[llvm-dev] inttoptr->add->ptrtoint capturing pointer?

[Jeroen Dobbelaere via llvm-dev]

> So if some restrict pointer 'x' is casted to int, adds 1, then casted back to pointer, it nullifies the restrict qualifier, despite the results having no other uses?
yes.

int * restrict x = ...;

bad usage:
    *(int*)((int)x + 1) = 42;

valid usage:
    x = (int*)((int)x + 1);
   *x = 42;

Greetings,

Jeroen Dobbelaere

From: Ryan Taylor <ryta1203 at gmail.com>
Sent: Thursday, February 18, 2021 18:51
To: Jeroen Dobbelaere <dobbel at synopsys.com>
Cc: Johannes Doerfert <johannesdoerfert at gmail.com>; llvm-dev <llvm-dev at lists.llvm.org>; Juneyoung Lee <juneyoung.lee at sf.snu.ac.kr>; Philip Reames <listmail at philipreames.com>
Subject: Re: [llvm-dev] inttoptr->add->ptrtoint capturing pointer?

So if some restrict pointer 'x' is casted to int, adds 1, then casted back to pointer, it nullifies the restrict qualifier, despite the results having no other uses?

Thanks.

On Thu, Feb 18, 2021 at 12:41 PM Jeroen Dobbelaere <Jeroen.Dobbelaere at synopsys.com<mailto:Jeroen.Dobbelaere at synopsys.com>> wrote:
> How will inttoptr work with the new restrict patches?  Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.

The full restrict patches see through 'inttoptr(ptrtoint( x ) )', Besides that, the analysis stops at 'ptr2int(x)' and 'anything can happen'.
Because of this, all other 'inttoptr' usages will never introduce a restrict provenance.

(aka, a restrict pointer converted to an int + some computations and then converted back to a pointer will normally not retain the 'restrictness' and that can trigger undefined behavior)

Greetings,

Jeroen Dobbelaere

From: Ryan Taylor <ryta1203 at gmail.com<mailto:ryta1203 at gmail.com>>
Sent: Thursday, February 18, 2021 18:31
To: Johannes Doerfert <johannesdoerfert at gmail.com<mailto:johannesdoerfert at gmail.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Jeroen Dobbelaere <dobbel at synopsys.com<mailto:dobbel at synopsys.com>>; Juneyoung Lee <juneyoung.lee at sf.snu.ac.kr<mailto:juneyoung.lee at sf.snu.ac.kr>>; Philip Reames <listmail at philipreames.com<mailto:listmail at philipreames.com>>
Subject: Re: [llvm-dev] inttoptr->add->ptrtoint capturing pointer?

Juneyoung said he hadn't started working on it yet, so I'm going to take a look at it also.

How will inttoptr work with the new restrict patches?  Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.

Thanks.





On Thu, Feb 18, 2021 at 12:04 PM Johannes Doerfert <johannesdoerfert at gmail.com<mailto:johannesdoerfert at gmail.com>> wrote:
I think you are working with a custom llvm here but I will
make a few general statements that might help:

- The noalias intrinsic as you've shown it captures, unfortunately.
   We don't have the `nocapture_maybe_returned` attribute in IR yet that
the Attributor uses for these situations,
   IIRC, Juneyoung is working on making it an LLVM-IR enum attribute
already.

- int2ptr is assumed to capture in basically every analysis I've seen.
It doesn't intrinsically but it is really
   hard to get it right. That said, we could allow *very* special
patterns but I would argue those should be recognized
   in instcombine and replaced there.

- Philip and I are working to define capture better for the lang ref, we
might want to include some examples and
   rational around int2ptr when we have a coherent model.

I've CC'ed people that might correct me or augment my answer, hope this
helps :)

~ Johannes


On 2/18/21 9:17 AM, Ryan Taylor via llvm-dev wrote:
> I have an example:
>
> foo(float * restrict y, int off1, int off2) {
>    float * restrict x;
>    for (..) {
>      for (..) {
>        x = y+off1;
>      }
>      x = (const float *)((int)x+off2))
>
> I'm not sure why this should be capturing the pointer?
>
> For instance, looking at scoped noalias dbg info:
>
> SNA: Capture check for B/CSB UO:   %54 = inttoptr i32 %add83 to float*,
> !dbg !101
> SNA: Pointer   %1 = call float* @llvm.noalias.p0_float(float* %0, metadata
> !38), !dbg !41 might be captured!
>
> Is this implying that the noalias intrinsic might be capturing the pointer?
> It doesn't look like "noalias" intrinsic has NoCapture property on the
> pointer:
>
> def int_noalias       : Intrinsic<[llvm_anyptr_ty],
>                                    [LLVMMatchType<0>, llvm_metadata_ty],
>                                    [IntrArgMemOnly, Returned<0>]>;
>
> It should though right? From the definition of capture it is returning the
> pointer; however, we know nothing is happening here.
>
> I'm on llvm 10 with Hal's restrict patches.
>
> Thanks.
>
>
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