On 08/10/2019 17:33, Peter Zijlstra wrote: > On Tue, Oct 08, 2019 at 03:34:04PM +0100, Valentin Schneider wrote: >> 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... > > But -fno-strict-overflow mandates 2s complement for all such signed > issues. >
So then there really shouldn't be any ambiguity. I have no idea if -fno-strict-overflow then also lifts the undefinedness of the right shifts, gotta get my spade and dig some more.
