<div dir="ltr">I'd have figured optnone was "no optimizations" not "use the entire O0 code path"?<div><br></div><div>At least that seemed to be the intent when you added it Paul?</div><div><div><br></div><div>-eric</div></div></div><br><div class="gmail_quote"><div dir="ltr">On Wed, Nov 18, 2015 at 8:05 AM Robinson, Paul 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:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="color:#1f497d">Well, 'optnone' is already not identical to –O0, and given the nature of things, probably can't be; but I am persuaded that it's reasonable for it to honor the –fast-isel option as a debugging tactic. I'll take
an AI to make this happen.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:#1f497d">Thanks,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:#1f497d">--paulr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="color:#1f497d">P.S. One nit, the "O0 + optnone" case should not have an asterisk, the FastISel flag is not manipulated if the opt level is already zero. Does not affect the strength of your argument, of course.<u></u><u></u></span></p>
<p class="MsoNormal"><a name="msg-f:1518194089395188083__MailEndCompose"><span style="color:#1f497d"><u></u> <u></u></span></a></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""> Daniel Sanders [mailto:<a href="mailto:Daniel.Sanders@imgtec.com" target="_blank">Daniel.Sanders@imgtec.com</a>]
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<b>Sent:</b> Wednesday, November 18, 2015 2:19 AM<br>
<b>To:</b> Robinson, Paul; <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a></span></p></div></div></div></div></div><div lang="EN-US" link="blue" vlink="purple"><div><div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt"><div><div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0in 0in 0in"><p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""><br>
<b>Subject:</b> RE: Mips unconditionally uses fast-isel?<u></u><u></u></span></p></div></div></div></div></div><div lang="EN-US" link="blue" vlink="purple"><div><div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt">
<p class="MsoNormal"><u></u> <u></u></p>
<p><span lang="EN-GB">> </span>-----Original Message-----<span lang="EN-GB"><u></u><u></u></span></p>
<p><span lang="EN-GB">> </span>From: Robinson, Paul [<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank">mailto:Paul_Robinson@playstation.sony.com</a>]<span lang="EN-GB"><u></u><u></u></span></p>
<p><span lang="EN-GB">> </span>Sent: 17 November 2015 22:58<span lang="EN-GB"><u></u><u></u></span></p>
<p><span lang="EN-GB">> </span>To: Daniel Sanders; <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">
llvm-dev@lists.llvm.org</a><span lang="EN-GB"><u></u><u></u></span></p>
<p><span lang="EN-GB">> </span>Subject: RE: Mips unconditionally uses fast-isel?<span lang="EN-GB"><u></u><u></u></span></p>
<p><span lang="EN-GB">> <u></u><u></u></span></p>
<p><span lang="EN-GB">> > > The other thing that might work, is having TargetMachine remember how<u></u><u></u></span></p>
<p><span lang="EN-GB">> > > the fast-isel option got set, and make OptLevelChanger do the right<u></u><u></u></span></p>
<p><span lang="EN-GB">> > > thing. But that seems like a hack to work around Mips not obeying the<u></u><u></u></span></p>
<p><span lang="EN-GB">> > > specified optimization level, honestly.<u></u><u></u></span></p>
<p><span lang="EN-GB">> ><u></u><u></u></span></p>
<p><span lang="EN-GB">> > I think we should do that as well. I don't think it's right that optnone<u></u><u></u></span></p>
<p><span lang="EN-GB">> > enables Fast ISel even when it's been explicitly disabled. It should do<u></u><u></u></span></p>
<p><span lang="EN-GB">> > the same checks as addPassesToGenerateCode() does.<u></u><u></u></span></p>
<p><span lang="EN-GB">> <u></u><u></u></span></p>
<p><span lang="EN-GB">> Hm? What you're asking for is that "-O2" and "-O2 -fast-isel=none" are<u></u><u></u></span></p>
<p><span lang="EN-GB">> identical, unless you have an 'optnone' function. Do you really have a<u></u><u></u></span></p>
<p><span lang="EN-GB">> use-case for controlling the codegen path for an 'optnone' function?<u></u><u></u></span></p>
<p><span lang="EN-GB">> The whole point of 'optnone' is to avoid optimizations.<u></u><u></u></span></p>
<p><span lang="EN-GB">> --paulr<u></u><u></u></span></p>
<p><span lang="EN-GB">> <u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black"><u></u> <u></u></span></p>
<p><span lang="EN-GB" style="color:black">No, that's already true. -O2 doesn't try to enable Fast ISel (unless the optnone attribute is given) so –fast-isel=false has no effect.<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black"><u></u> <u></u></span></p>
<p><span lang="EN-GB" style="color:black">I'm saying that optnone means 'use –O0 for this function' and that optnone should<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">respect non-default values of the -fast-isel flag like –O0 does. This is the behaviour I'd expect:<u></u><u></u></span></p>
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<p><span style="color:black"><u></u> <u></u></span></p>
