From: Jonas Oberhauser > Sent: 07 October 2024 12:55 > > Am 10/3/2024 um 3:23 PM schrieb Mathieu Desnoyers: > > What _does_ work however are the following two approaches: > > > > 1) Perform the equality check on the original variables, creating > > new versions (with OPTIMIZER_HIDE_VAR) of both variables for the > > rest of their use, therefore making sure the pointer dereference > > are not derived from versions of the variables which were compared > > with another pointer. (as suggested by Boqun) > > This should not be guaranteed to work, because right after the > comparison the compiler can do b=a, then it doesn't matter how much you > hide afterwards. > > However it might work if you escape the addresses of a and b first, in > which case the compiler will not do b=a anymore, but it might force the > compiler to put a and b on the stack, which has some performance impact. Nope, as pointed out last week, the compiler can move the 'a == b' check to before the OPTIMISER_HID_VAR() and then use the same register for both of them. > > 2) Perform the equality check on the versions resulting of hiding > > both variables, making sure those versions of the variables are > > not dereferenced afterwards. (as suggested by Linus) That (and other things) could usefully use: #define OPTIMISER_HIDE_VALUE(x) \ ({ __auto_type _x = x; OPTIMISER_HIDE_VAR(_x); _x; }) You'll almost certainly end up with a register-register move even if 'x' isn't used afterwards. The calling could just become: if (a == OPTIMISER_HIDE_VALUE(b) ... since it is likely that you only care about one of the pointers. (Actually isn't hiding one of them always enough?) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)