Re: [PATCH 07/14] Cleanup: avoid undefined shift operation

On 29-12-17 08:00, Linus Torvalds wrote:

On Thu, Dec 28, 2017 at 4:35 PM, Dirk Hohndel dirk@hohndel.org wrote:

...

     unsigned int signbit = (1 << (n - 1));

•   unsigned int mask = (0xFFFFFFFF << n);
  

•   unsigned int mask;
  

•   // shifting a 32bit constant by more than 31 bits has 
  

undefined behavior

•   if (n == 32)
  

•           mask = 0;
  

•   else
  

•           mask = (0xFFFFFFFF << n);
  

Ugh. That's ugly.

It's much simpler to just do

     unsigned int mask = ~(signbit-1);

instead.

Yes, that mask ends up including the sign bit (the old mask did not), but that doesn't matter for the end result.

That's indeed an easier and nicer fix!

The bitwise not can even be omitted from the mask, and moved to the return statements (where there is already both a mask and a ~mask).

Jef