2011/6/17 Zdenek Dvorak <rakd...@kam.mff.cuni.cz>:
> Hi,
>
>> >> > Index: tree-vrp.c
>> >> > ===================================================================
>> >> > --- tree-vrp.c (revision 173703)
>> >> > +++ tree-vrp.c (working copy)
>> >> > @@ -2273,7 +2273,12 @@ extract_range_from_binary_expr (value_ra
>> >> > {
>> >> > /* For pointer types, we are really only interested in
>> >> > asserting
>> >> > whether the expression evaluates to non-NULL. */
>> >> > - if (range_is_nonnull (&vr0) || range_is_nonnull (&vr1))
>> >> > + if (flag_delete_null_pointer_checks && nowrap_type_p
>> >> > (expr_type))
>> >>
>> >> the latter would always return true
>> >>
>> >> Btw, I guess you'll "miscompile" a load of code that is strictly
>> >> undefined. So I'm not sure we want to do this against our users ...
>> >
>> > Probably not, at least unless the user explicitly asks for it -- for
>> > example,
>> > we could have -fdelete-null-pointer-checks=2. In fact, it might be a good
>> > idea
>> > to implement this flag anyway, since some current uses of
>> > flag_delete_null_pointer_checks
>> > can lead to "miscompilations" when user makes an error in their code and
>> > would
>> > probably appreciate more having their program crash.
>> >
>> >> Oh, and of course it's even wrong. I thing it needs &&
>> >> !range_includes_zero (&vr1) (which we probably don't have). The
>> >> offset may be 0 and NULL + 0
>> >> is still NULL.
>> >
>> > actually, the result of NULL + 0 is undefined (pointer arithmetics is only
>> > defined
>> > for pointers to actual objects, and NULL cannot point to one).
>>
>> It's maybe undefined in C, but is it undefined in the middle-end? Thus,
>> are you sure we never generate it from (void *)((uintptr_t)p + obfuscated_0)?
>> I'm sure we simply fold that to p + obfuscated_0.
>
> if we do, we definitely should not -- the only point of such a construction is
> to bypass the pointer arithmetics restrictions,
Ok, we don't. Where does the C standard say that NULL + 0 is undefined?
Richard.
> Zdenek
>
