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<p class="MsoNormal">Jie,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Do you actually want a fat-pointer specifically, or do you just want an efficient way to associate metadata with C pointers? Because (as I‘m sure you know) fat pointers have serious compatibility problems with external libraries, and also
may break C programs in other ways due to lack of sound type information.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">John Criswell (copied) had created an improved version of Baggy Bounds which gives a efficient and compatible solution at low memory overhead. I suggest contacting him if you’re interested.<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">—Vikram Adve<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">+ Donald B. Gillies Professor of Computer Science, University of Illinois at Urbana-Champaign<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">+ Admin: Kimberly Baker – </span><u><span style="color:#0563C1"><a href="mailto:kabaker@illinois.edu"><span style="color:#0563C1">kabaker@illinois.edu</span></a></span></u><o:p></o:p></p>
<p class="MsoNormal"><span style="color:black">+ Skype: vikramadve || Zoom: <a href="https://zoom.us/j/2173900467">
<span style="color:#0563C1">https://zoom.us/j/2173900467</span></a><br>
+ Home page: <a href="http://vikram.cs.illinois.edu"><span style="color:#0563C1">http://vikram.cs.illinois.edu</span></a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">+ Center for Digital Agriculture: <a href="https://digitalag.illinois.edu/"><span style="color:#0563C1">https://digitalag.illinois.edu</span></a></span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Reply-To: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org>, "llvm-dev-request@lists.llvm.org" <llvm-dev-request@lists.llvm.org><br>
<b>Date: </b>Tuesday, January 7, 2020 at 8:06 AM<br>
<b>To: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org><br>
<b>Subject: </b>llvm-dev Digest, Vol 187, Issue 17<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Date: Tue, 7 Jan 2020 02:45:01 +0000<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">From: Jie Zhou via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">To: Eli Friedman via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Subject: [llvm-dev] Best way of implement a fat pointer for C<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Message-ID: <<a href="mailto:B6510CF7-CC75-4171-AF50-D6730EB068E8@cs.rochester.edu">B6510CF7-CC75-4171-AF50-D6730EB068E8@cs.rochester.edu</a>><o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Content-Type: text/plain; charset="utf-8"<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Dear All,<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">I’m working on a project that extends C. I’m adding a new type of pointer<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">that is a fat pointer. It has some metadata about the pointed object besides<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">the starting address of the object. Currently I implemented this pointer as<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">an llvm:StructType. In llvm::Type generation function<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">llvm::Type *CodeGenTypes::ConvertType(QualType T)<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">in the case for clang::Type::Pointer, instead of creating an llvm::PointerType<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">I create an llvm::StructType type for this new type of pointer. And I added some<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">helper code in llvm::StructType and in multiple places I added code to trick<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">the compiler to believe sometimes a struct is actually a pointer. Until now<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">it compile test programs fine with -O0 but I got lots of assertion failures when<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">compiling with -O1 or -O2 majorly because of the confusion of type mismatch.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">LLVM assumes that a PointerType is essentially an Integer (32 or 64 bit depending<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">on the architecture), and since this is quite a fundamental assumption, I started<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">to question whether my way of implementing the fat pointer is feasible.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">I thought about adding a new llvm type that inherits both llvm:PointerType<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">and llvm:StructType; but I’m not sure if this is the correct path. It looks like<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">this demands substantial changes to the compiler including adding code<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">for bitcode generation. Can you give me some advice on how to implement<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">a fat pointer in llvm?<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">Thanks,<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:"-webkit-standard",serif;color:black">- Jie<o:p></o:p></span></p>
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