<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p
{mso-style-priority:99;
mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.25in 1.0in 1.25in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks David,<o:p></o:p></span></a></p>
<p class="MsoNormal"><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.<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">Here is the sequence:<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" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Unoptimized code:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">define i32 @foo() #0 {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">entry:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %address.addr.i = alloca i32, align 4<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %value.i = alloca i32, align 4<o:p></o:p></span></p>
<p class="MsoNormal" style="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<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %0 = load i32* %address.addr.i, align 4<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %1 = inttoptr i32 %0 to i32 addrspace(1)*<o:p></o:p></span></p>
<p class="MsoNormal" style="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<o:p></o:p></span></p>
<p class="MsoNormal" style="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<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> %2 = load i32* %value.i, align 4<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> ret i32 %2<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">}<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">After optimization (early CSE):<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">define i32 @foo() #0 {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">entry:<o:p></o:p></span></p>
<p class="MsoNormal" style="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<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> ret i32 %std_ld.i<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><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"><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:<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" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white;mso-highlight:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
*llvm::ConstantFoldCastInstruction(</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white">unsigned</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">opc</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">,
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white;mso-highlight:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white"> *</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">V</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white;mso-highlight:white">Type</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white"> *</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">)
{<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white">...<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white">if</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white"> (</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">V</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">->isNullValue()
&& !</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">->isX86_MMXTy())<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue;background:white;mso-highlight:white">return</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF;background:white;mso-highlight:white">Constant</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">::getNullValue(</span><span style="font-size:9.5pt;font-family:Consolas;color:gray;background:white;mso-highlight:white">DestTy</span><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">);<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black;background:white;mso-highlight:white">...<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><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">Is this a bug?<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">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> Guy<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"><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 18:45<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>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">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>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<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>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:.5in;text-autospace:none">
“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>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">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>
</div>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<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>
</div>
<p>---------------------------------------------------------------------<br>
Intel Israel (74) Limited<o:p></o:p></p>
<p>This e-mail and any attachments may contain confidential material for<br>
the sole use of the intended recipient(s). Any review or distribution<br>
by others is strictly prohibited. If you are not the intended<br>
recipient, please contact the sender and delete all copies.<o:p></o:p></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu" target="_blank">http://llvm.cs.uiuc.edu</a><br>
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><o:p></o:p></p>
</blockquote>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
</div>
<p>---------------------------------------------------------------------<br>
Intel Israel (74) Limited</p>
<p>This e-mail and any attachments may contain confidential material for<br>
the sole use of the intended recipient(s). Any review or distribution<br>
by others is strictly prohibited. If you are not the intended<br>
recipient, please contact the sender and delete all copies.</p></body>
</html>