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

["lin1.hu at intel dot com" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #19 from Hu Lin <lin1.hu at intel dot com> ---
(In reply to Jakub Jelinek from comment #18)
> (In reply to Hu Lin from comment #17)
> > (In reply to Jakub Jelinek from comment #16)
> > > 
> > > 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).
> 

OK, so the part is one of your suggestions on how to test UB in a program. 
I have another question, -fsanitize=undefined disable this optimization, but
you said -ftrapv won't diagnose the trap. Why is the logic here different for
these two options？

> 
> 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.

Thanks for your advice.