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[MC][ELF] Eliminate some hash maps from ELFObjectWriter (#97421)
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Remove some maps. Mostly cleanup, only a slight performance win.

- Replace SectionIndexMap with layout order: The section layout order is
only used in MachO, so we can repurpose the field as section table
index.
- Store section offsets in MCSectionELF: No need for a map, and
especially not a std::map. Direct access to the underlying (and easily
modifyable) data structure is always faster.
- Improve storage of groups: There's no point in having a DenseMap, the
number of sections and groups are reasonably small to use vectors.
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@aengelke
aengelke authored Jul 3, 2024
1 parent 96c18a2 commit d548020
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Showing 2 changed files with 60 additions and 72 deletions.
12 changes: 12 additions & 0 deletions llvm/include/llvm/MC/MCSectionELF.h
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,10 @@ class MCSectionELF final : public MCSection {
/// section header index of the section where LinkedToSym is defined.
const MCSymbol *LinkedToSym;

/// Start/end offset in file, used by ELFWriter.
uint64_t StartOffset;
uint64_t EndOffset;

private:
friend class MCContext;

Expand Down Expand Up @@ -92,6 +96,14 @@ class MCSectionELF final : public MCSection {
}
const MCSymbol *getLinkedToSymbol() const { return LinkedToSym; }

void setOffsets(uint64_t Start, uint64_t End) {
StartOffset = Start;
EndOffset = End;
}
std::pair<uint64_t, uint64_t> getOffsets() const {
return std::make_pair(StartOffset, EndOffset);
}

static bool classof(const MCSection *S) {
return S->getVariant() == SV_ELF;
}
Expand Down
120 changes: 48 additions & 72 deletions llvm/lib/MC/ELFObjectWriter.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -66,8 +66,6 @@ using namespace llvm;

namespace {

using SectionIndexMapTy = DenseMap<const MCSectionELF *, uint32_t>;

class ELFObjectWriter;
struct ELFWriter;

Expand Down Expand Up @@ -136,8 +134,8 @@ struct ELFWriter {
unsigned SymbolTableIndex = ~0u;

// Sections in the order they are to be output in the section table.
std::vector<const MCSectionELF *> SectionTable;
unsigned addToSectionTable(const MCSectionELF *Sec);
std::vector<MCSectionELF *> SectionTable;
unsigned addToSectionTable(MCSectionELF *Sec);

// TargetObjectWriter wrappers.
bool is64Bit() const;
Expand Down Expand Up @@ -171,31 +169,21 @@ struct ELFWriter {
void writeSymbol(const MCAssembler &Asm, SymbolTableWriter &Writer,
uint32_t StringIndex, ELFSymbolData &MSD);

// Start and end offset of each section
using SectionOffsetsTy =
std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>;

// Map from a signature symbol to the group section index
using RevGroupMapTy = DenseMap<const MCSymbol *, unsigned>;

/// Compute the symbol table data
///
/// \param Asm - The assembler.
/// \param SectionIndexMap - Maps a section to its index.
/// \param RevGroupMap - Maps a signature symbol to the group section.
void computeSymbolTable(MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
const RevGroupMapTy &RevGroupMap,
SectionOffsetsTy &SectionOffsets);
void computeSymbolTable(MCAssembler &Asm, const RevGroupMapTy &RevGroupMap);

void writeAddrsigSection();

MCSectionELF *createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec);

void writeSectionHeader(const MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets);
void writeSectionHeader(const MCAssembler &Asm);

void writeSectionData(const MCAssembler &Asm, MCSection &Sec);

Expand All @@ -207,8 +195,7 @@ struct ELFWriter {
void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);

uint64_t writeObject(MCAssembler &Asm);
void writeSection(const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
void writeSection(uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
const MCSectionELF &Section);
};

Expand Down Expand Up @@ -330,7 +317,7 @@ uint64_t ELFWriter::align(Align Alignment) {
return NewOffset;
}

unsigned ELFWriter::addToSectionTable(const MCSectionELF *Sec) {
unsigned ELFWriter::addToSectionTable(MCSectionELF *Sec) {
SectionTable.push_back(Sec);
StrTabBuilder.add(Sec->getName());
return SectionTable.size();
Expand Down Expand Up @@ -612,9 +599,7 @@ bool ELFWriter::isInSymtab(const MCAssembler &Asm, const MCSymbolELF &Symbol,
}

void ELFWriter::computeSymbolTable(MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
const RevGroupMapTy &RevGroupMap,
SectionOffsetsTy &SectionOffsets) {
const RevGroupMapTy &RevGroupMap) {
MCContext &Ctx = Asm.getContext();
SymbolTableWriter Writer(*this, is64Bit());

