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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Just to add, there is also the ‘zext’ – ‘shuffle with zero’ duality which can broaden the discussion.<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">--Zvi<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>
<p class="MsoNormal"><a name="_____replyseparator"></a><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Sanjay Patel [mailto:spatel@rotateright.com]
<br>
<b>Sent:</b> Thursday, January 12, 2017 20:19<br>
<b>To:</b> Friedman, Eli <efriedma@codeaurora.org><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org>; Rackover, Zvi <zvi.rackover@intel.com><br>
<b>Subject:</b> Re: [llvm-dev] IR canonicalization: shufflevector or vector trunc?<o:p></o:p></span></p>
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<p class="MsoNormal">On Thu, Jan 12, 2017 at 11:06 AM, Friedman, Eli <<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On 1/12/2017 9:04 AM, Sanjay Patel via llvm-dev wrote:<o:p></o:p></p>
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<p class="MsoNormal">It's time for another round of "What is the canonical IR?"<br>
<br>
Credit for this episode to Zvi and PR31551. :)<br>
<a href="https://llvm.org/bugs/show_bug.cgi?id=31551" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=31551</a>
<o:p></o:p></p>
<pre>define <4 x i16> @shuffle(<16 x i16> %x) {<o:p></o:p></pre>
<pre> %shuf = shufflevector <16 x i16> %x, <16 x i16> undef, <4 x i32> <i32 0, i32 4, i32 8, i32 12><o:p></o:p></pre>
<pre> ret <4 x i16> %shuf<o:p></o:p></pre>
<pre>}<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>define <4 x i16> @trunc(<16 x i16> %x) {<o:p></o:p></pre>
<pre> %bc = bitcast <16 x i16> %x to <4 x i64><o:p></o:p></pre>
<pre> %tr = trunc <4 x i64> %bc to <4 x i16><o:p></o:p></pre>
<pre> ret <4 x i16> %tr<o:p></o:p></pre>
<pre>}<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<p class="MsoNormal">Potential reasons to prefer one or the other:<br>
1. Shuffle is the most compact.<br>
2. Trunc is easier to read.<br>
3. One of these is easier for value tracking.<br>
4. Compatibility with existing IR transforms (eg, InterleavedAccess recognizes the shuffle form).<o:p></o:p></p>
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<p class="MsoNormal">5. We don't create arbitrary shuffle masks in IR because that's bad for a lot of targets (but maybe this mask pattern should always be recognized as special?).<o:p></o:p></p>
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<p class="MsoNormal">Hmm... not sure what the right answer is, but a couple more observations:<br>
1. If we're going to canonicalize, we should probably canonicalize the same way independent of the original argument type (so we would introduce bitcasts either way).<o:p></o:p></p>
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<p class="MsoNormal">Ah, right - kill #1 in my list.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">2. Those two functions are only equivalent on little-endian platforms.<o:p></o:p></p>
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<p class="MsoNormal">I was wondering about that. So yes, if we do want to canonicalize (until the recent compile-time complaints, I always thought this was the objective of InstCombine...maybe it still is), then the masks we're matching or generating will differ
based on endianness.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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