Re: [avr-chat] Type promotion and shift operators

[Michael Hennebry]

On Wed, 2 Feb 2011, Rick Mann wrote:

> On Feb 2, 2011, at 08:15:40, Michael Hennebry wrote:
>
> > On Wed, 2 Feb 2011, Rick Mann wrote:
> >
> > > I was working with some uint32_t variables, using shifts, like this:
> > >
> > > uint32_t foo = 0;
> > >
> > > for (...)
> > > {
> > >   uint32_t v = // some value between [0, 31)
> > >
> > >   foo |= 1 << v;
             ^
The type of RHS is int regardless of the type of v.

> > > }
> > >
> > > This didn't work correctly for values of v greater than 15 until I changed the 
> > > line to:
> > >
> > >   foo |= 1UL << v;
             ^^^
The type of RHS is unsigned long regardless of the type of v.

> > > From what I understand of C, it should've promoted 1 to long, but maybe it's a 
> > > signed-vs-unsigned issue? What's happening on an ATmega644A with this code?
> >
> > 'Twas a quiet change.
> > the type of x<<y is now the type of x after the usual integer promotions.
>
> So, it looks like this?
>
>         foo = (int) ((long) 1 << v)

If foo is still uint32_t, don't coerce to int.
Not only is it not what you want,
it's not guaranteed to produce anything in particular.
It's an example of the general rule that
non-value-preserving conversions to a
signed integer have no guaranteed semantics.

--
Michael   address@hidden
"Pessimist: The glass is half empty.
Optimist:   The glass is half full.
Engineer:   The glass is twice as big as it needs to be."