| Some attempts to come up with a case where gcc or clang optimize in a way not easily describable as a specific "do_anything()": - printf (or any other external call) before UB - both gcc & clang keep the printf. - write to atomic before UB - easy to reverse by writing the old value, the interim value needn't ever be visible. - write to atomic/volatile, spinlock, UB - cannot be optimized out as the loop may be infinite (even in C++ as atomic & volatile are exceptions to "no infinite loops allowed") - write to volatile before UB - both gcc and clang keep the write. - read from volatile before UB - gcc keeps the read, but clang removes it. This is the closest I've got, but it's quite far from something you'd actually encounter (and could be easily countered by expecting volatile accesses to potentially exit(), at which point removing them is incorrect) Now, granted, C doesn't guarantee that all UB time travel must be of the easily-reversed kind, but, seemingly, basically nothing would be lost if it were. |