: Re: [RFA] Setting long long bitfields
Paul Hilfinger
hilfingr@EECS.Berkeley.EDU
Mon Nov 1 01:16:00 GMT 2004
Oops, sorry: I got the problematic case backwards. The problem is
that a bitsize of 0 is legal, and that causes the result of your
suggested tweak to be officially undefined.
Paul
> > > One possible tweak, several of us have an aversion to "?:", it would be
> > > nice if it wasn't there :-)
> >
> > Like this perhaps:
> >
> > + ULONGEST mask = (ULONGEST) -1 >> (8 * sizeof (ULONGEST) - bitsize);
>
> Andreas,
>
> I had considered exactly that line, but unfortunately ran across the
> following really irritating provision in the C standard:
>
> "If the value of the right operand is negative or is
> *greater than or equal to* the width of the promoted left
> operand, the behavior is undefined."
>
> [I know why the provision is there, of course: many machines treat a
> shift of wordsize bits as 0, because they mask off bits to the left,
> but it's STILL irritating.]
>
> Now, I presume we will never encounter a bitfield size of 8 * sizeof
> (ULONGEST), but the original code apparently tried to bullet-proof
> against this possibility, so I just went along.
>
> Paul
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