<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 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;}
/* 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:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
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;
font-family:"Calibri",sans-serif;
color:windowtext;}
span.EmailStyle19
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.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="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Edit. Predecessor -> successor.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b>From:</b> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org] <b>On Behalf Of </b>Balaram Makam via llvm-dev<br><b>Sent:</b> Friday, June 30, 2017 10:47 AM<br><b>To:</b> llvm-dev@lists.llvm.org<br><b>Subject:</b> [llvm-dev] LoopSimplify pass prevents loop unrolling<o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Hi All,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>In the attached test case there, is an unnested loop with 2 iterations. The loop latch block is terminated by an unconditional branch, so simplifycfg folds the almost empty latch block into its <b>successor</b> which is the loop header. This results in an additional backedge in the CFG, so when LoopRotate pass is called it canonicalizes the loop into a nested loop. However, now the loop trip count is unpredictable as the BackedgeTakenCount for the outer loop is not loop invariant. As a result the loop cannot be unrolled. Is this the intended canonicalization for this loop or is the loopsimplify canonicalizing incorrectly? Should simplifycfg skip folding the latch block into the loop header if this results in additional backedges and let the empty blocks be folded during CGP? More details in <a href="https://bugs.llvm.org/show_bug.cgi?id=33605">https://bugs.llvm.org/show_bug.cgi?id=33605</a>.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>FWIW, this prevents unrolling of a hot loop in spec2017/gcc and also prevents loop-interleave of a loop in spec2017/perlbench. Appreciate any suggestions on how to fix this.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Attached testcase:<o:p></o:p></p><p class=MsoNormal>$cat test.c<o:p></o:p></p><p class=MsoNormal>void foo();<o:p></o:p></p><p class=MsoNormal>bool test(int a, int b, int *c) {<o:p></o:p></p><p class=MsoNormal> bool changed = false;<o:p></o:p></p><p class=MsoNormal> for (unsigned int i = 2; i--;) {<o:p></o:p></p><p class=MsoNormal> int r = a | b;<o:p></o:p></p><p class=MsoNormal> if ( r != c[i]) {<o:p></o:p></p><p class=MsoNormal> c[i] = r;<o:p></o:p></p><p class=MsoNormal> foo();<o:p></o:p></p><p class=MsoNormal> changed = true;<o:p></o:p></p><p class=MsoNormal> }<o:p></o:p></p><p class=MsoNormal> }<o:p></o:p></p><p class=MsoNormal> return changed;<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks,<o:p></o:p></p><p class=MsoNormal>Balaram<o:p></o:p></p></div></body></html>