<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:x="urn:schemas-microsoft-com:office:excel" xmlns:p="urn:schemas-microsoft-com:office:powerpoint" xmlns:a="urn:schemas-microsoft-com:office:access" xmlns:dt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882" xmlns:s="uuid:BDC6E3F0-6DA3-11d1-A2A3-00AA00C14882" xmlns:rs="urn:schemas-microsoft-com:rowset" xmlns:z="#RowsetSchema" xmlns:b="urn:schemas-microsoft-com:office:publisher" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" xmlns:c="urn:schemas-microsoft-com:office:component:spreadsheet" xmlns:odc="urn:schemas-microsoft-com:office:odc" xmlns:oa="urn:schemas-microsoft-com:office:activation" xmlns:html="http://www.w3.org/TR/REC-html40" xmlns:q="http://schemas.xmlsoap.org/soap/envelope/" xmlns:rtc="http://microsoft.com/officenet/conferencing" xmlns:D="DAV:" xmlns:Repl="http://schemas.microsoft.com/repl/" xmlns:mt="http://schemas.microsoft.com/sharepoint/soap/meetings/" xmlns:x2="http://schemas.microsoft.com/office/excel/2003/xml" xmlns:ppda="http://www.passport.com/NameSpace.xsd" xmlns:ois="http://schemas.microsoft.com/sharepoint/soap/ois/" xmlns:dir="http://schemas.microsoft.com/sharepoint/soap/directory/" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dsp="http://schemas.microsoft.com/sharepoint/dsp" xmlns:udc="http://schemas.microsoft.com/data/udc" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:sub="http://schemas.microsoft.com/sharepoint/soap/2002/1/alerts/" xmlns:ec="http://www.w3.org/2001/04/xmlenc#" xmlns:sp="http://schemas.microsoft.com/sharepoint/" xmlns:sps="http://schemas.microsoft.com/sharepoint/soap/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:udcs="http://schemas.microsoft.com/data/udc/soap" xmlns:udcxf="http://schemas.microsoft.com/data/udc/xmlfile" xmlns:udcp2p="http://schemas.microsoft.com/data/udc/parttopart" xmlns:wf="http://schemas.microsoft.com/sharepoint/soap/workflow/" xmlns:dsss="http://schemas.microsoft.com/office/2006/digsig-setup" xmlns:dssi="http://schemas.microsoft.com/office/2006/digsig" xmlns:mdssi="http://schemas.openxmlformats.org/package/2006/digital-signature" xmlns:mver="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns:mrels="http://schemas.openxmlformats.org/package/2006/relationships" xmlns:spwp="http://microsoft.com/sharepoint/webpartpages" xmlns:ex12t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ex12m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:pptsl="http://schemas.microsoft.com/sharepoint/soap/SlideLibrary/" xmlns:spsl="http://microsoft.com/webservices/SharePointPortalServer/PublishedLinksService" xmlns:Z="urn:schemas-microsoft-com:" xmlns:tax="http://schemas.microsoft.com/sharepoint/taxonomy/soap/" xmlns:tns="http://schemas.microsoft.com/sharepoint/soap/recordsrepository/" xmlns:spsup="http://microsoft.com/webservices/SharePointPortalServer/UserProfileService" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:st="" 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 14 (filtered medium)">
<style><!--
/* Font Definitions */
@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;}
/* 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;
margin:0in;
margin-bottom:.0001pt;
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-size:10.0pt;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
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"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi David,<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">This is PR37240 (<a href="https://bugs.llvm.org/show_bug.cgi?id=37240">https://bugs.llvm.org/show_bug.cgi?id=37240</a>). I suspect this problem affects all
targets; your patch D68076 would address it only for AArch64. Although I would suggest you do some careful measurements to determine the runtime performance effect, to decide whether this is acceptable.<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 more complete approach in your steps 2 + 3 would solve this for all targets, assuming the solution did not have to be very target-specific. This would
benefit the entire community.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt">
<div>
<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""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>David Tellenbach via llvm-dev<br>
<b>Sent:</b> Thursday, September 26, 2019 5:57 AM<br>
<b>To:</b> llvm-dev<br>
<b>Cc:</b> nd<br>
<b>Subject:</b> [llvm-dev] [AArch64] Generated assembly differs depending on whether debug information is generated or not<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">Hi,</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">we at Arm have noticed that assembly can differ when compiling for AArch64</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">depending on whether debug information is generated or not.
</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">The issue is reproducible for the following small example compiled with `-O1`</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">for `aarch64-arm-linux-gnu`:</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> a() {</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> b(a);</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> for (;;)</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> c("", b);</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> }</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">The reason for the difference is that AArch64 frame lowering emits CFI</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">instructions if debug information is enabled but not if not. CFI instructions</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">act as scheduling boundaries during instruction scheduling and therefore lead to</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">differing scheduling regions and an overall different instruction scheduling.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">We see several ways to fix the issue and would welcome comments on this:</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> 1. Enabling unwind tables by default for AArch64: By enabling unwind tables</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> by default CFI instructions will be inserted in both, debug and non-debug</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> mode. This should lead to smaller scheduling regions and probably to less</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> scheduling potential.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> However, I've measured the average size of scheduling regions for randomly</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> generated programs with and without default unwind tables and found an</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> average difference of 0.5 to 1 instruction. Other architectures such as x86</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> do exactly this and therefore don't face the issue.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> The following patch on Phabricator introduces the said change:</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> https://reviews.llvm.org/D68076</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> 2. Postpone insertion of CFI instructions until after instruction scheduling.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> This would require a new pass running after instruction scheduling that
</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> inserts CFI instructions if needed. The downside I see is increased</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> compile-time and probably some code duplication with frame lowering.
</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> 3. Change instruction scheduling such that CFI instructions get tied together</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> with relevant instructions in such a way that they get scheduled together.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> If this could work it would probably the cleanest solution.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">To summarize:</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">1. would make scheduling in the non-debug case behave like in the</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">debug case and therefore probably cost some scheduling potential. However, it</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">would be by far the most easy to implement. 2. + 3. would probably lead to</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">better scheduling but seem to be more complex to implement.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black">Comments and additional ideas are welcome.</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New";color:black"> David</span><span style="font-family:"Calibri","sans-serif";color:black"><o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
</div>
</div>
</body>
</html>