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.
