pointer comparison in GIMPLE

[Martin Uecker]

Am Donnerstag, dem 11.12.2025 um 04:12 +0000 schrieb Krister Walfridsson via Gcc:
> On Wed, 10 Dec 2025, Richard Biener wrote:
> 
> > > The problem is that in GIMPLE, a pointer does not need to be in bounds. The caller could call the function with a value of i such that p + i happens to be equal to &a. So, as I understand it, the GIMPLE semantics do not allow the pass to conclude that p + i == &a is false, unless p + i is dereferenced (because dereferencing a through p + i would be UB due to provenance).
> > 
> > GIMPLE adopts most of the C pointer restrictions here thus we can (and do) conclude that pointers stay within an object when advanced.  This is used by the PTA pass which results are used when we optimize your example.  You have to divert to integer arithmetic to circumvent this and the PTA pass, while tracking provenance through integers as well, does the right thing with this.
> 
> Great, that is much better for smtgcc than the semantics I have currently 
> implemented!
> 
> But it is not completely clear to me what "most of the C pointer 
> restrictions" implies. Is the following a correct interpretation?
> 
> 1. A pointer must contain a value that points into (or one past) an object 
> corresponding to its provenance (where a pointer may have multiple 
> provenances). Otherwise it invokes undefined behavior.
> 
> 2. The provenance used for the result of POINTER_PLUS is the union of the 
> provenances for the two arguments.

Note that in C one argument would be an integer and there is no
provenance on integers in C as this can not work consistently.

(and I think GCC gets this wrong)

Martin


> 
> 3. The POINTER_PLUS operation is UB if the calculation overflows and 
> TYPE_OVERFLOW_WRAPS(ptr_type) is false.
> 
> 4. The rules are the same for the calculations done in MEM_REF and 
> TARGET_MEM_REF as for POINTER_PLUS.
> 
> Question: For the TARGET_MEM_REF calculation:
>    BASE + STEP * INDEX + INDEX2 + OFFSET
> Is it treated as one POINTER_PLUS, i.e.
>    BASE + (STEP * INDEX + INDEX2 + OFFSET)
> or as two (i.e. do we care about overflow and OOB between the two index 
> calculations)?
> 
> 
> FWIW, the vectorizer and ivopts do introduce pointers that are outside the 
> object (which is why I allowed it in my current semantics)...
> 
>     /Krister