pointer comparison in GIMPLE

[Krister Walfridsson]

On Wed, 10 Dec 2025, Martin Uecker wrote:

> Date: Wed, 10 Dec 2025 09:11:14 +0100
> From: Martin Uecker <muecker@gwdg.de>
> To: Krister Walfridsson <krister.walfridsson@gmail.com>, gcc@gcc.gnu.org
> Cc: Richard Biener <richard.guenther@gmail.com>
> Subject: Re: pointer comparison in GIMPLE
> 
> Am Mittwoch, dem 10.12.2025 um 03:51 +0000 schrieb Krister Walfridsson via Gcc:
>> I have a problem in smtgcc with the semantics of pointer comparison in
>> GIMPLE.
>>
>> Consider the function foo:
>>
>>    void foo(char *p, long long i)
>>    {
>>      char a;
>>      if (p + i == &a)
>>        __builtin_abort();
>>    }
>>
>> The FRE pass determines that the condition is always false, and the
>> if-statement is removed. This is correct in the C semantics, as the only
>> way the condition can be true invokes undefined behavior. But GIMPLE is
>> more permissive, so p + i may in fact evaluate to the address of a. So
>> something else is needed to allow this optimization for GIMPLE.
>>
>> This could easily be fixed by letting EQ_EXPR evaluate to false for
>> pointers with different provenance. But that makes other optimizations
>> invalid because then p > q, p == q, and p < q can all be false
>> simultaneously.
>>
>> So I guess I am missing something... Why is this optimization allowed on
>> GIMPLE?
>
> Are the precise GIMPLE semantics documented somewhere?   I would guess that
> some notion of provenance would be criticial also for GIMPLE.

It is, to my knowledge, not conveniently documented in one place, even 
though the information is probably available spread out across the 
internals manual, various .def files, and comments in the source code.

I plan to write some documentation when I have fully figured out the 
rules. The following two earlier mail threads and bug report are part of 
that effort:
* https://gcc.gnu.org/pipermail/gcc/2025-March/245763.html
* https://gcc.gnu.org/pipermail/gcc/2025-April/245870.html
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120980

    /Krister