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

[Ryan Taylor via llvm-dev]

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> 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.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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