<div dir="ltr">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?<div><div><br></div><div>Thanks.</div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Feb 18, 2021 at 12:41 PM Jeroen Dobbelaere <<a href="mailto:Jeroen.Dobbelaere@synopsys.com">Jeroen.Dobbelaere@synopsys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">> How will inttoptr work with the new restrict patches? Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">The full restrict patches see through 'inttoptr(ptrtoint( x ) )', Besides that, the analysis stops at 'ptr2int(x)' and 'anything can happen'.<u></u><u></u></p>
<p class="MsoNormal">Because of this, all other 'inttoptr' usages will never introduce a restrict provenance.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">(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)<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Greetings,<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Jeroen Dobbelaere<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b> Ryan Taylor <<a href="mailto:ryta1203@gmail.com" target="_blank">ryta1203@gmail.com</a>> <br>
<b>Sent:</b> Thursday, February 18, 2021 18:31<br>
<b>To:</b> Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com" target="_blank">johannesdoerfert@gmail.com</a>><br>
<b>Cc:</b> llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>>; Jeroen Dobbelaere <<a href="mailto:dobbel@synopsys.com" target="_blank">dobbel@synopsys.com</a>>; Juneyoung Lee <<a href="mailto:juneyoung.lee@sf.snu.ac.kr" target="_blank">juneyoung.lee@sf.snu.ac.kr</a>>; Philip Reames <<a href="mailto:listmail@philipreames.com" target="_blank">listmail@philipreames.com</a>><br>
<b>Subject:</b> Re: [llvm-dev] inttoptr->add->ptrtoint capturing pointer?<u></u><u></u></p>
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<p class="MsoNormal">Juneyoung said he hadn't started working on it yet, so I'm going to take a look at it also.<u></u><u></u></p>
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<p class="MsoNormal">How will inttoptr work with the new restrict patches? Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.<u></u><u></u></p>
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<p class="MsoNormal">Thanks.<u></u><u></u></p>
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<p class="MsoNormal">On Thu, Feb 18, 2021 at 12:04 PM Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com" target="_blank">johannesdoerfert@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">I think you are working with a custom llvm here but I will<br>
make a few general statements that might help:<br>
<br>
- The noalias intrinsic as you've shown it captures, unfortunately.<br>
We don't have the `nocapture_maybe_returned` attribute in IR yet that <br>
the Attributor uses for these situations,<br>
IIRC, Juneyoung is working on making it an LLVM-IR enum attribute <br>
already.<br>
<br>
- int2ptr is assumed to capture in basically every analysis I've seen. <br>
It doesn't intrinsically but it is really<br>
hard to get it right. That said, we could allow *very* special <br>
patterns but I would argue those should be recognized<br>
in instcombine and replaced there.<br>
<br>
- Philip and I are working to define capture better for the lang ref, we <br>
might want to include some examples and<br>
rational around int2ptr when we have a coherent model.<br>
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I've CC'ed people that might correct me or augment my answer, hope this <br>
helps :)<br>
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~ Johannes<br>
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On 2/18/21 9:17 AM, Ryan Taylor via llvm-dev wrote:<br>
> I have an example:<br>
><br>
> foo(float * restrict y, int off1, int off2) {<br>
> float * restrict x;<br>
> for (..) {<br>
> for (..) {<br>
> x = y+off1;<br>
> }<br>
> x = (const float *)((int)x+off2))<br>
><br>
> I'm not sure why this should be capturing the pointer?<br>
><br>
> For instance, looking at scoped noalias dbg info:<br>
><br>
> SNA: Capture check for B/CSB UO: %54 = inttoptr i32 %add83 to float*,<br>
> !dbg !101<br>
> SNA: Pointer %1 = call float* @llvm.noalias.p0_float(float* %0, metadata<br>
> !38), !dbg !41 might be captured!<br>
><br>
> Is this implying that the noalias intrinsic might be capturing the pointer?<br>
> It doesn't look like "noalias" intrinsic has NoCapture property on the<br>
> pointer:<br>
><br>
> def int_noalias : Intrinsic<[llvm_anyptr_ty],<br>
> [LLVMMatchType<0>, llvm_metadata_ty],<br>
> [IntrArgMemOnly, Returned<0>]>;<br>
><br>
> It should though right? From the definition of capture it is returning the<br>
> pointer; however, we know nothing is happening here.<br>
><br>
> I'm on llvm 10 with Hal's restrict patches.<br>
><br>
> Thanks.<br>
><br>
><br>
> _______________________________________________<br>
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