<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=us-ascii">
<meta name="Generator" content="Microsoft Word 12 (filtered medium)">
<style>
<!--
/* Font Definitions */
@font-face
{font-family:SimSun;
panose-1:2 1 6 0 3 1 1 1 1 1;}
@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:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
@font-face
{font-family:"\@SimSun";
panose-1:2 1 6 0 3 1 1 1 1 1;}
/* 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;}
span.EmailStyle17
{mso-style-type:personal-reply;
font-family:"Calibri","sans-serif";
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;}
@page Section1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.Section1
{page:Section1;}
-->
</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="Section1">
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">So fence only forces ordering of atomic instructions.<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">Let me change my question then.<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">If I have a target-specific intrinsic which forces ordering of ordinary load/store instructions. Then should it also force ordering of load/stores to noalias
pointers in caller functions?<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">Xiaoyi<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>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<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""> Eli Friedman [mailto:eli.friedman@gmail.com]
<br>
<b>Sent:</b> Wednesday, June 12, 2013 7:06 PM<br>
<b>To:</b> Guo, Xiaoyi<br>
<b>Cc:</b> LLVM Dev<br>
<b>Subject:</b> Re: [LLVMdev] A question w.r.t fence instruction vs. noalias pointer<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">On Wed, Jun 12, 2013 at 6:17 PM, Guo, Xiaoyi <<a href="mailto:Xiaoyi.Guo@amd.com" target="_blank">Xiaoyi.Guo@amd.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Hi,<br>
<br>
I have the following test case:<br>
<br>
define void @foo(<2 x float>* noalias nocapture %out, <2 x float>* noalias nocapture %data0) nounwind {<br>
entry:<br>
%val1 = load <2 x float>* %data0, align 8<br>
store <2 x float> %val1, <2 x float>* %out, align 8<br>
fence acq_rel<br>
%val2 = load <2 x float>* %data0, align 8<br>
store <2 x float> %val2, <2 x float>* %out, align 8<br>
ret void<br>
}<br>
<br>
If I run it though GVN with BasicAliasAnalysis, GVN does not remove the load after the fence.<o:p></o:p></p>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
According to the LLVM atomics rules, the compiler is actually completely free to either hoist the fence to the top of the function or sink it to the bottom because your loads/stores aren't atomic. The fact that LLVM doesn't actually do this is just because
I didn't put much effort into alias analysis for fences; see <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/AliasAnalysis.h?revision=182755&view=markup#l431">
http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/AliasAnalysis.h?revision=182755&view=markup#l431</a> .
<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">-Eli<o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</body>
</html>