<div dir="ltr"><div>So for us we use SLEEF to actually implement the libcalls (LLVM intrinsics) that LLVM by default would generate - and since SLEEF has highly optimal 8-wide pow, optimized for AVX and AVX2, we really want to use that.</div><div><br></div><div>So we would not see 4/8 libcalls and instead see 1 call to something that lights up the ymm registers. I guess the problem then is that the default expectation is that pow would be implemented using N scalar libcalls?</div><div><br></div><div>Cheers,</div><div>-Neil.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jul 16, 2020 at 6:08 PM Sanjay Patel <<a href="mailto:spatel@rotateright.com">spatel@rotateright.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>The debug spew for loop vectorization shows:</div><div>LV: Found an estimated cost of 49 for VF 4 For instruction: %14 = tail call float @llvm.pow.f32(float %10, float %13)</div>LV: Vector loop of width 4 costs: 13.<br><div><br></div><div>LV: Found an estimated cost of 107 for VF 8 For instruction: %14 = tail call float @llvm.pow.f32(float %10, float %13)</div>LV: Vector loop of width 8 costs: 14.<br><div>LV: Selecting VF: 4.</div><div><br></div><div>So rounding of the integer division could be to blame? <br></div><div><br></div><div>But before we focus on that, there's a lot of hand-waving involved in creating these costs beginning with the base cost implementation:</div><div> unsigned SingleCallCost = 10; // Library call cost. Make it expensive.<br></div><div><br></div><div>But before we focus on that... :)</div><div><br></div><div>Are we modeling the right thing? Ie, are you not expecting to see 4 or 8 libcalls when the vector pow call gets expanded on this example? If we are doing those libcalls, then it's not clear to me how anything else in the loop matters for performance.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jul 16, 2020 at 10:20 AM Neil Henning via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Tried a bunch of them there (x86-64, haswell, znver2) and they all defaulted to 4-wide - haswell additionally caused some extra loop unrolling but still with 8-wide pows.</div><div><br></div><div>Cheers,</div><div>-Neil.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jul 16, 2020 at 2:39 PM Roman Lebedev <<a href="mailto:lebedev.ri@gmail.com" target="_blank">lebedev.ri@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><div>Did you specify the target CPU the code should be optimized for?<br></div><div>For clang that is -march=native/znver2/... / -mtune=<same></div><div>For opt/llc that is <span style="color:rgb(0,0,0);font-family:monospace">--mcpu=<same></span></div></div>I would expect that by default, some generic baseline is picked.<div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jul 16, 2020 at 4:25 PM Neil Henning via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Hey list,</div><div><br></div><div>I've recently done the first test run of bumping our Burst compiler from LLVM 10 -> 11 now that the branch has been cut, and have noticed an apparent loop vectorization codegen regression for X86 with AVX or AVX2 enabled. The following IR example is vectorized to 4 wide with LLVM 11 and trunk whereas in LLVM 10 it (correctly as per what we want) vectorized it 8 wide matching the ymm registers.</div><div><br></div><div><span style="font-family:monospace"><font size="1">; ModuleID = '../test.ll'<br>source_filename = "main"<br>target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"<br>target triple = "x86_64-pc-windows-msvc-coff"<br><br>%"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0" = type { float*, i32, [4 x i8] }<br><br>; Function Attrs: nofree<br>define dllexport void @func(float* noalias nocapture %output, %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* nocapture nonnull readonly dereferenceable(16) %a, %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* nocapture nonnull readonly dereferenceable(16) %b) local_unnamed_addr #0 !ubaa. !1 {<br>entry:<br> %0 = getelementptr %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0", %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* %a, i64 0, i32 1<br> %1 = load i32, i32* %0, align 1<br> %.not = icmp eq i32 %1, 0<br> br i1 %.not, label %BL.0042, label %<a href="http://BL.0005.lr.ph" target="_blank">BL.0005.lr.ph</a><br><br><a href="http://BL.0005.lr.ph" target="_blank">BL.0005.lr.ph</a>: ; preds = %entry<br> %2 = bitcast %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* %a to i8**<br> %3 = load i8*, i8** %2, align 1<br> %4 = bitcast %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* %b to i8**<br> %5 = load i8*, i8** %4, align 1<br> %wide.trip.count = zext i32 %1 to i64<br> br label %BL.0005<br><br>BL.0005: ; preds = %BL.0005, %<a href="http://BL.0005.lr.ph" target="_blank">BL.0005.lr.ph</a><br> %indvars.iv = phi i64 [ 0, %<a href="http://BL.0005.lr.ph" target="_blank">BL.0005.lr.ph</a> ], [ %indvars.iv.next, %BL.0005 ]<br> %6 = shl nuw nsw i64 %indvars.iv, 2<br> %7 = getelementptr float, float* %output, i64 %indvars.iv<br> %8 = getelementptr i8, i8* %3, i64 %6<br> %9 = bitcast i8* %8 to float*<br> %10 = load float, float* %9, align 4<br> %11 = getelementptr i8, i8* %5, i64 %6<br> %12 = bitcast i8* %11 to float*<br> %13 = load float, float* %12, align 4<br> %14 = tail call float @llvm.pow.f32(float %10, float %13)<br> store float %14, float* %7, align 4<br> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1<br> %exitcond.not = icmp eq i64 %indvars.iv.next, %wide.trip.count<br> br i1 %exitcond.not, label %BL.0042, label %BL.0005<br><br>BL.0042: ; preds = %BL.0005, %entry<br> ret void<br>}<br><br>; Function Attrs: norecurse readnone<br>define dllexport void @burst.initialize(i8* (i8*)* nocapture readnone %callback) local_unnamed_addr #1 !ubaa. !0 {<br>entry:<br> ret void<br>}<br><br>; Function Attrs: nounwind readnone speculatable willreturn<br>declare float @llvm.pow.f32(float, float) #2<br><br>attributes #0 = { nofree }<br>attributes #1 = { norecurse readnone }<br>attributes #2 = { nounwind readnone speculatable willreturn }<br><br>!ubaa.Burst.Compiler.IL.Tests.VectorsMaths\2FFloatPointer.0 = !{!0, !0, !0, !0}<br><br>!0 = !{i1 false}<br>!1 = !{i1 true, i1 false, i1 false}</font></span></div><div><span style="font-family:monospace"><font size="1"><br></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif">If I run this with ../llvm-project/llvm/build/bin/opt.exe -o - -S -O3 ../avx_sad_4.ll -mattr=avx -debug, I can see that the loop vectorizer correctly considers using 8-wide ymm registers for this, but has decided that the 4-wide variant is cheaper based on some cost modelling I don't understand.</font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif"><br></font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif">So is this expected behaviour? I know there was some cost model changes in the 10->11 timeframe.<br></font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif"><br></font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif">Thanks for any help,</font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif"><br></font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif">Cheers,</font></font></font></span></div><div><span style="font-family:monospace"><font size="1"><font size="2"><font face="arial,sans-serif">-Neil.</font></font></font></span></div></div></blockquote><div>Roman</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>-- <br><div dir="ltr"><div dir="ltr"><table style="border-collapse:collapse;border-spacing:0px;color:rgb(90,90,91);font-size:13px;margin:0px 0px 20px;padding:0px" width="100%" cellspacing="0" cellpadding="0" border="0"><tbody style="margin:0px;padding:0px"><tr style="margin:0px;padding:0px"><td style="border-collapse:collapse;font-size:0px;line-height:1.5em;padding:0px 0px 20px;vertical-align:top" align="left"><table style="border-collapse:collapse;border-spacing:0px;margin:0px;padding:0px" cellspacing="0" cellpadding="0" border="0" align="left"><tbody style="margin:0px;padding:0px"><tr style="margin:0px;padding:0px"><td style="border-collapse:collapse;font-size:1.12em;line-height:1.5em;padding:0px;vertical-align:top;width:64px"><img style="border: medium none; border-radius: 0px; display: block; font-size: 13px; height: auto; line-height: 100%; margin: 0px; max-width: 100%; outline-style: none; outline-width: medium; padding: 20px 0px 0px; width: 100%;" alt="" src="https://unity3d.com/profiles/unity3d/themes/unity/images/ui/other/unity-logo-dark-email.png" width="64" height="auto"></td></tr></tbody></table></td></tr><tr style="margin:0px;padding:0px"><td style="border-collapse:collapse;font-size:0px;line-height:1.5em;padding:0px;vertical-align:top" align="left"><div style="color:rgb(0,0,0);font-family:Roboto,Arial;font-size:14px;font-weight:600;line-height:15px;margin:0px;padding:0px">Neil Henning</div></td></tr><tr style="margin:0px;padding:0px"><td style="border-collapse:collapse;font-size:0px;line-height:1.5em;padding:0px;vertical-align:top" align="left"><div style="color:rgb(0,0,0);font-family:Roboto,Arial;font-size:14px;line-height:15px;margin:0px;padding:0px 0px 10px">Senior Software Engineer Compiler</div></td></tr><tr style="margin:0px;padding:0px"><td style="border-collapse:collapse;font-size:0px;line-height:1.5em;padding:0px;vertical-align:top" align="left"><div style="color:rgb(0,0,0);font-family:Roboto,Arial;font-size:12px;line-height:15px;margin:0px;padding:0px"><a href="http://unity.com" target="_blank">unity.com</a></div></td></tr></tbody></table></div></div></div></div>
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