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<p class="MsoNormal">> 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?<o:p></o:p></p>
<p class="MsoNormal">yes.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">int * restrict x = ...;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">bad usage:<o:p></o:p></p>
<p class="MsoNormal"> *(int*)((int)x + 1) = 42;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">valid usage:<o:p></o:p></p>
<p class="MsoNormal"> x = (int*)((int)x + 1);<o:p></o:p></p>
<p class="MsoNormal"> *x = 42;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Greetings,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Jeroen Dobbelaere<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Ryan Taylor <ryta1203@gmail.com> <br>
<b>Sent:</b> Thursday, February 18, 2021 18:51<br>
<b>To:</b> Jeroen Dobbelaere <dobbel@synopsys.com><br>
<b>Cc:</b> Johannes Doerfert <johannesdoerfert@gmail.com>; llvm-dev <llvm-dev@lists.llvm.org>; Juneyoung Lee <juneyoung.lee@sf.snu.ac.kr>; Philip Reames <listmail@philipreames.com><br>
<b>Subject:</b> Re: [llvm-dev] inttoptr->add->ptrtoint capturing pointer?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Feb 18, 2021 at 12:41 PM Jeroen Dobbelaere <<a href="mailto:Jeroen.Dobbelaere@synopsys.com">Jeroen.Dobbelaere@synopsys.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">> How will inttoptr work with the new restrict patches? Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The full restrict patches see through 'inttoptr(ptrtoint( x ) )', Besides that, the analysis stops at 'ptr2int(x)' and 'anything can happen'.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Because of this, all other 'inttoptr' usages will never introduce a restrict provenance.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">(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)<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Greetings,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Jeroen Dobbelaere<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Juneyoung said he hadn't started working on it yet, so I'm going to take a look at it also.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">How will inttoptr work with the new restrict patches? Certainly the int2ptr capturing shouldn't nullify the restrict qualifier.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Thu, Feb 18, 2021 at 12:04 PM Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com" target="_blank">johannesdoerfert@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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>
<br>
I've CC'ed people that might correct me or augment my answer, hope this <br>
helps :)<br>
<br>
~ Johannes<br>
<br>
<br>
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>
> LLVM Developers mailing list<br>
> <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
> <a href="https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!Pm5BQtdh_gU6pe-WvhApIs2FOjI1V7vJDj6H93m0sNUItsa5T6CbzW5JL1cixruSF_kY7ywW$" target="_blank">
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></p>
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