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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">+llvm-dev<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Yes. The question then becomes, does LLVM handle the case where the cast of the value 0 to a non-void* pointer?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Think of this case:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">int *<b>ptr = (int</b>*)0;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">based on a strict reading of the spec, ptr itself technically is not the null pointer constant. If ptr points to an object in the local address space in OpenCL,
or any address space where 0 is a valid address and memory exists, does LLVM handle this correctly?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I don’t believe that it does, but I can’t say for certain.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Micah<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Michael LIAO [mailto:michael.hliao@gmail.com]
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<b>Sent:</b> Sunday, November 17, 2013 10:46 PM<br>
<b>To:</b> Micah Villmow<br>
<b>Subject:</b> Re: [LLVMdev] Null pointers with a non-0 representation<o:p></o:p></span></p>
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<p class="MsoNormal">Hi Micah & Michael<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">By C standard, section 6.3.2.2, "An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer,
called a null pointer, is guaranteed to compare unequal to a pointer to any object or function." As LLVM IR is modelled after C, I assume null pointer in LLVM IR is equivalent to 0 as well. The folding from 0 to null pointer in LLVM follows that behaviour
very similar.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">- michael<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal">On Sun, Nov 17, 2013 at 1:00 PM, Micah Villmow <<a href="mailto:micah.villmow@smachines.com" target="_blank">micah.villmow@smachines.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"><span style="color:#1F497D">The reason why I suggested looking there is because the same questions came up as address 0 in some of the OpenCL address spaces are legal addresses.
So in order to solve this problem, they would have to find answers to your questions, mainly your ‘c)’ question. </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">I took a look at LangRef and it does look like there is a ‘null’ representation of the null pointer constant, which wasn’t part of LLVM back when I worked
on this problem. So maybe you are just running into an issue where the code hasn’t been updated yet because it works on everyones platform but yours.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"><br>
Micah</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Kuperstein, Michael M [mailto:<a href="mailto:michael.m.kuperstein@intel.com" target="_blank">michael.m.kuperstein@intel.com</a>]
<br>
<b>Sent:</b> Sunday, November 17, 2013 3:28 AM<br>
<b>To:</b> Micah Villmow; <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a></span><o:p></o:p></p>
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<p class="MsoNormal"><br>
<b>Subject:</b> RE: Null pointers with a non-0 representation<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>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Hi Micah,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Thanks a lot for the reference.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Unfortunately, it looks like the discussion there is dealing with a somewhat different issue – modeling OpenCL address spaces. It’s true that the null
pointer issue may arise in a similar context (e.g. null pointers in different address spaces may have different internal representations), but it’s not entirely related. Or have I missed something in the thread?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Thanks,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> Michael</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Micah Villmow [<a href="mailto:micah.villmow@smachines.com" target="_blank">mailto:micah.villmow@smachines.com</a>]
<br>
<b>Sent:</b> Sunday, November 17, 2013 11:42<br>
<b>To:</b> Kuperstein, Michael M; <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">
LLVMdev@cs.uiuc.edu</a><br>
<b>Subject:</b> RE: Null pointers with a non-0 representation</span><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>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">I ran into the same problem in OpenCL with Local/Private address spaces and I believe it was discussed here in the past. So I would suggest looking through
the archives as I believe some of your questions are answered there.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">This post might be a good starting point:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a href="http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/044101.html" target="_blank">http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/044101.html</a><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"><span style="color:#1F497D">Also the SPIR provisional spec as of last November deals with this issue if I remember correctly, so maybe you can discuss with Boaz Ouriel if he is
still at Intel Israel about how they went about handling this problem, or look at the SPIR related posts from last year.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Hope this helps,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Micah</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a> [<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">mailto:llvmdev-bounces@cs.uiuc.edu</a>]
<b>On Behalf Of </b>Kuperstein, Michael M<br>
<b>Sent:</b> Sunday, November 17, 2013 1:15 AM<br>
<b>To:</b> <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a><br>
<b>Subject:</b> [LLVMdev] Null pointers with a non-0 representation</span><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>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I feel a bit silly asking this, but here goes.<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 C spec does not require the null pointer to be implemented as an integer with the 0 bit-pattern. The implementation may arbitrarily choose any other (integer, but not necessarily)
representation, as long as it’s distinct from any legal pointer. The only requirement is that casting an integer 0 to a pointer type results in a null pointer. <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">LLVM IR appears to have similar the casting behavior. That is, if I try to create a pointer with integer value 0 (e.g. by constructing an inttoptr constant expression with a i32
0 argument), it gets immediately folded into a null pointer constant. Furthermore, a PtrToInt from a null pointer results in 0 (which is even “stricter” than the C spec.)
<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">So, that raises a few question:<o:p></o:p></p>
<p>a)<span style="font-size:7.0pt"> </span>Are those the desired semantics? I guess if clang generates this kind of IntToPtr instructions (as opposed to directly resolving the cast to a null pointer) this is unavoidable. But otherwise, why does LLVM treat
null pointers and pointers with the integer value 0 as equivalent? <br>
It’s completely natural for most platforms, but isn’t necessarily the right thing to do.<o:p></o:p></p>
<p>b)<span style="font-size:7.0pt"> </span>Assuming this is really desirable, should the fact that casting an integer 0 to a pointer type results in a null pointer be documented in the langref (as part of the inttoptr/ptrtoint documentation)? Or is it
too “low-level”?<o:p></o:p></p>
<p>c)<span style="font-size:7.0pt"> </span>Let’s say I want to create a pointer with the numeric value 0, distinct from the null pointer, because 0 really does represent a valid pointer on my platform. Should this possible at the IR level? If it should
be, then how?<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">Anyone has any inputs, except laughing me out of the (virtual) room? :-)<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">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Michael<o:p></o:p></p>
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