[Bug middle-end/114700] middle-end optimization generates wrong code with -fsanitize=undefined

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114700

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Hu Lin from comment #11)
> I think it doesn't mean that's not a bug with -ftrapv, it should preserve
> all overflow traps. Because it doesn't work, we use -fsanitize=undefined
> instead of it.
> 
> refer: Gcc's trapv is known not always to work correctly.

No, -ftrapv isn't a debugging tool.  There is no overflow in the expression
that GCC actually evaluates (into which the expression has been optimized).
If you have overflow in an expression that is never used, GCC with -ftrapv will
also
eliminate it as unused and won't diagnose the trap.
-fsanitize=undefined behaves in that case actually the same with -O1 and higher
(intentionally, to decrease the cost of the sanitization).  So, one needs to
use -O0 -fsanitize=undefined to get as many cases of UB in the program
diagnosed as possible.