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<p class="MsoNormal"><span lang="EN-US">Hi Teresa,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks for the detailed reply!<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> How are you creating your bitcode files?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I create the bitcode with `-flto=thin -c` and sure it has a GLOBALVAL_SUMMARY_BLOCK. And there’s no RegularLTO partition only ThinLTO bicode.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> Where is it aborting in the backend?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">It aborts at ` report_fatal_error("Failed to setup codegen")` in of codegen() of LTOBackend.cpp. And before that in createTargetMachine() it also warns
</span>‘<span lang="EN-US">xxx is not a recognized processor</span>’<span lang="EN-US">. But the program should return before run into backend() IIUC since all I want is using lld with `-thinlto-index-only` option to generate *.thinlto.bc and feed it and bitocde
to clang to invoke target-specific optimizations(we use gnu as and ld).Do you have any suggestion about this?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Mindong<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Teresa Johnson [mailto:tejohnson@google.com]
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<b>Sent:</b> Wednesday, November 27, 2019 11:12 PM<br>
<b>To:</b> chenmindong <chenmindong1@huawei.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org; Yuchao (Michael) <michael.yuchao@huawei.com><br>
<b>Subject:</b> Re: [llvm-dev] ThinLTO Problem<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Hi Mindong,<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Wed, Nov 27, 2019 at 3:29 AM chenmindong via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Hi,<br>
<br>
I'm working on enabling thinLTO for our custom backend on LLVM-8 with lld to get code size benefits from dead symbol elimination. The code in LTO::run() of LTO.cpp confuses me that, even though thinLTO is specified, runRegularLTO() will be run first and its
return value determines whether runThinLTO() will be executed. <br>
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My question is if it's clearly known that thinLTO is used, is it still necessary to execute runRegularLTO()?If it is, what's the reason behind?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">This is to handle the case where the LTO link is given a mix of regular and thin LTO bitcode - it should do regular LTO on that subset and ThinLTO on the other. The other case this handles is when a bitcode object is
split into regular and thin LTO halves - this is enabled for things like CFI but shouldn't be the default currently (you'd have to build with -fsplit-lto-unit to get it unless you are building with CFI).<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Whether the bitcode is added to the regular or Thin LTO partition is determined in LTO::addModule, and is based on a flag set when the bitcode is read which is based in turn on whether the bitcode has a summary block,
and whether that is a thinlto or regular (full) LTO summary block. How are you creating your bitcode files? If you run llvm-dis on it does it have summary entries? You can also see if you run llvm-bcanalyzer whether it has a GLOBALVAL_SUMMARY_BLOCK or a FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
or neither. If you are compiling with -flto=thin you should realistically have the former, which would make it a ThinLTO bitcode. But it sounds like you have some objects that either don't or have the full LTO summary.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">For now our custom backend, where distributed thinLTO is adopted, it works fine as I removed the line executing runRegularLTO(). But if I preserve it, the code fails the `if (Conf.PostInternalizeModuleHook &&!Conf.PostInternalizeModuleHook(0,
*RegularLTO.CombinedModule))`, which I also don't understand, and fall through to backend() and abort there. I believe something is missed during adding the target support but cannot figure it out. Could anyone help?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">By default there should not be a PostInternalizeModuleHook set (it is set to support cases like -save-temps), so it isn't surprising that it would fail that test and fall through to the backend() call, which is what you
would want if there was a regular LTO partition. Where is it aborting in the backend?<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Teresa<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><br>
Regards,<br>
Mindong Chen<br>
_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></span></p>
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<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">-- <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-family:"Arial",sans-serif;color:#555555">Teresa Johnson |<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-family:"Arial",sans-serif;color:#555555"> Software Engineer |<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-family:"Arial",sans-serif;color:#555555"> </span><span lang="EN-US"><a href="mailto:tejohnson@google.com" target="_blank"><span style="font-family:"Arial",sans-serif">tejohnson@google.com</span></a></span><span lang="EN-US" style="font-family:"Arial",sans-serif;color:#555555"> |<o:p></o:p></span></p>
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