Is it a good idea to add int/uint/long/ulong functions for the "<<" left shift operation here?

https://github.com/D-Programming-Language/druntime/blob/master/src/core/checkedint.d

Bye,
bearophile

On Thu, Jul 31, 2014 at 05:00:22PM +0000, bearophile via Digitalmars-d-learn wrote:
> Is it a good idea to add int/uint/long/ulong functions for the "<<" left shift operation here?
> 
> https://github.com/D-Programming-Language/druntime/blob/master/src/core/checkedint.d
[...]

What would you check for? Shifting something that already has its high bit set?


T

-- 
Don't drink and derive. Alcohol and algebra don't mix.

H. S. Teoh:

> What would you check for? Shifting something that already has its high bit set?

If you have a uint where the 3 most significant bits are 1, and you shift it 3 bits on the left, you lose those three bits, you have an overflow. The point of checkedint functions/intrinsics is to reveal overflows.

Bye,
bearophile

On Thu, Jul 31, 2014 at 06:08:56PM +0000, bearophile via Digitalmars-d-learn wrote:
> H. S. Teoh:
> 
> >What would you check for? Shifting something that already has its high bit set?
> 
> If you have a uint where the 3 most significant bits are 1, and you shift it 3 bits on the left, you lose those three bits, you have an overflow. The point of checkedint functions/intrinsics is to reveal overflows.
[...]

OK, makes sense. But what about if only the most significant bit is 1? Wouldn't that also be an overflow? So you're essentially checking if the topmost n bits are zero, where n is the number of bits you wish to shift by.


T

-- 
BREAKFAST.COM halted...Cereal Port Not Responding. -- YHL

H. S. Teoh:

> OK, makes sense. But what about if only the most significant bit is 1?
> Wouldn't that also be an overflow? So you're essentially checking if the
> topmost n bits are zero, where n is the number of bits you wish to shift by.

Of course.

https://issues.dlang.org/show_bug.cgi?id=13231

Bye,
bearophile