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<p class="MsoPlainText">> <span lang="EN-US">-----Original Message-----</span></p>
<p class="MsoPlainText">> <span lang="EN-US">From: Robinson, Paul [mailto:Paul_Robinson@playstation.sony.com]</span></p>
<p class="MsoPlainText">> <span lang="EN-US">Sent: 17 November 2015 22:58</span></p>
<p class="MsoPlainText">> <span lang="EN-US">To: Daniel Sanders; llvm-dev@lists.llvm.org</span></p>
<p class="MsoPlainText">> <span lang="EN-US">Subject: RE: Mips unconditionally uses fast-isel?</span></p>
<p class="MsoPlainText">> </p>
<p class="MsoPlainText">> > > The other thing that might work, is having TargetMachine remember how</p>
<p class="MsoPlainText">> > > the fast-isel option got set, and make OptLevelChanger do the right</p>
<p class="MsoPlainText">> > > thing. But that seems like a hack to work around Mips not obeying the</p>
<p class="MsoPlainText">> > > specified optimization level, honestly.</p>
<p class="MsoPlainText">> ></p>
<p class="MsoPlainText">> > I think we should do that as well. I don't think it's right that optnone</p>
<p class="MsoPlainText">> > enables Fast ISel even when it's been explicitly disabled. It should do</p>
<p class="MsoPlainText">> > the same checks as addPassesToGenerateCode() does.</p>
<p class="MsoPlainText">> </p>
<p class="MsoPlainText">> Hm? What you're asking for is that "-O2" and "-O2 -fast-isel=none" are</p>
<p class="MsoPlainText">> identical, unless you have an 'optnone' function. Do you really have a</p>
<p class="MsoPlainText">> use-case for controlling the codegen path for an 'optnone' function?</p>
<p class="MsoPlainText">> The whole point of 'optnone' is to avoid optimizations.</p>
<p class="MsoPlainText">> --paulr</p>
<p class="MsoPlainText">> </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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<p class="MsoPlainText"><span style="color:black">-fast-isel=false<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-fast-isel=default<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-fast-isel=true<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O0<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">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O0 + optnone attribute<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">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O1 + optnone attribute<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">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O2 + optnone attribute<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">FastISel<o:p></o:p></span></p>
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<p class="MsoPlainText"><span style="color:black">-O3 + optnone attribute<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">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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<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">-O2<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">-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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