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<p class="MsoNormal">(reply inline)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b>From:</b> Chris Lattner <clattner@nondot.org>
<br>
<b>Sent:</b> Sunday, April 12, 2020 3:28 PM<br>
<b>To:</b> Eli Friedman <efriedma@quicinc.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] Re: [llvm-dev] LLVM multithreading support<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in">On Apr 12, 2020, at 2:23 PM, Eli Friedman <<a href="mailto:efriedma@quicinc.com">efriedma@quicinc.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">So probably I’d recommend two things:<o:p></o:p></p>
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<![if !supportLists]><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>If locking uncontended locks is showing up on profiles as a performance bottleneck, it’s probably worth looking into ways to reduce that overhead in both single-threaded and multi-threaded contexts. (Reducing the number of locks taken
in frequently called code, or using a better lock implementation).<o:p></o:p></p>
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<![if !supportLists]><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>If you want some mechanism to disable MLIR locking, it should probably be a boolean attached to the MLIR context in question, not a global variable.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Ok, but let me argue the other way. We currently have a cmake flag that sets LLVM_ENABLE_THREADS, and that flag enables an across the board speedup. That cmake flag is the *worst* possible thing for library composability.
:-). Are you suggesting that we remove it?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
<p class="MsoNormal">Yes, I would like to remove LLVM_ENABLE_THREADS. Assuming you’re not building for some exotic target that doesn’t have threads, there isn’t any reason to randomly shut off all thread-related functionality in the LLVM support libraries.
There isn’t any significant performance or codesize gain to be had outside of MLIR, as far as I know, and it increases the number of configurations we have to worry about. I have no idea if turning it off even works on master; I don’t know of any buildbots
or users using that configuration.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If you want to support some sort of lockless mode in MLIR, I think that burden should be carried as part of MLIR, instead of infecting the entire LLVM codebase.<o:p></o:p></p>
<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal">-Eli <o:p></o:p></p>
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