<div dir="ltr"><div>Hey JinGu!</div><div><br></div><div>I don't know the answer to this from a MemorySSA / LICM perspective (I'm not deeply aware of their internals) - but I do remember having to fix this exact kind of issue with our Burst HPC# compiler with our own custom alias analysis. >From our perspective it was safe to assume that any pointer deriving from @a couldn't alias with it, and we could make this optimization (this was really important in LLVM 6/7 timeframe if I'm remembering it right because we had a lot of cases where these kind of things would disable vectorization).<br></div><div><br></div><div>Cheers,</div><div>-Neil.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Oct 29, 2021 at 7:27 PM Jingu Kang via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">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">
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<p class="MsoNormal">Ping<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org" target="_blank">llvm-dev-bounces@lists.llvm.org</a>> <b>On Behalf Of
</b>Jingu Kang via llvm-dev<br>
<b>Sent:</b> 27 October 2021 15:15<br>
<b>To:</b> <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> [llvm-dev] Question about hoisting LoadInst in LICM pass using MemorySSA/AliasAnalysis<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Hi All,<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">I have a question about hoisting LoadInst in LICM pass. I am looking at below IR code.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">@a = dso_local local_unnamed_addr global i32** null, align 8<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">define dso_local void @foo(i32 %max) {<u></u><u></u></p>
<p class="MsoNormal">entry:<u></u><u></u></p>
<p class="MsoNormal"> br label %for.cond<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.cond: ; preds = %for.body, %entry<u></u><u></u></p>
<p class="MsoNormal"> %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]<u></u><u></u></p>
<p class="MsoNormal"> %cmp.not = icmp sgt i32 %i.0, %max<u></u><u></u></p>
<p class="MsoNormal"> br i1 %cmp.not, label %for.cond.cleanup, label %for.body<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.cond.cleanup: ; preds = %for.cond<u></u><u></u></p>
<p class="MsoNormal"> ret void<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.body: ; preds = %for.cond<u></u><u></u></p>
<p class="MsoNormal"> %0 = load i32**, i32*** @a, align 8, !tbaa !8<u></u><u></u></p>
<p class="MsoNormal"> %idxprom = zext i32 %i.0 to i64<u></u><u></u></p>
<p class="MsoNormal"> %arrayidx = getelementptr inbounds i32*, i32** %0, i64 %idxprom<u></u><u></u></p>
<p class="MsoNormal"> store i32* null, i32** %arrayidx, align 8, !tbaa !8<u></u><u></u></p>
<p class="MsoNormal"> %inc = add nuw nsw i32 %i.0, 1<u></u><u></u></p>
<p class="MsoNormal"> br label %for.cond, !llvm.loop !12<u></u><u></u></p>
<p class="MsoNormal">}<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">I have expected the `%0 = load` is hoisted to entry block because I think it is loop invariant. However, LICM pass fails to hoist it because the MemorySSA is as below.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">define dso_local void @foo(i32 %max) local_unnamed_addr #0 {<u></u><u></u></p>
<p class="MsoNormal">entry:<u></u><u></u></p>
<p class="MsoNormal"> br label %for.cond<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.cond: ; preds = %for.body, %entry<u></u><u></u></p>
<p class="MsoNormal">; 2 = MemoryPhi({entry,liveOnEntry},{for.body,1})<u></u><u></u></p>
<p class="MsoNormal"> %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]<u></u><u></u></p>
<p class="MsoNormal"> %cmp.not = icmp sgt i32 %i.0, %max<u></u><u></u></p>
<p class="MsoNormal"> br i1 %cmp.not, label %for.cond.cleanup, label %for.body<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.cond.cleanup: ; preds = %for.cond<u></u><u></u></p>
<p class="MsoNormal"> ret void<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">for.body: ; preds = %for.cond<u></u><u></u></p>
<p class="MsoNormal">; MemoryUse(2) MayAlias<u></u><u></u></p>
<p class="MsoNormal"> %0 = load i32**, i32*** @a, align 8, !tbaa !8<u></u><u></u></p>
<p class="MsoNormal"> %idxprom = zext i32 %i.0 to i64<u></u><u></u></p>
<p class="MsoNormal"> %arrayidx = getelementptr inbounds i32*, i32** %0, i64 %idxprom<u></u><u></u></p>
<p class="MsoNormal">; 1 = MemoryDef(2)<u></u><u></u></p>
<p class="MsoNormal"> store i32* null, i32** %arrayidx, align 8, !tbaa !8<u></u><u></u></p>
<p class="MsoNormal"> %inc = add nuw nsw i32 %i.0, 1<u></u><u></u></p>
<p class="MsoNormal"> br label %for.cond, !llvm.loop !12<u></u><u></u></p>
<p class="MsoNormal">}<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">As we can see there is `MemoryUse(2) MayAlias` above the `%0 = load` because AliasAnalysis returns MayAlias between the ‘%0 = load’ and ‘store i32* null’. I think it could be `MemoryUse(liveOnEntry)`. How do you think about it? If I missed
something, please let me know.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Thanks,<u></u><u></u></p>
<p class="MsoNormal">JinGu Kang<u></u><u></u></p>
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</blockquote></div><br clear="all"><br>-- <br><div dir="ltr" class="gmail_signature"><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>