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<p><span style="color:black">-fast-isel=false<u></u><u></u></span></p>
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<p><span style="color:black">-fast-isel=default<u></u><u></u></span></p>
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<p><span style="color:black">-fast-isel=true<u></u><u></u></span></p>
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<p><span style="color:black">-O0<u></u><u></u></span></p>
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<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O0 + optnone attribute<u></u><u></u></span></p>
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<p><span style="color:black">SelectionDAG*<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<td width="151" valign="top" style="width:90.85pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O1 + optnone attribute<u></u><u></u></span></p>
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<td width="164" valign="top" style="width:98.55pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG*<u></u><u></u></span></p>
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<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<td width="151" valign="top" style="width:90.85pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O2 + optnone attribute<u></u><u></u></span></p>
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<td width="164" valign="top" style="width:98.55pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG*<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O3 + optnone attribute<u></u><u></u></span></p>
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<p><span style="color:black">SelectionDAG*<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O1<u></u><u></u></span></p>
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<td width="164" valign="top" style="width:98.55pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="151" valign="top" style="width:90.85pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O2<u></u><u></u></span></p>
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<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="151" valign="top" style="width:90.85pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span style="color:black">-O3<u></u><u></u></span></p>
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<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<td width="153" valign="top" style="width:91.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt">
<p><span style="color:black">SelectionDAG<u></u><u></u></span></p>
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<p><span style="color:black">FastISel<u></u><u></u></span></p>
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<p><span lang="EN-GB" style="color:black">The cells marked with '*' differ from the current behaviour.<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"><u></u> <u></u></span></p>
<p><span lang="EN-GB" style="color:black">In terms of code, I think this part of OptLevelChanger::OptLevelChanger():<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> if (NewOptLevel == CodeGenOpt::None) {<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> DEBUG(dbgs() << "\nEnable FastISel for Function "<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> << IS.MF->getFunction()->getName() << "\n");<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> IS.TM.setFastISel(true);<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> }<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">Should be:<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> if (NewOptLevel == CodeGenOpt::None) {<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> DEBUG(dbgs() << "\nEnable FastISel for Function "<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> << IS.MF->getFunction()->getName() << "\n");<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> IS.TM.setFastISel(EnableFastISelOption != cl::BOU_FALSE);<u></u><u></u></span></p>
<p><span lang="EN-GB" style="font-family:"Courier New";color:black"> }<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">Where EnableFastISelOption has the same value as the global in LLVMTargetMachine.cpp<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black"><u></u> <u></u></span></p>
<p><span lang="EN-GB" style="color:black">The main reason I'm asking for this is that I think it's weird to for optnone to use a different code generator<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">than –O0. These hidden overrides exist to help us debug code generation problems and, faced with a code<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">generation bug, –fast-isel=false is useful for quickly determining whether it's in FastISel or somewhere else.<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">The current behaviour allows optnone to overrule the hidden option to force-disable FastISel which will give<u></u><u></u></span></p>
<p><span lang="EN-GB" style="color:black">misleading guidance for bugs that lie in functions with optnone.<u></u><u></u></span></p>
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