Write sections mostly in one pass.
During ELF writing, there is no need to further relax the sections, so we
should not be creating fragments. This patch avoids doing so in all cases
but debug section compression (that is next).
Also, the ELF format is fairly simple to write. We can do a single pass over
the sections to write them out and compute the section header table.
llvm-svn: 236235
diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index ae57f95..725461d 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -78,10 +78,6 @@
static bool isInSymtab(const MCAsmLayout &Layout, const MCSymbolData &Data,
bool Used, bool Renamed);
static bool isLocal(const MCSymbolData &Data, bool isUsedInReloc);
- static bool IsELFMetaDataSection(const MCSectionData &SD);
-
- void writeDataSectionData(MCAssembler &Asm, const MCAsmLayout &Layout,
- const MCSectionData &SD);
/// Helper struct for containing some precomputed information on symbols.
struct ELFSymbolData {
@@ -193,7 +189,8 @@
const MCAsmLayout &Layout);
// Start and end offset of each section
- typedef std::vector<std::pair<uint64_t, uint64_t>> SectionOffsetsTy;
+ typedef std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>
+ SectionOffsetsTy;
void WriteSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
std::vector<const MCSectionELF *> &Sections,
@@ -224,32 +221,17 @@
const SectionIndexMapTy &SectionIndexMap,
const RevGroupMapTy &RevGroupMap);
- void maybeAddToGroup(MCAssembler &Asm,
- ArrayRef<const MCSectionELF *> Sections,
- const RevGroupMapTy &RevGroupMap,
- const MCSectionELF &Section, unsigned Index);
-
- void computeIndexMap(MCAssembler &Asm,
- std::vector<const MCSectionELF *> &Sections,
- SectionIndexMapTy &SectionIndexMap,
- const RevGroupMapTy &RevGroupMap);
-
- void createRelocationSection(MCAssembler &Asm, const MCSectionELF &Sec);
+ const MCSectionELF *createRelocationSection(MCAssembler &Asm,
+ const MCSectionELF &Sec);
void CompressDebugSections(MCAssembler &Asm, MCAsmLayout &Layout);
- void
+ const MCSectionELF *
createSectionHeaderStringTable(MCAssembler &Asm,
std::vector<const MCSectionELF *> &Sections);
- void createStringTable(MCAssembler &Asm,
- std::vector<const MCSectionELF *> &Sections);
-
- // Create the sections that show up in the symbol table. Currently
- // those are the .note.GNU-stack section and the group sections.
- void createIndexedSections(MCAssembler &Asm, const MCAsmLayout &Layout,
- RevGroupMapTy &RevGroupMap,
- std::vector<const MCSectionELF *> &Sections,
- SectionIndexMapTy &SectionIndexMap);
+ const MCSectionELF *
+ createStringTable(MCAssembler &Asm,
+ std::vector<const MCSectionELF *> &Sections);
void ExecutePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
@@ -628,7 +610,7 @@
}
uint64_t SecEnd = OS.tell();
- SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
+ SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
if (ShndxIndexes.empty())
@@ -644,7 +626,7 @@
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = OS.tell();
- SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
+ SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
}
// It is always valid to create a relocation with a symbol. It is preferable
@@ -948,37 +930,6 @@
return true;
}
-void ELFObjectWriter::maybeAddToGroup(MCAssembler &Asm,
- ArrayRef<const MCSectionELF *> Sections,
- const RevGroupMapTy &RevGroupMap,
- const MCSectionELF &Section,
- unsigned Index) {
- const MCSymbol *Sym = Section.getGroup();
- if (!Sym)
- return;
