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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi Chris,<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'>It's not broken, but the performance is crippled.<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'>I noticed that the code still contains some MMX instructions,
but several operations get expanded (apparently swizzling and such get expanded
to a large number of byte moves).<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'>I could use intrinsics, but they wouldn't be optimized like
other vector operations. I could use SSE operations, but they would increase
SSE register pressure while MMX registers are left unused.<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'>So ideally I would like to inform LLVM that selecting MMX
instructions is fine. I'm inserting emms instructions in the right spots
myself.<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'>Thanks,<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'>Nicolas<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>
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<p class=MsoNormal style='margin-left:.5in'><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"'> Chris Lattner [mailto:clattner@apple.com] <br>
<b>Sent:</b> Tuesday, September 07, 2010 17:42<br>
<b>To:</b> Nicolas Capens<br>
<b>Cc:</b> llvmdev@cs.uiuc.edu<br>
<b>Subject:</b> Re: [LLVMdev] LLVM 2.8 and MMX<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
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<p class=MsoNormal style='margin-left:.5in'>On Sep 7, 2010, at 7:45 AM, Nicolas
Capens wrote:<o:p></o:p></p>
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<br>
<o:p></o:p></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'>Hi all,<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'>I've tested a recent revision and noticed
that using 64-bit vectors became very slow. It looks like they are expanded to
non-MMX instructions to avoid breaking code which does not clear the MMX state
using emms?<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'>For my project I'm already manually
inserting emms instructions in the right places, so I'd really like 64-bit
vector operations to be lowered to MMX instructions for best performance. So is
there some way to get that behavior back? I tried making IsVerbotenVectorType
return false but that didn't appear to help.<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif"'>I would be very grateful if someone would
add a switch to disable the expanding before the 2.8 release is finalized.<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
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<p class=MsoNormal style='margin-left:.5in'>The implementation of MMX is in a
state of flux: the plan is to stop generic vectors from being selected to mmx
operations, and add intrinsics for every mmx operations (including add). However,
2.8 shouldn't be broken, that would be a serious regression. Please file
a bug with a small example. Thanks!<o:p></o:p></p>
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<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
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<p class=MsoNormal style='margin-left:.5in'>-Chris<o:p></o:p></p>
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