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author | David Blaikie <dblaikie@gmail.com> | 2014-04-10 21:53:53 +0000 |
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committer | David Blaikie <dblaikie@gmail.com> | 2014-04-10 21:53:53 +0000 |
commit | fe255214516f62b9ec2d37dc44eab879f5576ea6 (patch) | |
tree | 96660d816ea028d50d34d20d024f9cf5a1353f3a /lib | |
parent | 22f3236dd688ec4109ed02a93c3f0ada8887d682 (diff) | |
download | llvm-fe255214516f62b9ec2d37dc44eab879f5576ea6.tar.gz llvm-fe255214516f62b9ec2d37dc44eab879f5576ea6.tar.bz2 llvm-fe255214516f62b9ec2d37dc44eab879f5576ea6.tar.xz |
Reimplement debug info compression by compressing the whole section, rather than a fragment.
To support compressing the debug_line section that contains multiple
fragments (due, I believe, to variation in choices of line table
encoding depending on the size of instruction ranges in the actual
program code) we needed to support compressing multiple MCFragments in a
single pass.
This patch implements that behavior by mutating the post-relaxed and
relocated section to be the compressed form of its former self,
including renaming the section.
This is a more flexible (and less invasive, to a degree) implementation
that will allow for other features such as "use compression only if it's
smaller than the uncompressed data".
Compressing debug_frame would be a possible further extension to this
work, but I've left it for now. The hurdle there is alignment sections -
which might require going as far as to refactor
MCAssembler.cpp:writeFragment to handle writing to a byte buffer or an
MCObjectWriter (there's already a virtual call there, so it shouldn't
add substantial compile-time cost) which could in turn involve
refactoring MCAsmBackend::writeNopData to use that same abstraction...
which involves touching all the backends. This would remove the limited
handling of fragment writing seen in
ELFObjectWriter.cpp:getUncompressedData which would be nice - but it's
more invasive.
I did discover that I (perhaps obviously) don't need to handle
relocations when I rewrite the fragments - since the relocations have
already been applied and computed (and stored into
ELFObjectWriter::Relocations) by this stage (necessarily, because we
need to have written any immediate values or assembly-time relocations
into the data already before we compress it, which we have). The test
case doesn't necessarily cover that in detail - I can add more test
coverage if that's preferred.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205990 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib')
-rw-r--r-- | lib/MC/ELFObjectWriter.cpp | 114 | ||||
-rw-r--r-- | lib/MC/MCContext.cpp | 15 |
2 files changed, 129 insertions, 0 deletions
diff --git a/lib/MC/ELFObjectWriter.cpp b/lib/MC/ELFObjectWriter.cpp index e9b8fe2965..e802c01fea 100644 --- a/lib/MC/ELFObjectWriter.cpp +++ b/lib/MC/ELFObjectWriter.cpp @@ -17,6 +17,7 @@ #include "llvm/ADT/SmallString.h" #include "llvm/ADT/StringMap.h" #include "llvm/MC/MCAsmBackend.h" +#include "llvm/MC/MCAsmInfo.h" #include "llvm/MC/MCAsmLayout.h" #include "llvm/MC/MCAssembler.h" #include "llvm/MC/MCContext.h" @@ -27,6 +28,7 @@ #include "llvm/MC/MCObjectWriter.h" #include "llvm/MC/MCSectionELF.h" #include "llvm/MC/MCValue.h" +#include "llvm/Support/Compression.h" #include "llvm/Support/Debug.h" #include "llvm/Support/Endian.h" #include "llvm/Support/ELF.h" @@ -253,6 +255,8 @@ class ELFObjectWriter : public MCObjectWriter { void CreateRelocationSections(MCAssembler &Asm, MCAsmLayout &Layout, RelMapTy &RelMap); + void CompressDebugSections(MCAssembler &Asm, MCAsmLayout &Layout); + void WriteRelocations(MCAssembler &Asm, MCAsmLayout &Layout, const RelMapTy &RelMap); @@ -1168,6 +1172,114 @@ void ELFObjectWriter::CreateRelocationSections(MCAssembler &Asm, } } +static SmallVector<char, 128> getUncompressedData(MCAsmLayout &Layout, MCSectionData::FragmentListType &Fragments) { + SmallVector<char, 128> UncompressedData; + for (const MCFragment &F : Fragments) { + const SmallVectorImpl<char> *Contents; + switch (F.getKind()) { + case MCFragment::FT_Data: + Contents = &cast<MCDataFragment>(F).getContents(); + break; + case MCFragment::FT_Dwarf: + Contents = &cast<MCDwarfLineAddrFragment>(F).getContents(); + break; + case MCFragment::FT_DwarfFrame: + Contents = &cast<MCDwarfCallFrameFragment>(F).getContents(); + break; + default: + llvm_unreachable( + "Not expecting any other fragment types in a debug_* section"); + } + UncompressedData.append(Contents->begin(), Contents->end()); + } + return UncompressedData; +} + +// Include the debug info compression header: +// "ZLIB" followed by 8 bytes