On Mon, 7 May 2007, Esben Nielsen wrote:
>
> What is (long)(a-b) ? I have tried to look it up in the C99 standeard but I
> can't find it. Maybe it is in the referred LIA-1 standeard, which I can't find
> with google.
I don't worry about non-2's-complement machines (they don't exist, and
likely won't exist in the future either).
So I worry about compilers rewriting my code.
So "(long)(a-b) < 0" (with "a" and "b" being unsigned long) is basically a
portable way of testing the high bit of the result.
> I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b" as
> that should be completely portable.
That certainly works too, but the difference is irrelevant, since Linux is
unlikely to work on insane machines anyway (ie we do make a lot of other
assumptions about the architecture, being two's-complement is the least of
those).
So you basically shouldn't worry about hardware: everybody is pretty much
the same. You should worry about *compilers* - that's where the
differences show up.
So "(long)(a-b)" may be "implementation defined" (but since
implementations are all 2's complement, we don't care), but a signed
"(a-b)" that over/overflows is *undefined*, and that is much worse because
it means that the compiler can do some funky stuff, and _that_ is a real
practical worry.
And no, I also don't worry about porting Linux to 18-bit machines, or to
ternary CPU's.
Linus
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