On Tue, 19 Dec 2006, Robert Dewar wrote:
> > GCC does not use the latitude given in C99 only to treat certain
> > aspects of signed @samp{<<} as undefined, but this is subject to
> > change.
>
> That hardly seems sufficient documentation, when documenting undefined,
> you had better say what the semantics is. Saying it is not treated as
> undefined, and then failing to define it is a bit of a contradiction
> in terms :-)
That's documented in the previous paragraphs:
@item
@cite{The results of some bitwise operations on signed integers (C90
6.3, C99 6.5).}
Bitwise operators act on the representation of the value including
both the sign and value bits, where the sign bit is considered
immediately above the highest-value value bit. Signed @samp{>>} acts
on negative numbers by sign extension.
> What is (a*2)/2 optimized to? certainly it has the value a if you wrap, so you
> are not necessarily depending on undefined here.
It's optimized to a (and has been since at least 2.7.2.3, the earliest
version I have around - though it wasn't until 3.0 that this optimization
was fixed to apply only to signed types and not unsigned types as well).
But if you wrap, (INT_MIN*2)/2 would be 0 not INT_MIN, for example.
--
Joseph S. Myers
[EMAIL PROTECTED]