- const MCSectionELF *Group = Sections[RevGroupMap.lookup(Sym) - 1];
- MCSectionData &Data = Asm.getOrCreateSectionData(*Group);
- // FIXME: we could use the previous fragment
- MCDataFragment *F = new MCDataFragment(&Data);
- write(*F, Index);
-}
-
-void ELFObjectWriter::computeIndexMap(
- MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections,
- SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap) {
- for (const MCSectionData &SD : Asm) {
- const MCSectionELF &Section =
- static_cast<const MCSectionELF &>(SD.getSection());
- if (Section.getType() == ELF::SHT_GROUP)
- continue;
- Sections.push_back(&Section);
- unsigned Index = Sections.size();
- SectionIndexMap[&Section] = Index;
- maybeAddToGroup(Asm, Sections, RevGroupMap, Section, Index);
- createRelocationSection(Asm, Section);
- }
-}
-
void ELFObjectWriter::computeSymbolTable(
MCAssembler &Asm, const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap,
@@ -1124,10 +1075,11 @@
UndefinedSymbolData[i].SymbolData->setIndex(Index++);
}
-void ELFObjectWriter::createRelocationSection(MCAssembler &Asm,
- const MCSectionELF &Sec) {
+const MCSectionELF *
+ELFObjectWriter::createRelocationSection(MCAssembler &Asm,
+ const MCSectionELF &Sec) {
if (Relocations[&Sec].empty())
- return;
+ return nullptr;
MCContext &Ctx = Asm.getContext();
const StringRef SectionName = Sec.getSectionName();
@@ -1149,6 +1101,7 @@
Flags, EntrySize, Sec.getGroup(), &Sec);
MCSectionData &RelSD = Asm.getOrCreateSectionData(*RelaSection);
RelSD.setAlignment(is64Bit() ? 8 : 4);
+ return RelaSection;
}
static SmallVector<char, 128>
@@ -1356,7 +1309,7 @@
}
}
-void ELFObjectWriter::createSectionHeaderStringTable(
+const MCSectionELF *ELFObjectWriter::createSectionHeaderStringTable(
MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
const MCSectionELF *ShstrtabSection = Sections[ShstrtabIndex - 1];
@@ -1369,9 +1322,10 @@
}
ShStrTabBuilder.finalize(StringTableBuilder::ELF);
OS << ShStrTabBuilder.data();
+ return ShstrtabSection;
}
-void ELFObjectWriter::createStringTable(
+const MCSectionELF *ELFObjectWriter::createStringTable(
MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
MCContext &Ctx = Asm.getContext();
const MCSectionELF *StrtabSection =
@@ -1380,43 +1334,7 @@
Sections.push_back(StrtabSection);
StringTableIndex = Sections.size();
OS << StrTabBuilder.data();
-}
-
-void ELFObjectWriter::createIndexedSections(
- MCAssembler &Asm, const MCAsmLayout &Layout, RevGroupMapTy &RevGroupMap,
- std::vector<const MCSectionELF *> &Sections,
- SectionIndexMapTy &SectionIndexMap) {
- MCContext &Ctx = Asm.getContext();
-
- const MCSectionELF *ShstrtabSection =
- Ctx.getELFSection(".shstrtab", ELF::SHT_STRTAB, 0);
- Sections.push_back(ShstrtabSection);
- ShstrtabIndex = Sections.size();
- assert(ShstrtabIndex == 1);
-
- // Build the groups
- for (const MCSectionData &SD : Asm) {
- const MCSectionELF &Section =
- static_cast<const MCSectionELF &>(SD.getSection());
- if (!(Section.getFlags() & ELF::SHF_GROUP))
- continue;
-
- const MCSymbol *SignatureSymbol = Section.getGroup();
- Asm.getOrCreateSymbolData(*SignatureSymbol);
- unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
- if (!GroupIdx) {
- const MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
- Sections.push_back(Group);
- GroupIdx = Sections.size();
-
- MCSectionData &Data = Asm.getOrCreateSectionData(*Group);
- Data.setAlignment(4);
- MCDataFragment *F = new MCDataFragment(&Data);
- write(*F, uint32_t(ELF::GRP_COMDAT));
