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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Our specialists (Intel) say that “vmovaps” and “vmovsd” have the same
</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">throughput</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> and latency, but “vmovsd” reduces chance of 4k aliasing, so it is preferable.<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"><b><i><span style="color:teal">- Elena</span></i></b><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"><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""> llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Cameron McInally<br>
<b>Sent:</b> Thursday, July 26, 2012 17:50<br>
<b>To:</b> Jan Sjodin<br>
<b>Cc:</b> dag@cray.com; llvmdev@cs.uiuc.edu<br>
<b>Subject:</b> Re: [LLVMdev] X86 FMA4<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hey Jan and Dave,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It's not obvious, but there is a significant scalar performance issue following the GCC intrinsics.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Let's look at the VFMADDSD pattern. We're operating on scalars with undefineds as the remaining vector elements of the operands. This sounds okay, but when one looks closer...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"> vmovsd fp4_+1088(%rip), %xmm3 # fpppp.f:647<br>
vmovaps %xmm3, 18560(%rsp) # fpppp.f:647 <= 16-byte spill<br>
vfmaddsd %xmm5, fp4_+3288(%rip), %xmm3, %xmm3 # fpppp.f:647<o:p></o:p></p>
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<p class="MsoNormal">The spill here is 16-bytes. But, we're only using the low 8-bytes of xmm3. Changing the intrinsics and patterns to accept scalar operands, we end up with...<o:p></o:p></p>
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<p class="MsoNormal"><br>
vmovsd fp4_+1056(%rip), %xmm0 # fpppp.f:666<br>
vmovsd %xmm0, 10088(%rsp) # fpppp.f:666 <= 8-byte spill<br>
vfmaddsd %xmm3, fp4_+3288(%rip), %xmm0, %xmm3 # fpppp.f:666<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I do not know the actual number of cycles offhand, but I believe on Interlagos and Sandybridge, a vmovaps takes roughly 3x as many micro-ops as a vmovsd if it involves memory.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-Cameron<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Jul 26, 2012 at 9:41 AM, Jan Sjodin <<a href="mailto:jan_sjodin@yahoo.com" target="_blank">jan_sjodin@yahoo.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Because the intrinsics uses vector types (same as gcc).<br>
<br>
<br>
- Jan<br>
<br>
<br>
<br>
----- Original Message -----<br>
> From: "<a href="mailto:dag@cray.com">dag@cray.com</a>" <<a href="mailto:dag@cray.com">dag@cray.com</a>><br>
> To: <a href="mailto:llvmdev@cs.uiuc.edu">llvmdev@cs.uiuc.edu</a><br>
> Cc:<br>
> Sent: Wednesday, July 25, 2012 3:26 PM<br>
> Subject: [LLVMdev] X86 FMA4<br>
><br>
> We're migrating to LLVM 3.1 and trying to use the upstream FMA patterns.<br>
><br>
> Why is VFMADDSD4 defined with vector types? Is this simply because the<br>
> gcc intrinsic uses vector types? It's quite unnatural if you have a<br>
> compiler that generates FMAs as opposed to requiring user intrinsics.<o:p></o:p></p>
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