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

[Rick Mann]

On Feb 2, 2011, at 10:49:11, Michael Hennebry wrote:

> 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.

No, I wasn't proposing that as the code I should write. I was trying to codify 
what you said. You said, "the type of x<<y is now the type of x after the usual 
integer promotions":

1) usual integer promotions (which I take to mean, in this case, the LHS is 
promoted to the size of RHS)
2) result of shift operation is type of LHS argument

That seems to say that my original expression:

        foo |= 1 << v;

Behaves as if I had written this:

        foo |= (int) (((long) 1) << v);    // extra parens added for clarity

Which strikes me as terribly wrong.

When was this change adopted into the language?

-- 
Rick

> 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."