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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I’m not sure how much people actually use these, but the AVX-512 versions of these, at least, can be very useful internally to implement AVX-512 intrinsics.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For AVX-512, we use the same GCC builtin for all 3 versions of the intrinsic (pass-through masked, set to zero masked, and unmasked). This is the same implementation
that’s used in GCC, and is fairly clean, since the only difference is in the desired pass-through values (actual value, zero, or undef).<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">However, since we don’t actually have the undef intrinsics right now, we put a zero in the unmasked version as well, which is definitely a pessimization.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The plan is to change them to use undef once the undef intrinsics are implemented.<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"><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""> Eric Christopher [mailto:echristo@gmail.com]
<br>
<b>Sent:</b> Monday, August 17, 2015 21:33<br>
<b>To:</b> reviews+D12052+public+a6057f04f570e35c@reviews.llvm.org; llvm-dev@redking.me.uk; craig.topper@gmail.com; Kuperstein, Michael M<br>
<b>Cc:</b> david.majnemer@gmail.com; Badouh, Asaf; cfe-commits@lists.llvm.org; Richard Smith<br>
<b>Subject:</b> Re: [PATCH] D12052: [X86][SSE] Add _mm_undefined_* intrinsics<o:p></o:p></span></p>
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<p class="MsoNormal" style="line-height:115%">On Sun, Aug 16, 2015 at 3:05 AM Simon Pilgrim <<a href="mailto:llvm-dev@redking.me.uk">llvm-dev@redking.me.uk</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt;line-height:115%">RKSimon added a comment.<br>
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Yes using that uninitialized value has worried me as well. I originally set it to zero (and considered using __ LINE __ or __ COUNTER __) but both introduce defined behaviour that I could see causing all sorts of problems further down the line in debug vs release
builds. How undefined do we want our undefined to be? ;-)<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%">Yeah, this is why I hadn't implemented them yet either.<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%"> <o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt;line-height:115%">I can create __builtin_ia32_undef64mmx / __builtin_ia32_undef128 / __builtin_ia32_undef256 / __builtin_ia32_undef512 if nobody can think of a better alternative?<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%">This seems fairly heavyweight, but I don't have any better ideas. I'll assume we don't want to try to expose undef as a value in C (making it as something we could just add), if not then this seems to make the most
sense. It's pretty painful/ugly though.<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%">Are people using these or did they just notice for completeness? We probably _could_ define them to zero and leave it at that. It's not pleasant and slower than it needs to be but not crazy.<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%"><o:p> </o:p></p>
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<p class="MsoNormal" style="line-height:115%">-eric<o:p></o:p></p>
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<p class="MsoNormal" style="line-height:115%"> <o:p></o:p></p>
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Repository:<br>
rL LLVM<br>
<br>
<a href="http://reviews.llvm.org/D12052" target="_blank">http://reviews.llvm.org/D12052</a><br>
<br>
<br>
<o:p></o:p></p>
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