmetic' <ftp://crl.dec.com/pub/misc/lia-1-dis.ps.Z>)
>if INT_MODULO is true (which is the only plausible value for INT_MODULO
>for real-world C compilers).  I doubt whether Wood's compiler vendor
>claims LIA-1 conformance today, but this may become more important in
>the future, and perhaps Wood can point his vendor at LIA-1 and say that
>it's time to read the handwriting on the wall.

It will also be true for any implementation conforming to IEEE Std 1754.
I urge all users to insist that their vendors' implementations conform to
IEEE Std 1754.

					Sincerely,
					Robert Corbett