[Bug middle-end/114700] middle-end optimization generates causes -fsanitize=undefined not to happen in some cases

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

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

--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Hu Lin from comment #17)
> (In reply to Jakub Jelinek from comment #16)
> > (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.
> 
> OK, that look like GCC's -ftrapv is not the same as clang's. Then my added
> condition should be (optimize || !TYPE_OVERFLOW_SANITIZED (type)). 

Why?  Just !TYPE_OVERFLOW_SANITIZED (type).

> > When a pattern already has one if, can't you just add that to the preexisting if rather than adding yet another one.
> 
> I made a mistake on this line, it should be
> +   (if (!TYPE_OVERFLOW_SANITIZED (type))
>      (if (!ANY_INTEGRAL_TYPE_P (type)
>          || TYPE_OVERFLOW_WRAPS (type))
>       (negate (view_convert @1))
>       (view_convert (negate @1))))
> 
> I can't just modify the preexisting if, the optimization shouldn't be used
> with -fsanitize=undefined.

TYPE_OVERFLOW_SANITIZED is
#define TYPE_OVERFLOW_SANITIZED(TYPE)                   \
  (INTEGRAL_TYPE_P (TYPE)                               \
   && !TYPE_OVERFLOW_WRAPS (TYPE)                       \
   && (flag_sanitize & SANITIZE_SI_OVERFLOW))
so, it isn't true for non-integral types, nor for TYPE_OVERFLOW_WRAPS types.
So, if you want to avoid the (view_convert (negate @1)), just add (if
!TYPE_OVERFLOW_SANITIZED (type)) above the (view_convert (negate @1)).  But in
each case, you want to be careful which exact type you want to check, type is
the type of
the outermost expression, otherwise TREE_TYPE (@0) etc.