representing the uncompressed size of the section, +// useful for consumers to preallocate a buffer to decompress into. +static void prependCompressionHeader(uint64_t Size, SmallVectorImpl<char> &CompressedContents) { + static const StringRef Magic = "ZLIB"; + if (sys::IsLittleEndianHost) + Size = sys::SwapByteOrder(Size); + CompressedContents.insert(CompressedContents.begin(), + Magic.size() + sizeof(Size), 0); + std::copy(Magic.begin(), Magic.end(), CompressedContents.begin()); + std::copy(reinterpret_cast<char *>(&Size), + reinterpret_cast<char *>(&Size + 1), + CompressedContents.begin() + Magic.size()); +} + +// Return a single fragment containing the compressed contents of the whole +// section. Null if the section was not compressed for any reason. +static std::unique_ptr<MCDataFragment> getCompressedFragment(MCAsmLayout &Layout, MCSectionData::FragmentListType &Fragments) { + std::unique_ptr<MCDataFragment> CompressedFragment(new MCDataFragment()); + + // Gather the uncompressed data from all the fragments, recording the + // alignment fragment, if seen, and any fixups. + SmallVector<char, 128> UncompressedData = + getUncompressedData(Layout, Fragments); + + SmallVectorImpl<char> &CompressedContents = CompressedFragment->getContents(); + + zlib::Status Success = zlib::compress( + StringRef(UncompressedData.data(), UncompressedData.size()), + CompressedContents); + if (Success != zlib::StatusOK) + return nullptr; + + prependCompressionHeader(UncompressedData.size(), CompressedContents); + + return CompressedFragment; +} + +static void CompressDebugSection(MCAssembler &Asm, MCAsmLayout &Layout, + const MCSectionELF &Section, + MCSectionData &SD) { + StringRef SectionName = Section.getSectionName(); + MCSectionData::FragmentListType &Fragments = SD.getFragmentList(); + + std::unique_ptr<MCDataFragment> CompressedFragment = + getCompressedFragment(Layout, Fragments); + + // Leave the section as-is if the fragments could not be compressed. + if (!CompressedFragment) + return; + + // Invalidate the layout for the whole section since it will have new and + // different fragments now. + Layout.invalidateFragmentsFrom(&Fragments.front()); + Fragments.clear(); + + // Complete the initialization of the new fragment + CompressedFragment->setParent(&SD); + CompressedFragment->setLayoutOrder(0); + Fragments.push_back(CompressedFragment.release()); + + // Rename from .debug_* to .zdebug_* + Asm.getContext().renameELFSection(&Section, + (".z" + SectionName.drop_front(1)).str()); +} + +void ELFObjectWriter::CompressDebugSections(MCAssembler &Asm, + MCAsmLayout &Layout) { + if (!Asm.getContext().getAsmInfo()->compressDebugSections()) + return; + + for (MCSectionData &SD : Asm) { + const MCSectionELF &Section = static_cast<const MCSectionELF&>(SD.getSection()); + StringRef SectionName = Section.getSectionName(); + + // Compressing debug_frame requires handling alignment fragments which is + // more work (possibly generalizing MCAssembler.cpp:writeFragment to allow + // for writing to arbitrary buffers) for little benefit. + if (!SectionName.startswith(".debug_") || SectionName == ".debug_frame") + continue; + + CompressDebugSection(Asm, Layout, Section, SD); + } +} + void ELFObjectWriter::WriteRelocations(MCAssembler &Asm, MCAsmLayout &Layout, const RelMapTy &RelMap) { for (MCAssembler::const_iterator it = Asm.begin(), @@ -1652,6 +1764,8 @@ void ELFObjectWriter::WriteObject(MCAssembler &Asm, unsigned NumUserSections = Asm.size(); + CompressDebugSections(Asm, const_cast<MCAsmLayout&>(Layout)); + DenseMap<const MCSectionELF*, const MCSectionELF*> RelMap; CreateRelocationSections(Asm, const_cast<MCAsmLayout&>(Layout), RelMap); diff --git a/lib/MC/MCContext.cpp b/lib/MC/MCContext.cpp index 9dfd6367ed..8c2b509530 100644 --- a/lib/MC/MCContext.cpp +++ b/lib/MC/MCContext.cpp @@ -251,6 +251,21 @@ getELFSection(StringRef Section, unsigned Type, unsigned Flags, return getELFSection(Section, Type, Flags, Kind, 0, ""); } +void MCContext::renameELFSection(const MCSectionELF *Section, StringRef Name) { + if (ELFUniquingMap == 0) + ELFUniquingMap = new ELFUniqueMapTy(); + ELFUniqueMapTy &Map = *(ELFUniqueMapTy*)ELFUniquingMap; + + StringRef GroupName; + if (const MCSymbol *Group = Section->getGroup()) + GroupName = Group->getName(); + + Map.erase(SectionGroupPair(Section->getSectionName(), GroupName)); + auto I = Map.insert(std::make_pair(SectionGroupPair(Name, GroupName), + Section)).first; + const_cast<MCSectionELF*>(Section)->setSectionName(I->first.first); +} + const MCSectionELF *MCContext:: getELFSection(StringRef Section, unsigned Type, unsigned Flags, SectionKind Kind, unsigned EntrySize, StringRef Group) { |