<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:"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;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri","sans-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-compose;
font-family:"Calibri","sans-serif";
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;}
@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">Hello!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during<o:p></o:p></p>
<p class="MsoNormal">debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its
<o:p></o:p></p>
<p class="MsoNormal">liverange to the end of the function.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler,
<o:p></o:p></p>
<p class="MsoNormal">is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are<o:p></o:p></p>
<p class="MsoNormal">proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value.
<o:p></o:p></p>
<p class="MsoNormal">The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level.
<o:p></o:p></p>
<p class="MsoNormal">Code emission would simply ignore the new opcode.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option<o:p></o:p></p>
<p class="MsoNormal">(e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection<o:p></o:p></p>
<p class="MsoNormal">throughout the entire function body.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be
<o:p></o:p></p>
<p class="MsoNormal">eventually support for a -Og mode (optimize for debugging), that mode could enable it.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any comments or alternative ideas are appreciated.<o:p></o:p></p>
</div>
</body>
</html>