Expand Down Expand Up @@ -697,7 +682,7 @@ void ELFWriter::computeSymbolTable(MCAssembler &Asm,

if (Mode == NonDwoOnly && isDwoSection(Section))
continue;
MSD.SectionIndex = SectionIndexMap.lookup(&Section);
MSD.SectionIndex = Section.getOrdinal();
assert(MSD.SectionIndex && "Invalid section index!");
if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
HasLargeSectionIndex = true;
Expand Down Expand Up @@ -775,7 +760,7 @@ void ELFWriter::computeSymbolTable(MCAssembler &Asm,
}

uint64_t SecEnd = W.OS.tell();
SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
SymtabSection->setOffsets(SecStart, SecEnd);

ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
if (ShndxIndexes.empty()) {
Expand All @@ -785,12 +770,11 @@ void ELFWriter::computeSymbolTable(MCAssembler &Asm,
assert(SymtabShndxSectionIndex != 0);

SecStart = W.OS.tell();
const MCSectionELF *SymtabShndxSection =
SectionTable[SymtabShndxSectionIndex - 1];
MCSectionELF *SymtabShndxSection = SectionTable[SymtabShndxSectionIndex - 1];
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = W.OS.tell();
SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
SymtabShndxSection->setOffsets(SecStart, SecEnd);
}

void ELFWriter::writeAddrsigSection() {
Expand Down Expand Up @@ -1030,8 +1014,7 @@ void ELFWriter::writeRelocations(const MCAssembler &Asm,
}
}

void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex, uint64_t Offset,
void ELFWriter::writeSection(uint32_t GroupSymbolIndex, uint64_t Offset,
uint64_t Size, const MCSectionELF &Section) {
uint64_t sh_link = 0;
uint64_t sh_info = 0;
Expand All @@ -1050,7 +1033,7 @@ void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
sh_link = SymbolTableIndex;
assert(sh_link && ".symtab not found");
const MCSection *InfoSection = Section.getLinkedToSection();
sh_info = SectionIndexMap.lookup(cast<MCSectionELF>(InfoSection));
sh_info = InfoSection->getOrdinal();
break;
}

Expand All @@ -1075,10 +1058,8 @@ void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
// If the value in the associated metadata is not a definition, Sym will be
// undefined. Represent this with sh_link=0.
const MCSymbol *Sym = Section.getLinkedToSymbol();
if (Sym && Sym->isInSection()) {
const MCSectionELF *Sec = cast<MCSectionELF>(&Sym->getSection());
sh_link = SectionIndexMap.lookup(Sec);
}
if (Sym && Sym->isInSection())
sh_link = Sym->getSection().getOrdinal();
}

WriteSecHdrEntry(StrTabBuilder.getOffset(Section.getName()),
Expand All @@ -1087,9 +1068,7 @@ void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
Section.getEntrySize());
}

void ELFWriter::writeSectionHeader(const MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets) {
void ELFWriter::writeSectionHeader(const MCAssembler &Asm) {
const unsigned NumSections = SectionTable.size();

// Null section first.
Expand All @@ -1105,16 +1084,14 @@ void ELFWriter::writeSectionHeader(const MCAssembler &Asm,
else
GroupSymbolIndex = Section->getGroup()->getIndex();

const std::pair<uint64_t, uint64_t> &Offsets =
SectionOffsets.find(Section)->second;
std::pair<uint64_t, uint64_t> Offsets = Section->getOffsets();
uint64_t Size;
if (Type == ELF::SHT_NOBITS)
Size = Asm.getSectionAddressSize(*Section);
else
Size = Offsets.second - Offsets.first;

writeSection(SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
*Section);
writeSection(GroupSymbolIndex, Offsets.first, Size, *Section);
}
}

Expand All @@ -1127,17 +1104,15 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
StringTableIndex = addToSectionTable(StrtabSection);

RevGroupMapTy RevGroupMap;
SectionIndexMapTy SectionIndexMap;

DenseMap<const MCSymbol *, SmallVector<const MCSectionELF *, 0>> GroupMembers;

// Write out the ELF header ...
writeHeader(Asm);

