Re: Fold pointer range checks with equal spans

[Marc Glisse <marc.glisse@inria.fr>]

On Tue, 31 Jul 2018, Richard Biener wrote:

>>> Also, when @2 == @0 + (@1+1) then the original condition is true but
>>> ((sizetype) @0 - (sizetype) @2 + @1) > (@1 * 2) is not?
>>>    (sizetype) @0 - (sizetype) (@0 + @1 + 1) + @1 > @1 * 2
>>> -> -1 > @1 * 2
>>>
>>> which is false.  So I can't really see how you can apply this transform in
>>> general (it would be fine for generating alias checks but a bit more
>>> pessimistic).
>>>
>>> But maybe I am missing something?
>>
>> It relies on sizetype being unsigned: (sizetype)-1 > @1 * 2 is true.
>
> Hmm, so mathematically this is
>
>  (@0 - @2) % modreduce + @1 > @1 * 2
>
> then, but I don't want to think too much about this since Marc didn't
> object here ;)

We already transform abs(x)<=3 into (unsigned)x+3u<=6u, that's the usual
way we do range checking so I didn't pay much attention to that part.
(tempted to say: "I didn't want to think too much about this since
Richard was going to do it anyway" ;-)

Turning multiple comparisons of the form P + cst CMP Q + cst into a
range check on P - Q sounds good (we don't really have to restrict to
the case where the range is symmetric). Actually, just turning P + cst
CMP Q + cst into P - Q CMP cst should do it, we should already have code
to handle range checking on integers (modulo the issue of CSE P-Q and
Q-P). But I don't know if a couple :s is sufficient to make this
transformation a good canonicalization.

If we start from a comparison of pointer_plus, I think it would make
sense to use pointer_diff.

I believe currently we try to use pointer operations (pointer_plus,
pointer_diff, lt) only for related pointers and we cast to some integer
type for wilder cases (implementation of std::less in C++ for instance).
On the other hand, in an alias check, the 2 pointers are possibly
unrelated, so maybe the code emitted for an alias check should be
changed.

-- 
Marc Glisse