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

[Jeroen Dobbelaere via llvm-dev]

> 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>
Sent: Thursday, February 18, 2021 18:31
To: Johannes Doerfert <johannesdoerfert at gmail.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Jeroen Dobbelaere <dobbel at synopsys.com>; Juneyoung Lee <juneyoung.lee at sf.snu.ac.kr>; Philip Reames <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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