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<p class="MsoNormal">> Well, 'optnone' is already not identical to –O0, and given the nature of things, probably can't be<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It's not important to the topic but I'm curious about this since, in instruction selection at least, they both set SelectionDAGISel::OptLevel to CodeGenOpt::None and MCCodeGenInfo::OptLevel by one means or another. Could you point me at
a difference between the two? I assume it's outside of instruction selection.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> 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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Bugs aside, you're right. I made this table using the Mips target so it was switching from O2 to O0.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I'll try to fix that everything-is-O2 bug today/tomorrow. It will probably be tomorrow due to other commitments.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> This is a different motivation than Daniel expressed, which is a more principled idea coming from the "optnone should exactly match –O0" misconception<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It doesn't change anything but just for the record: While I do think that this should be true as far as possible, my opinion on -fast-isel=false is more rooted in the idea that emergency overrides shouldn't be overridable by normal use.
As an analogy, circuit breakers shouldn't be overridable by a light switch.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Robinson, Paul [mailto:Paul_Robinson@playstation.sony.com]
<br>
<b>Sent:</b> 18 November 2015 16:04<br>
<b>To:</b> Daniel Sanders; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> RE: Mips unconditionally uses fast-isel?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US" 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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" 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.<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"></a><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Daniel Sanders [mailto:Daniel.Sanders@imgtec.com]
<br>
<b>Sent:</b> Wednesday, November 18, 2015 2:19 AM<br>
<b>To:</b> Robinson, Paul; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> RE: Mips unconditionally uses fast-isel?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoPlainText">> <span lang="EN-US">-----Original Message-----</span><o:p></o:p></p>
<p class="MsoPlainText">> <span lang="EN-US">From: Robinson, Paul [<a href="mailto:Paul_Robinson@playstation.sony.com">mailto:Paul_Robinson@playstation.sony.com</a>]</span><o:p></o:p></p>
<p class="MsoPlainText">> <span lang="EN-US">Sent: 17 November 2015 22:58</span><o:p></o:p></p>
<p class="MsoPlainText">> <span lang="EN-US">To: Daniel Sanders; <a href="mailto:llvm-dev@lists.llvm.org">
llvm-dev@lists.llvm.org</a></span><o:p></o:p></p>
<p class="MsoPlainText">> <span lang="EN-US">Subject: RE: Mips unconditionally uses fast-isel?</span><o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
<p class="MsoPlainText">> > > The other thing that might work, is having TargetMachine remember how<o:p></o:p></p>
<p class="MsoPlainText">> > > the fast-isel option got set, and make OptLevelChanger do the right<o:p></o:p></p>
<p class="MsoPlainText">> > > thing. But that seems like a hack to work around Mips not obeying the<o:p></o:p></p>
<p class="MsoPlainText">> > > specified optimization level, honestly.<o:p></o:p></p>
<p class="MsoPlainText">> ><o:p></o:p></p>
<p class="MsoPlainText">> > I think we should do that as well. I don't think it's right that optnone<o:p></o:p></p>
<p class="MsoPlainText">> > enables Fast ISel even when it's been explicitly disabled. It should do<o:p></o:p></p>
<p class="MsoPlainText">> > the same checks as addPassesToGenerateCode() does.<o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
<p class="MsoPlainText">> Hm? What you're asking for is that "-O2" and "-O2 -fast-isel=none" are<o:p></o:p></p>
<p class="MsoPlainText">> identical, unless you have an 'optnone' function. Do you really have a<o:p></o:p></p>
<p class="MsoPlainText">> use-case for controlling the codegen path for an 'optnone' function?<o:p></o:p></p>
<p class="MsoPlainText">> The whole point of 'optnone' is to avoid optimizations.<o:p></o:p></p>
<p class="MsoPlainText">> --paulr<o:p></o:p></p>
<p class="MsoPlainText">> <o:p></o:p></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span 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.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">I'm saying that optnone means 'use –O0 for this function' and that optnone should<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">respect non-default values of the -fast-isel flag like –O0 does. This is the behaviour I'd expect:<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
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<td width="131" valign="top" style="width:98.55pt;border:solid windowtext 1.0pt;border-left:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-fast-isel=false<o:p></o:p></span></p>
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<td width="122" valign="top" style="width:91.8pt;border:solid windowtext 1.0pt;border-left:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-fast-isel=default<o:p></o:p></span></p>
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<td width="121" valign="top" style="width:90.85pt;border:solid windowtext 1.0pt;border-left:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-fast-isel=true<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O0<o:p></o:p></span></p>
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<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O0 + optnone attribute<o:p></o:p></span></p>
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<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG*<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O1 + optnone attribute<o:p></o:p></span></p>
</td>
<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG*<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O2 + optnone attribute<o:p></o:p></span></p>
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<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG*<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O3 + optnone attribute<o:p></o:p></span></p>
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<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG*<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O1<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<td width="161" valign="top" style="width:120.5pt;border:solid windowtext 1.0pt;border-top:none;background:#D9D9D9;padding:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">-O2<o:p></o:p></span></p>
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<td width="131" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<td width="122" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<td width="121" 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:0cm 5.4pt 0cm 5.4pt">
<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O3<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">SelectionDAG<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">The cells marked with '*' differ from the current behaviour.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="color:black">In terms of code, I think this part of OptLevelChanger::OptLevelChanger():<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> if (NewOptLevel == CodeGenOpt::None) {<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> DEBUG(dbgs() << "\nEnable FastISel for Function "<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> << IS.MF->getFunction()->getName() << "\n");<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> IS.TM.setFastISel(true);<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> }<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">Should be:<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> if (NewOptLevel == CodeGenOpt::None) {<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> DEBUG(dbgs() << "\nEnable FastISel for Function "<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> << IS.MF->getFunction()->getName() << "\n");<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> IS.TM.setFastISel(EnableFastISelOption != cl::BOU_FALSE);<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Courier New";color:black"> }<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">Where EnableFastISelOption has the same value as the global in LLVMTargetMachine.cpp<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span 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<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">than –O0. These hidden overrides exist to help us debug code generation problems and, faced with a code<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">generation bug, –fast-isel=false is useful for quickly determining whether it's in FastISel or somewhere else.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">The current behaviour allows optnone to overrule the hidden option to force-disable FastISel which will give<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="color:black">misleading guidance for bugs that lie in functions with optnone.<o:p></o:p></span></p>
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