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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Hal> Noting that all of the above use the same number of IR instructions, I prefer this third option:<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">as does RecurrenceDescriptor::isMinMaxSelectCmpPattern(), which looks for “Select(ICmp(X, Y), X, Y)”.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Sanjay></span>
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Another possible consideration is the structure/accuracy of the cost models used by the vectorizers and other passes. I don't think they ever special-case the cmp+sel pair as a possibly
unified (and therefore cheaper than the sum of the parts) operation.<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">They call the above to identify min/max reductions; and use TTI::getCmpSelInstrCost(). Special-casing may be redundant if the costs of scalar-vs-vector min/max
correspond to the costs of scalar-vs-vector cmp&sel.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Ayal.<o:p></o:p></span></p>
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<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"> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Sanjay Patel via llvm-dev<br>
<b>Sent:</b> Tuesday, November 08, 2016 02:13<br>
<b>To:</b> Hal Finkel <hfinkel@anl.gov><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] should we have IR intrinsics for integer min/max?<o:p></o:p></span></p>
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<p class="MsoNormal">Thanks, Hal and Matt for the feedback. As usual, my instincts about canonicalization were probably wrong. :)<o:p></o:p></p>
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I thought that @max1 vs. @max3 would be viewed as an unknowable trade-off between reducing the dependency chain and the pseudo-canonical min/max form, so we'd add intrinsics, and defer that decision to the backend.<o:p></o:p></p>
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<p class="MsoNormal">I'll wait to see if there are any other arguments presented.<o:p></o:p></p>
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<p class="MsoNormal">@max2 vs. @max3 is a straightforward commute that we should have been doing anyway, so I can start there. Assuming we go with @max3, we need to add something to DAGCombine to turn that back into @max1 (PPC w/ isel and AArch64 do better
with @max1; x86 is the same).<o:p></o:p></p>
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<p class="MsoNormal"> <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 Mon, Nov 7, 2016 at 2:47 PM, Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-family:"Helvetica",sans-serif;color:black">From:
</span></b><span style="font-family:"Helvetica",sans-serif;color:black">"Sanjay Patel via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>To: </b>"llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>Sent: </b>Monday, November 7, 2016 1:01:27 PM<br>
<b>Subject: </b>[llvm-dev] should we have IR intrinsics for integer min/max?<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black">Hi -<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black">The answer to this question may help to resolve larger questions about intrinsics and vectorization that were discussed at the dev mtg last week,
but let's start with the basics:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black">Which, if any, of these is the canonical IR?<o:p></o:p></span></p>
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; ret = x < y ? 0 : x-y<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black">define i32 @max1(i32 %x, i32 %y) {<br>
%sub = sub nsw i32 %x, %y<br>
%cmp = icmp slt i32 %x, %y ; cmp is independent of sub<br>
%sel = select i1 %cmp, i32 0, i32 %sub<br>
ret i32 %sel<br>
}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black">; ret = (x-y) < 0 ? 0 : x-y<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black">define i32 @max2(i32 %x, i32 %y) {<br>
%sub = sub nsw i32 %x, %y<br>
%cmp = icmp slt i32 %sub, 0 ; cmp depends on sub, but this looks more like a max?<br>
%sel = select i1 %cmp, i32 0, i32 %sub<br>
ret i32 %sel<br>
}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black">; ret = (x-y) > 0 ? x-y : 0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black">define i32 @max3(i32 %x, i32 %y) {<br>
%sub = sub nsw i32 %x, %y<br>
%cmp = icmp sgt i32 %sub, 0 ; canonicalize cmp+sel - looks even more like a max?<br>
%sel = select i1 %cmp, i32 %sub, i32 0<br>
ret i32 %sel<br>
}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Noting that all of the above use the same number of IR instructions, I prefer this third option:<br>
<br>
1. It uses fewer values in the icmp/select, so the live range of the x and y, individually, is shorter. This seems like a reasonable metric for simplicity.<br>
2. Using a comparison of (x-y) against zero likely makes it easier for computing known bits to simply the answer (you only need to compute the sign bit).<br>
3. The constant of the select, 0, is the second argument (which seems to reflect our general canonical choice).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><br>
define i32 @max4(i32 %x, i32 %y) {<br>
%sub = sub nsw i32 %x, %y<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"> %max = llvm.smax.i32(i32 %sub, i32 0) ; this intrinsic doesn't exist today<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black"> ret i32 %max<br>
}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">I don't currently see the need for a new intrinsic.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black">FWIW, InstCombine doesn't canonicalize any of the first 3 options currently. Codegen suffers because of that (depending on the target machine and data types). Regardless of the
IR choice, some backend fixes are needed.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><br>
Another possible consideration is the structure/accuracy of the cost models used by the vectorizers and other passes. I don't think they ever special-case the cmp+sel pair as a possibly unified (and therefore cheaper than the sum of the parts) operation.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">We don't have a facility currently for the target to provide a cost for combined operations. We should, but there's design work to be done.<br>
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-Hal<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Helvetica",sans-serif;color:black"><br>
Note that we added FP variants for min/max ops with:<br>
<a href="https://reviews.llvm.org/rL220341" target="_blank">https://reviews.llvm.org/rL220341</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><br>
_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></span></p>
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<br>
<br>
<span class="hoenzb">-- <o:p></o:p></span></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#888888">Hal Finkel<br>
Lead, Compiler Technology and Programming Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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