2011/6/17 Zdenek Dvorak <rakd...@kam.mff.cuni.cz>:
>> >> Interesting. I'd never thought about the generation/use angle to prove
>> >> a pointer was non-null. ISTM we could use that same logic to infer that
>> >> more pointers are non-null in extract_range_from_binary_expr.
>> >>
>> >> Interested in tackling that improvement, obviously as an independent
>> >> patch?
>> >>
>> >
>> > I'm not familiar with vrp code, but.. something like this?
>> >
>> > 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.
Richard.
> Zdenek
>
