Hi shahid,<br><br>Having looked into this in the past, we're not taking into account the fact that integer promotions and truncations will disappear during code generation. The canonicalisation phase likes to create i32 and greater type arithmetic, which creates extensions and promotions . However of course the vectoriser can use i8 arithmetic I vectors, so those should go away. <br><br>Often this is the cause of insufficient vectorisation factors being picked, and we should fix it. <br><br>James<br><div class="gmail_quote">On Sat, 13 Dec 2014 at 16:09, Shahid, Asghar-ahmad <<a href="mailto:Asghar-ahmad.Shahid@amd.com">Asghar-ahmad.Shahid@amd.com</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">So IMO, if we modify the VF calculation for targets/subtargets using TTI where higher VF is supported<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">The vectorizer’s scope will become wider.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Did/do you foresee any issue with this?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Thanks,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Shahid<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
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<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""> Nadav Rotem [mailto:<a href="mailto:nrotem@apple.com" target="_blank">nrotem@apple.com</a>]
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<b>Sent:</b> Saturday, December 13, 2014 2:47 AM<br>
<b>To:</b> Shahid, Asghar-ahmad<br>
<b>Cc:</b> <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [LLVMdev] Vectorization factor limitation in Loop Vectorizer<u></u><u></u></span></p>
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<p class="MsoNormal">Hi Shahid, <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">On Dec 10, 2014, at 10:48 PM, Shahid, Asghar-ahmad <<a href="mailto:Asghar-ahmad.Shahid@amd.com" target="_blank">Asghar-ahmad.Shahid@amd.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Hi Nadav/Devs<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I am exploring Loop Vectorizer to vectorize i8 scalar operations into 8xi8 vector operation.<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I was expecting the Loop Vectorizer to analyze the profitability for vectorization factor(VF) of 8,<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">However it is not doing so due to the widest type calculation done for the blocks inside the loop.<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">May be I am missing something, however, I am curious to know why Loop Vectorizer limits the<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">profitability check to widest type and not allowing for other narrower type?<u></u><u></u></span></p>
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<p class="MsoNormal">The vectorizer stops the search of profitable vectorization factors at the widest type because higher vectorization factors would require the compiler to split the vectorized value into multiple registers. The vectorizer’s cost model first
tries to optimize for SIMD instruction utilization. Later, we optimize for ILP by doubling the vectorization factor (we call it “interleave”) and exposing ILP. <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Thanks, <u></u><u></u></p>
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<p class="MsoNormal">Nadav <u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Regards,<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Shahid<u></u><u></u></span></p>
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