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<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Well, address 0 is pretty well aligned for whatever alignment we like.<br>
<br>
So I understand, that “load volatile i32, i32 addrspace(1)* null” is a valid memory access, accessing address 0?<o:p></o:p></span></a></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">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Guy</span><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"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> David Majnemer [mailto:david.majnemer@gmail.com]
<br>
<b>Sent:</b> Tuesday, June 09, 2015 22:55<br>
<b>To:</b> Benyei, Guy<br>
<b>Cc:</b> llvmdev@cs.uiuc.edu<br>
<b>Subject:</b> Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?<o:p></o:p></span></p>
<p class="MsoNormal"><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 Tue, Jun 9, 2015 at 12:32 PM, David Majnemer <<a href="mailto:david.majnemer@gmail.com" target="_blank">david.majnemer@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">'load volatile i32 addrspace(1)* null' seems fine to me. However, it looks like instcombine will turn:<o:p></o:p></p>
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<p class="MsoNormal">define i32 @foo() {<o:p></o:p></p>
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<p class="MsoNormal">entry:<o:p></o:p></p>
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<p class="MsoNormal"> %std_ld.i = load volatile i32, i32 addrspace(1)* null<o:p></o:p></p>
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<p class="MsoNormal"> ret i32 %std_ld.i<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">into:<o:p></o:p></p>
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<p class="MsoNormal">define i32 @foo() {<o:p></o:p></p>
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<p class="MsoNormal">entry:<o:p></o:p></p>
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<p class="MsoNormal"> %std_ld.i = load volatile i32, i32 addrspace(1)* null, align 536870912<o:p></o:p></p>
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<p class="MsoNormal"> ret i32 %std_ld.i<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">which is not ok.<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 second thought, I think that high alignment is benign. It simply indicates that your backend is free to load the value as-if it were aligned to an arbitrary boundary (because it is 1 byte, 2 byte, 4 byte, etc. aligned).<o:p></o:p></p>
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<p class="MsoNormal"> <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 Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <<a href="mailto:guy.benyei@intel.com" target="_blank">guy.benyei@intel.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"><a name="14dd9cf34fd0f69f_14dd99a8b3023cdb__MailE"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks David,</span></a><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">It turns out, that the address space I was using was not 0, and yet the pointer was constant folded
to null.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Here is the sequence:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Unoptimized code:</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">define i32 @foo() #0 {</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">entry:</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %address.addr.i = alloca i32, align 4</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %value.i = alloca i32, align 4</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> store i32 0, i32* %address.addr.i, align 4</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %0 = load i32* %address.addr.i, align 4</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %1 = inttoptr i32 %0 to i32 addrspace(1)*</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %std_ld.i = load volatile i32 addrspace(1)* %1</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> store i32 %std_ld.i, i32* %value.i, align 4</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %2 = load i32* %value.i, align 4</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> ret i32 %2</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">}</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">After optimization (early CSE):</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">define i32 @foo() #0 {</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">entry:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %std_ld.i = load volatile i32 addrspace(1)* null, align 536870912</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> ret i32 %std_ld.i</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">}</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The contant folder doesn’t seem to check for address space, it simply checks if the integer in question
is zero, and folds the inttoptr to null:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white"> *llvm::ConstantFoldCastInstruction(</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white">unsigned</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">opc</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">,
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white"> *</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">V</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white">Type</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white"> *</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">)
{</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white">...</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white">if</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white"> (</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">V</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">->isNullValue()
&& !</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">->isX86_MMXTy())</span><o:p></o:p></p>
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<span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white">return</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">::getNullValue(</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">);</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:black;background:white">...</span><o:p></o:p></p>
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<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in">
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Is this a bug?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;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="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Guy</span><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="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> David Majnemer [mailto:<a href="mailto:david.majnemer@gmail.com" target="_blank">david.majnemer@gmail.com</a>]
<br>
<b>Sent:</b> Tuesday, June 09, 2015 18:45<br>
<b>To:</b> Benyei, Guy<br>
<b>Cc:</b> <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?</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>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Tue, Jun 9, 2015 at 5:57 AM, Benyei, Guy <<a href="mailto:guy.benyei@intel.com" target="_blank">guy.benyei@intel.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">Hello,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">It seems that ConstantFoldCastInstruction in ConstantFold.cpp folds inttoptr instruction with 0 as operand to a null pointer. It makes sense, when talking about a C-style frontend,
as the C99 spec (6.3.2.3) states:<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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“An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.”<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">On the other hand, some architectures use 0 as a valid memory location, and this constant folding seems to be possibly harmful when the code actually tries to access the memory
location at address 0.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Is this behavior intentional? Do I miss something? Will a load from address null try to access address 0, or may it become an undef value?<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">LLVM assumes that the null pointer in address space zero can never be successfully dereferenced. You must utilize some other address space to dereference a null 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"> <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"> Guy<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"> <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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