[llvm] r236267 - Simplify the creation of compressed debug sections.
David Blaikie
dblaikie at gmail.com
Thu Apr 30 14:59:12 PDT 2015
On Thu, Apr 30, 2015 at 2:51 PM, Rafael Espindola <
rafael.espindola at gmail.com> wrote:
> Author: rafael
> Date: Thu Apr 30 16:51:58 2015
> New Revision: 236267
>
> URL: http://llvm.org/viewvc/llvm-project?rev=236267&view=rev
> Log:
> Simplify the creation of compressed debug sections.
>
Awesome, thanks a bunch!
It was pretty confusing figuring out how to do this when I implemented it -
creating fragments, etc.
>
> This is actually fairly simple in the current code layout: Check if we
> should
> compress just before writing out and everything else just works.
>
> This removes the last case in which the object writer was creating a
> fragment.
>
> Modified:
> llvm/trunk/lib/MC/ELFObjectWriter.cpp
>
> Modified: llvm/trunk/lib/MC/ELFObjectWriter.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/ELFObjectWriter.cpp?rev=236267&r1=236266&r2=236267&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/MC/ELFObjectWriter.cpp (original)
> +++ llvm/trunk/lib/MC/ELFObjectWriter.cpp Thu Apr 30 16:51:58 2015
> @@ -182,8 +182,6 @@ class ELFObjectWriter : public MCObjectW
> support::endian::Writer<support::big>(OS).write(Val);
> }
>
> - template <typename T> void write(MCDataFragment &F, T Value);
> -
> void writeHeader(const MCAssembler &Asm);
>
> void WriteSymbol(SymbolTableWriter &Writer, ELFSymbolData &MSD,
> @@ -224,8 +222,6 @@ class ELFObjectWriter : public MCObjectW
> const MCSectionELF *createRelocationSection(MCAssembler &Asm,
> const MCSectionELF &Sec);
>
> - void CompressDebugSections(MCAssembler &Asm, MCAsmLayout &Layout);
> -
> const MCSectionELF *createSectionHeaderStringTable();
> const MCSectionELF *createStringTable(MCContext &Ctx);
>
> @@ -236,10 +232,13 @@ class ELFObjectWriter : public MCObjectW
> const SectionIndexMapTy &SectionIndexMap,
> const SectionOffsetsTy &SectionOffsets);
>
> + void writeSectionData(const MCAssembler &Asm, const MCSectionData &SD,
> + const MCAsmLayout &Layout);
> +
> void WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
> - uint64_t Address, uint64_t Offset,
> - uint64_t Size, uint32_t Link, uint32_t Info,
> - uint64_t Alignment, uint64_t EntrySize);
> + uint64_t Address, uint64_t Offset, uint64_t
> Size,
> + uint32_t Link, uint32_t Info, uint64_t
> Alignment,
> + uint64_t EntrySize);
>
> void writeRelocations(const MCAssembler &Asm, const MCSectionELF
> &Sec);
>
> @@ -267,15 +266,6 @@ unsigned ELFObjectWriter::addToSectionTa
> return SectionTable.size();
> }
>
> -template <typename T> void ELFObjectWriter::write(MCDataFragment &F, T
> Val) {
> - if (IsLittleEndian)
> - Val = support::endian::byte_swap<T, support::little>(Val);
> - else
> - Val = support::endian::byte_swap<T, support::big>(Val);
> - const char *Start = (const char *)&Val;
> - F.getContents().append(Start, Start + sizeof(T));
> -}
> -
> void SymbolTableWriter::createSymtabShndx() {
> if (!ShndxIndexes.empty())
> return;
> @@ -1105,7 +1095,7 @@ ELFObjectWriter::createRelocationSection
>
> static SmallVector<char, 128>
> getUncompressedData(const MCAsmLayout &Layout,
> - MCSectionData::FragmentListType &Fragments) {
> + const MCSectionData::FragmentListType &Fragments) {
> SmallVector<char, 128> UncompressedData;
> for (const MCFragment &F : Fragments) {
> const SmallVectorImpl<char> *Contents;
> @@ -1148,104 +1138,43 @@ prependCompressionHeader(uint64_t Size,
> return true;
> }
>
> -// 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(const MCAsmLayout &Layout,
> - MCSectionData::FragmentListType &Fragments) {
> - std::unique_ptr<MCDataFragment> CompressedFragment(new
> MCDataFragment());
> +void ELFObjectWriter::writeSectionData(const MCAssembler &Asm,
> + const MCSectionData &SD,
> + const MCAsmLayout &Layout) {
> + 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 (!Asm.getContext().getAsmInfo()->compressDebugSections() ||
> + !SectionName.startswith(".debug_") || SectionName ==
> ".debug_frame") {
> + Asm.writeSectionData(&SD, Layout);
> + return;
> + }
>
> - // Gather the uncompressed data from all the fragments, recording the
> - // alignment fragment, if seen, and any fixups.
