<html><head></head><body>One last thing I remembered. Most of the regression is because of massive branch mis prediction, which is not surprising in a hot binary search.<br>
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In <a href="http://cglab.ca/~morin/misc/arraylayout-v2">http://cglab.ca/~morin/misc/arraylayout-v2</a>/ there more efficient algorithms that don't need mis predicted branches.<br>
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Last time I tried clang was creating more branches than expected, but that might be fixed now.<br>
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Cheers,<br>
Rafael<br>
<br><br><div class="gmail_quote">On November 3, 2017 1:21:42 PM PDT, Rafael Avila de Espindola <rafael.espindola@gmail.com> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<pre class="k9mail">I found a fascinating fact about what regressed and what improved with<br />the cache:<br /><br />Testcases that are compiled by gcc and have debug info benefit<br />tremendously: scylla got 20% faster and mozilla 15%.<br /><br />Tests without debug information benefited a bit.<br /><br />The one test we have with clang debug info (clang-gdb-index) regressed a<br />bit (1.3%).<br /><br />So there is some difference that causes gcc produced debug info to hit a<br />lot more relocations to a symbol in a merge section. Not sure what that<br />is.<br /><br />Cheers,<br />Rafael<br /><br />Rui Ueyama via llvm-commits <llvm-commits@lists.llvm.org> writes:<br /><br /><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;"> Author: ruiu<br /> Date: Tue Oct 31 12:14:06 2017<br /> New Revision: 317026<br /><br /> URL: <a href="http://llvm.org/viewvc/llvm-project?rev=317026&view=rev">http://llvm.org/viewvc/llvm-project?rev=317026&view=rev</a><br /> Log:<br /> Revert r316305: Remove a fast lookup table from MergeInputSection.<br /><br /> This reverts commit r316305 because performance regression was observed.<br /><br /> Modified:<br /> lld/trunk/ELF/InputSection.cpp<br /> lld/trunk/ELF/InputSection.h<br /><br /> Modified: lld/trunk/ELF/InputSection.cpp<br /> URL: <a href="http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=317026&r1=317025&r2=317026&view=diff">http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=317026&r1=317025&r2=317026&view=diff</a><br /><hr /><br /> --- lld/trunk/ELF/InputSection.cpp (original)<br /> +++ lld/trunk/ELF/InputSection.cpp Tue Oct 31 12:14:06 2017<br /> @@ -968,9 +968,24 @@ uint64_t MergeInputSection::getOffset(ui<br /> if (!Live)<br /> return 0;<br /> <br /> - const SectionPiece &Piece = *getSectionPiece(Offset);<br /> + // Initialize OffsetMap lazily.<br /> + llvm::call_once(InitOffsetMap, [&] {<br /> + OffsetMap.reserve(Pieces.size());<br /> + for (size_t I = 0; I < Pieces.size(); ++I)<br /> + OffsetMap[Pieces[I].InputOff] = I;<br /> + });<br /> +<br /> + // Find a string starting at a given offset.<br /> + auto It = OffsetMap.find(Offset);<br /> + if (It != OffsetMap.end())<br /> + return Pieces[It->second].OutputOff;<br /> +<br /> + // If Offset is not at beginning of a section piece, it is not in the map.<br /> + // In that case we need to search from the original section piece vector.<br /> + const SectionPiece &Piece = *this->getSectionPiece(Offset);<br /> if (!Piece.Live)<br /> return 0;<br /> +<br /> uint64_t Addend = Offset - Piece.InputOff;<br /> return Piece.OutputOff + Addend;<br /> }<br /><br /> Modified: lld/trunk/ELF/InputSection.h<br /> URL: <a href="http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=317026&r1=317025&r2=317026&view=diff">http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.h?rev=317026&r1=317025&r2=317026&view=diff</a><br /><hr /><br /> --- lld/trunk/ELF/InputSection.h (original)<br /> +++ lld/trunk/ELF/InputSection.h Tue Oct 31 12:14:06 2017<br /> @@ -263,6 +263,9 @@ private:<br /> void splitStrings(ArrayRef<uint8_t> A, size_t Size);<br /> void splitNonStrings(ArrayRef<uint8_t> A, size_t Size);<br /> <br /> + mutable llvm::DenseMap<uint32_t, uint32_t> OffsetMap;<br /> + mutable llvm::once_flag InitOffsetMap;<br /> +<br /> llvm::DenseSet<uint64_t> LiveOffsets;<br /> };<br /> <br /><br /><br /><hr /><br /> llvm-commits mailing list<br /> llvm-commits@lists.llvm.org<br /> <a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits</a><br /></blockquote></pre></blockquote></div><br>
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