Re: make sure struct real_value's bitfields are packed

[Segher Boessenkool <segher at kernel dot crashing dot org>]

> > > +#define REAL_EXP(REAL) \
> > > +  ((int)((REAL)->uexp ^ (unsigned int)(1 << (EXP_BITS - 1))) \
> > > +   - (1 << (EXP_BITS - 1)))
> >
> > > I hadn't come across this "(x ^ signbit) - signbit" idiom for
> > > implementing sign-extension before.  Nice.
> >
> > Yeah, I found it great as well when I learned about it in the binutils
> > list :-)
>
> I don't really like the idiom. It's obfuscated and requires a large
> constant, which is costly on some targets. Plain old
> (x << EXP_BITS) >> EXP_BITS is much more readable and already gets
> recognized by gcc as sign extension.

But it doesn't mean the same thing!  To use your idiom, you need
to know the size in bits of int.  Messy.


Segher