<div dir="ltr"><div>I think inttoptr should drop noalias-ness of the input pointer because it may collide with existing optimizations.</div><div><br></div><div>int i = ptr2int(p) + ofs<br></div><div>int j = ptr2int(p') + ofs'</div><div>if (i == j) use(int2ptr(i));</div><div><br></div><div>Optimization can transform use(int2ptr(i)) inside the branch into use(int2ptr(j)) because i == j holds.</div><div>If int2ptr preserves noalias-ness of the input pointer, this transformation becomes invalid.</div><div>For example, if p was a normal pointer and p' was a noalias pointer, the replacement makes the program more undefined.</div><div>In general, it is hard to detect whether an integer variable is derived from a pointer or not; imagine that i and j were complex expressions.</div><div><br></div>In terms of correctness, it is okay for LLVM to define the inttoptr cast drop the noalias information because it makes the program more defined.<div>noalias (and restrict as well) guarantees disjointness of memory accesses by defining its violation as undefined behavior. Removing noalias is thus okay.</div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Feb 19, 2021 at 12:54 PM Ryan Taylor <<a href="mailto:ryta1203@gmail.com" target="_blank">ryta1203@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div dir="ltr">Juneyoung,<div><br></div><div> It seems from the spec the intention (in best practice) is to allow restrict to follow through to any pointers based on the original restrict pointer providing </div><div> that the new pointer is based on the original restrict pointer and only the original restrict pointer (ie, no other pointers), so on and so forth (ie propagated). </div><div> So in the example, the original pointer's restrict qualifier, in this case, should follow through the ptr2int->add->int2ptr to the new pointer. </div><div><br></div><div> Yes, currently (and with the new patches) this doesn't work, as int2ptr assumes capture, which prevents noalias, but should that be correct? It doesn't seem</div><div> like it from the spec.</div><div><br></div><div>Thanks.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Feb 18, 2021 at 10:37 PM Juneyoung Lee <<a href="mailto:juneyoung.lee@sf.snu.ac.kr" target="_blank">juneyoung.lee@sf.snu.ac.kr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr">Hi,<div><br></div><div>I think what Jeroen says is about the behavior of C programs.</div><div>In C, an expression can be restrict-qualified, which isn't the case in LLVM IR; there is no 'i8* restrict' type or something like that.</div><div>I guess noalias intrinsics are inserted to preserve the information when translating restrict pointers in C to IR.</div><div><br></div><div>To summarize, the return value of inttoptr (which is an IR instruction) itself won't work as a restrict-qualified pointer because there is no restrict qualifier in IR.</div><div>If it is somehow used by the noalias intrinsics, it should be able to gain the power to work as a restrict pointer.</div><div><br></div><div><br></div><div><div>> x = (const float *)((int)x+off2))</div><div>></div><div>> I'm not sure why this should be capturing the pointer? </div></div><div><br></div><div>The reason is that (int)x + off2 may accidentally point into a different object.</div><div>A possible workaround is to use (const float*)((const char*)x+off2), which is guaranteed to preserve the provenance.</div><div>clang-tidy has an option to detect integer-to-pointer casts to warn about possible performance degradation: <a href="https://clang.llvm.org/extra/clang-tidy/checks/performance-no-int-to-ptr.html" target="_blank">https://clang.llvm.org/extra/clang-tidy/checks/performance-no-int-to-ptr.html</a><br></div><div><br></div><div>Juneyoung</div></div></div></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Feb 19, 2021 at 4:00 AM Ryan Taylor <<a href="mailto:ryta1203@gmail.com" target="_blank">ryta1203@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div dir="ltr">Yes, so if you have an int2ptr->add+(non-pointer)->ptr2int that should retain restrict qualifier, but you are saying it doesn't, or that the new patches won't support that, correct?<div><br></div><div>Thanks.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Feb 18, 2021 at 1:22 PM Jeroen Dobbelaere <<a href="mailto:Jeroen.Dobbelaere@synopsys.com" target="_blank">Jeroen.Dobbelaere@synopsys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">
<div lang="EN-US">
<div>
