<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<style type="text/css" style="display:none;"> P {margin-top:0;margin-bottom:0;} </style>
</head>
<body dir="ltr">
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Hi Markus,</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
It's fair to expect there's going to be slight differences between each target, so HardwareLoopInfo is there to communicate these between the backend and the transform. I would suggest adding a flag in that struct to prevent the HardwareLoops pass from adding
one to the 'ExitCount' as this is cleaner than adding another intrinsic.</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Regards,</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
sam</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div id="Signature">
<div id="divtagdefaultwrapper" dir="ltr" style="font-size:12pt; color:rgb(0,0,0); background-color:rgb(255,255,255); font-family:Calibri,Arial,Helvetica,sans-serif,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols">
<p style="margin-top: 0px; margin-bottom: 0px;"></p>
<p style="margin-top: 0px; margin-bottom: 0px;font-family:"Times New Roman""><span style="font-family:Calibri,Helvetica,sans-serif">Sam Parker</span></p>
<span style="font-family:Calibri,Helvetica,sans-serif"></span>
<p style="margin-top: 0px; margin-bottom: 0px;font-family:"Times New Roman""><span style="font-family:Calibri,Helvetica,sans-serif">Compilation Tools Engineer | Arm</span></p>
<span style="font-family:Calibri,Helvetica,sans-serif"></span>
<p style="margin-top: 0px; margin-bottom: 0px;font-family:"Times New Roman""><span style="font-family:Calibri,Helvetica,sans-serif">. . . . . . . . . . . . . . . . . . . . . . . . . . .</span></p>
<span style="font-family:Calibri,Helvetica,sans-serif"></span>
<p style="margin-top: 0px; margin-bottom: 0px;font-family:"Times New Roman""><span style="font-family:Calibri,Helvetica,sans-serif">Arm.com</span></p>
<p style="margin-top: 0px; margin-bottom: 0px;"></p>
</div>
</div>
<div id="appendonsend"></div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Markus Lavin <markus.lavin@ericsson.com><br>
<b>Sent:</b> 11 July 2019 14:40<br>
<b>To:</b> llvm-dev@lists.llvm.org<br>
<b>Cc:</b> Sam Parker<br>
<b>Subject:</b> llvm.set.loop.iterations </font>
<div> </div>
</div>
<style type="text/css" style="display:none">
<!--
p
{margin-top:0;
margin-bottom:0}
-->
</style>
<div dir="ltr">
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
After playing a bit with the newly introduced hardware loop framework I realize that the llvm.set.loop.iterations intrinsic takes as argument the number of iterations the loop will execute. In fact it goes all the way to, on IR, insert an addition of constant
1 to the number of taken backedges returned by SCEV.</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
If the machine instruction realizing the loop is interested in the number of branches-to-make / backedges-taken then this is slightly awkward as we need to subtract the constant 1. Of course if the iteration count was constant this is trivial but if it is passed
in register then it is not so nice to have to insert these subtract instructions from a MIR pass (where the hwloop finalization is being done).</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
I wonder what would be the best way to deal with this.</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
One way would be to add a TTI hook gating the original addition but then the intrinsic will have two meanings depending on what this hook returns which is not good.</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
Another way would be to introduce a second intrinsic, say llvm.set.loop.backedges that corresponds to that value, but then we have yet another intrinsic.</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
A third option could be to have the original intrinsic take both values as arguments.</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
Any thoughts on this?</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
regards</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
Markus</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
<br>
</div>
</div>
</body>
</html>