// ... then the sections ...
SectionOffsetsTy SectionOffsets;
std::vector<MCSectionELF *> Groups;
std::vector<MCSectionELF *> Relocations;
SmallVector<std::pair<MCSectionELF *, SmallVector<unsigned>>, 0> Groups;
// Map from group section index to group
SmallVector<unsigned, 0> GroupMap;
SmallVector<MCSectionELF *> Relocations;
for (MCSection &Sec : Asm) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
if (Mode == NonDwoOnly && isDwoSection(Section))
Expand All @@ -1152,49 +1127,50 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
writeSectionData(Asm, Section);

uint64_t SecEnd = W.OS.tell();
SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
Section.setOffsets(SecStart, SecEnd);

MCSectionELF *RelSection = createRelocationSection(Ctx, Section);

unsigned *GroupIdxEntry = nullptr;
if (SignatureSymbol) {
unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
if (!GroupIdx) {
GroupIdxEntry = &RevGroupMap[SignatureSymbol];
if (!*GroupIdxEntry) {
MCSectionELF *Group =
Ctx.createELFGroupSection(SignatureSymbol, Section.isComdat());
GroupIdx = addToSectionTable(Group);
*GroupIdxEntry = addToSectionTable(Group);
Group->setAlignment(Align(4));
Groups.push_back(Group);

GroupMap.resize(*GroupIdxEntry + 1);
GroupMap[*GroupIdxEntry] = Groups.size();
Groups.emplace_back(Group, SmallVector<unsigned>{});
}
SmallVector<const MCSectionELF *, 0> &Members =
GroupMembers[SignatureSymbol];
Members.push_back(&Section);
if (RelSection)
Members.push_back(RelSection);
}

SectionIndexMap[&Section] = addToSectionTable(&Section);
Section.setOrdinal(addToSectionTable(&Section));
if (RelSection) {
SectionIndexMap[RelSection] = addToSectionTable(RelSection);
RelSection->setOrdinal(addToSectionTable(RelSection));
Relocations.push_back(RelSection);
}

if (GroupIdxEntry) {
auto &Members = Groups[GroupMap[*GroupIdxEntry]];
Members.second.push_back(Section.getOrdinal());
if (RelSection)
Members.second.push_back(RelSection->getOrdinal());
}

OWriter.TargetObjectWriter->addTargetSectionFlags(Ctx, Section);
}

for (MCSectionELF *Group : Groups) {
for (auto &[Group, Members] : Groups) {
// Remember the offset into the file for this section.
const uint64_t SecStart = align(Group->getAlign());

const MCSymbol *SignatureSymbol = Group->getGroup();
assert(SignatureSymbol);
write(uint32_t(Group->isComdat() ? unsigned(ELF::GRP_COMDAT) : 0));
for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
uint32_t SecIndex = SectionIndexMap.lookup(Member);
write(SecIndex);
}
W.write<unsigned>(Members);

uint64_t SecEnd = W.OS.tell();
SectionOffsets[Group] = std::make_pair(SecStart, SecEnd);
Group->setOffsets(SecStart, SecEnd);
}

if (Mode == DwoOnly) {
Expand All @@ -1210,7 +1186,7 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
}

// Compute symbol table information.
computeSymbolTable(Asm, SectionIndexMap, RevGroupMap, SectionOffsets);
computeSymbolTable(Asm, RevGroupMap);

for (MCSectionELF *RelSection : Relocations) {
// Remember the offset into the file for this section.
Expand All @@ -1220,27 +1196,27 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
cast<MCSectionELF>(*RelSection->getLinkedToSection()));

uint64_t SecEnd = W.OS.tell();
SectionOffsets[RelSection] = std::make_pair(SecStart, SecEnd);
RelSection->setOffsets(SecStart, SecEnd);
}

if (OWriter.EmitAddrsigSection) {
uint64_t SecStart = W.OS.tell();
writeAddrsigSection();
uint64_t SecEnd = W.OS.tell();
SectionOffsets[AddrsigSection] = std::make_pair(SecStart, SecEnd);
AddrsigSection->setOffsets(SecStart, SecEnd);
}
}

{
uint64_t SecStart = W.OS.tell();
StrTabBuilder.write(W.OS);
SectionOffsets[StrtabSection] = std::make_pair(SecStart, W.OS.tell());
StrtabSection->setOffsets(SecStart, W.OS.tell());
}

const uint64_t SectionHeaderOffset = align(is64Bit() ? Align(8) : Align(4));

// ... then the section header table ...
writeSectionHeader(Asm, SectionIndexMap, SectionOffsets);
writeSectionHeader(Asm);

uint16_t NumSections = support::endian::byte_swap<uint16_t>(
(SectionTable.size() + 1 >= ELF::SHN_LORESERVE) ? (uint16_t)ELF::SHN_UNDEF
Expand Down

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