<p class="MsoNormal">> Seems like as long as the pointer is based on the restrict pointer (and no other pointer)and follows the constraints, it to is restrict qualified?<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">as long as the pointer expression is based on a restrict pointer... its accesses are associated to that restrict pointer (and assumed to not alias with other restrict pointers in the same scope).<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>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><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 19:11<br>
<b>To:</b> Jeroen Dobbelaere <<a href="mailto:dobbel@synopsys.com" target="_blank">dobbel@synopsys.com</a>><br>
<b>Cc:</b> Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com" target="_blank">johannesdoerfert@gmail.com</a>>; llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</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"><u></u> <u></u></p>
<div>
<p class="MsoNormal">Ok, just clarifying. <u></u><u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Am I interrupting 6.7.3.1.4 incorrectly? <u></u><u></u></p>
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<div>
<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal">Seems like as long as the pointer is based on the restrict pointer (and no other pointer)and follows the constraints, it to is restrict qualified?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"> During each execution of B, let L be any lvalue that has &L based on P. If L is used to access the value of the object X that it designates, and X is also modified (by any means), then the following requirements apply: T shall not be
const-qualified. Every other lvalue used to access the value of X shall also have its address based on P. Every access that modifies X shall be considered also to modify P, for the purposes of this subclause.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal">Thanks. <u></u><u></u></p>
</div>
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<div>
<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal">On Thu, Feb 18, 2021 at 12:59 PM Jeroen Dobbelaere <<a href="mailto:Jeroen.Dobbelaere@synopsys.com" target="_blank">Jeroen.Dobbelaere@synopsys.com</a>> wrote:<u></u><u></u></p>
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<div>
<div>
<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?<u></u><u></u></p>
<p class="MsoNormal">yes.<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">int * restrict x = ...;<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">bad usage:<u></u><u></u></p>
<p class="MsoNormal"> *(int*)((int)x + 1) = 42;<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">valid usage:<u></u><u></u></p>
<p class="MsoNormal"> x = (int*)((int)x + 1);<u></u><u></u></p>
<p class="MsoNormal"> *x = 42;<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>
<p class="MsoNormal"> <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 18:51<br>
<b>To:</b> Jeroen Dobbelaere <<a href="mailto:dobbel@synopsys.com" target="_blank">dobbel@synopsys.com</a>><br>
<b>Cc:</b> Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com" target="_blank">johannesdoerfert@gmail.com</a>>; llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</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>
</div>
</div>
<p class="MsoNormal"> <u></u><u></u></p>
<div>
<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?<u></u><u></u></p>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<div>
<p class="MsoNormal">Thanks.<u></u><u></u></p>
</div>
</div>
</div>
<p class="MsoNormal"> <u></u><u></u></p>
<div>
<div>
<p class="MsoNormal">On Thu, Feb 18, 2021 at 12:41 PM Jeroen Dobbelaere <<a href="mailto:Jeroen.Dobbelaere@synopsys.com" target="_blank">Jeroen.Dobbelaere@synopsys.com</a>> wrote:<u></u><u></u></p>
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<div>
<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>
<p class="MsoNormal"> <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"> <u></u><u></u></p>
<div>
<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>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<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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<div>
<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal">Thanks.<u></u><u></u></p>
<div>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<p class="MsoNormal"> <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>
<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><u></u><u></u></p>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr"><div dir="ltr"><div><br></div><font size="1">Juneyoung Lee</font><div><font size="1">Software Foundation Lab, Seoul National University</font></div></div></div>
</blockquote></div>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr"><div dir="ltr"><div><br></div><font size="1">Juneyoung Lee</font><div><font size="1">Software Foundation Lab, Seoul National University</font></div></div></div>