> + // Gather the uncompressed data from all the fragments.
> + const MCSectionData::FragmentListType &Fragments = SD.getFragmentList();
> SmallVector<char, 128> UncompressedData =
> getUncompressedData(Layout, Fragments);
>
> - SmallVectorImpl<char> &CompressedContents =
> CompressedFragment->getContents();
> -
> + SmallVector<char, 128> CompressedContents;
> zlib::Status Success = zlib::compress(
> StringRef(UncompressedData.data(), UncompressedData.size()),
> CompressedContents);
> - if (Success != zlib::StatusOK)
> - return nullptr;
> -
> - if (!prependCompressionHeader(UncompressedData.size(),
> CompressedContents))
> - return nullptr;
> -
> - return CompressedFragment;
> -}
> -
> -typedef DenseMap<const MCSectionData *, std::vector<MCSymbolData *>>
> -DefiningSymbolMap;
> -
> -static void UpdateSymbols(const MCAsmLayout &Layout,
> - const std::vector<MCSymbolData *> &Symbols,
> - MCFragment &NewFragment) {
> - for (MCSymbolData *Sym : Symbols) {
> - Sym->setOffset(Sym->getOffset() +
> - Layout.getFragmentOffset(Sym->getFragment()));
> - Sym->setFragment(&NewFragment);
> + if (Success != zlib::StatusOK) {
> + Asm.writeSectionData(&SD, Layout);
> + return;
> }
> -}
>
> -static void CompressDebugSection(MCAssembler &Asm, MCAsmLayout &Layout,
> - const DefiningSymbolMap &DefiningSymbols,
> - 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)
> + if (!prependCompressionHeader(UncompressedData.size(),
> CompressedContents)) {
> + Asm.writeSectionData(&SD, Layout);
> return;
> -
> - // Update the fragment+offsets of any symbols referring to fragments in
> this
> - // section to refer to the new fragment.
> - auto I = DefiningSymbols.find(&SD);
> - if (I != DefiningSymbols.end())
> - UpdateSymbols(Layout, I->second, *CompressedFragment);
> -
> - // 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;
> -
> - DefiningSymbolMap DefiningSymbols;
> -
> - for (MCSymbolData &SD : Asm.symbols())
> - if (MCFragment *F = SD.getFragment())
> - DefiningSymbols[F->getParent()].push_back(&SD);
> -
> - 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, DefiningSymbols, Section, SD);
> - }
> + OS << CompressedContents;
> }
>
> void ELFObjectWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type,
> @@ -1408,8 +1337,6 @@ void ELFObjectWriter::writeSectionHeader
>
> void ELFObjectWriter::WriteObject(MCAssembler &Asm,
> const MCAsmLayout &Layout) {
> - CompressDebugSections(Asm, const_cast<MCAsmLayout &>(Layout));
> -
> MCContext &Ctx = Asm.getContext();
> const MCSectionELF *ShstrtabSection =
> Ctx.getELFSection(".shstrtab", ELF::SHT_STRTAB, 0);
> @@ -1453,7 +1380,7 @@ void ELFObjectWriter::WriteObject(MCAsse
> }
> writeRelocations(Asm, *Section.getAssociatedSection());
> } else {
> - Asm.writeSectionData(&SD, Layout);
> + writeSectionData(Asm, SD, Layout);
> }
>
> uint64_t SecEnd = OS.tell();
>
>
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