🚀 go-pugleaf

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.