On 19 January 2017 at 08:03, leon zadorin wrote: > Last time I checked in C, as well as in C++, standards -- shifting by > the number of bits greater *or equal* to the size of the type is > undefined behavior... > > e.g. something like "An expression is shifted by a negative number or > by an amount greater than or equal to the width of the promoted > expression (6.5.7)." from > https://www.securecoding.cert.org/confluence/display/c/CC.+Undefined+Behavior#CC.UndefinedBehavior-ub_51 > > The exact place in C++14 and or latest C standards escape me right > now, but the thesis of this UB stands correct I think... Yes, and it's easily confirmed by compiling with -fsanitize=undefined, so that it flags the error at runtime: 60=>0x2000000000000000 61=>0x4000000000000000 ub.c:11:25: runtime error: left shift of 1 by 63 places cannot be represented in type 'long int' 62=>0x8000000000000000 63=>0x0 0=>0x2 So it's not an optimization bug, the testcase has undefined behaviour.
