Re: [PATCH v3 04/10] sched/fair: rework load_balance

[Valentin Schneider]

On 08/10/2019 15:16, Peter Zijlstra wrote:
> On Wed, Oct 02, 2019 at 11:47:59AM +0100, Valentin Schneider wrote:
> 
>> Yeah, right shift on signed negative values are implementation defined.
> 
> Seriously? Even under -fno-strict-overflow? There is a perfectly
> sensible operation for signed shift right, this stuff should not be
> undefined.
> 

Mmm good point. I didn't see anything relevant in the description of that
flag. All my copy of the C99 standard (draft) says at 6.5.7.5 is:

"""
The result of E1 >> E2 [...] If E1 has a signed type and a negative value,
the resulting value is implementation-defined.
"""

Arithmetic shift would make sense, but I think this stems from twos'
complement not being imposed: 6.2.6.2.2 says sign can be done with
sign + magnitude, twos complement or ones' complement...

I suppose when you really just want a division you should ask for division
semantics - i.e. use '/'. I'd expect compilers to be smart enough to turn
that into a shift if a power of 2 is involved, and to do something else
if negative values can be involved.