- }
- }
-
- computeIndexMap(Asm, Sections, SectionIndexMap, RevGroupMap);
+ return StrtabSection;
}
void ELFObjectWriter::writeSection(MCAssembler &Asm,
@@ -1472,26 +1390,6 @@
Alignment, Section.getEntrySize());
}
-bool ELFObjectWriter::IsELFMetaDataSection(const MCSectionData &SD) {
- return SD.getOrdinal() == ~UINT32_C(0) &&
- !SD.getSection().isVirtualSection();
-}
-
-void ELFObjectWriter::writeDataSectionData(MCAssembler &Asm,
- const MCAsmLayout &Layout,
- const MCSectionData &SD) {
- if (IsELFMetaDataSection(SD)) {
- for (MCSectionData::const_iterator i = SD.begin(), e = SD.end(); i != e;
- ++i) {
- const MCFragment &F = *i;
- assert(F.getKind() == MCFragment::FT_Data);
- WriteBytes(cast<MCDataFragment>(F).getContents());
- }
- } else {
- Asm.writeSectionData(&SD, Layout);
- }
-}
-
void ELFObjectWriter::writeSectionHeader(
ArrayRef<const MCSectionELF *> Sections, MCAssembler &Asm,
const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
@@ -1512,7 +1410,8 @@
else
GroupSymbolIndex = getSymbolIndexInSymbolTable(Asm, Section.getGroup());
- const std::pair<uint64_t, uint64_t> &Offsets = SectionOffsets[i];
+ const std::pair<uint64_t, uint64_t> &Offsets =
+ SectionOffsets.find(&Section)->second;
uint64_t Size = Section.getType() == ELF::SHT_NOBITS
? Layout.getSectionAddressSize(&SD)
: Offsets.second - Offsets.first;
@@ -1524,56 +1423,107 @@
void ELFObjectWriter::WriteObject(MCAssembler &Asm,
const MCAsmLayout &Layout) {
+ CompressDebugSections(Asm, const_cast<MCAsmLayout &>(Layout));
+
+ std::vector<const MCSectionELF *> Sections;
+ MCContext &Ctx = Asm.getContext();
+ const MCSectionELF *ShstrtabSection =
+ Ctx.getELFSection(".shstrtab", ELF::SHT_STRTAB, 0);
+ Sections.push_back(ShstrtabSection);
+ ShstrtabIndex = Sections.size();
+
RevGroupMapTy RevGroupMap;
SectionIndexMapTy SectionIndexMap;
- CompressDebugSections(Asm, const_cast<MCAsmLayout &>(Layout));
- std::vector<const MCSectionELF *> Sections;
- createIndexedSections(Asm, Layout, RevGroupMap, Sections, SectionIndexMap);
-
- // Compute symbol table information.
- computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
-
- SectionOffsetsTy SectionOffsets;
+ std::map<const MCSymbol *, std::vector<const MCSectionELF *>> GroupMembers;
// Write out the ELF header ...
writeHeader(Asm);
// ... then the sections ...
- SectionOffsets.push_back(std::make_pair(0, 0));
- for (auto I = ++Sections.begin(), E = Sections.end(); I != E; ++I) {
- const MCSectionELF &Sec = **I;
- const MCSectionData &SD = Asm.getOrCreateSectionData(Sec);
+ SectionOffsetsTy SectionOffsets;
+ bool ComputedSymtab = false;
+ for (const MCSectionData &SD : Asm) {
+ const MCSectionELF &Section =
+ static_cast<const MCSectionELF &>(SD.getSection());
+
uint64_t Padding = OffsetToAlignment(OS.tell(), SD.getAlignment());
WriteZeros(Padding);
// Remember the offset into the file for this section.
uint64_t SecStart = OS.tell();
- unsigned Type = Sec.getType();
- if (Type != ELF::SHT_REL && Type != ELF::SHT_RELA)
- writeDataSectionData(Asm, Layout, SD);
- else
- writeRelocations(Asm, *Sec.getAssociatedSection());
+ const MCSymbol *SignatureSymbol = Section.getGroup();
+ unsigned Type = Section.getType();
+ if (Type == ELF::SHT_GROUP) {
+ assert(SignatureSymbol);
+ write(uint32_t(ELF::GRP_COMDAT));
+ for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
+ uint32_t SecIndex = SectionIndexMap.lookup(Member);
+ write(SecIndex);
+ }
+ } else if (Type == ELF::SHT_REL || Type == ELF::SHT_RELA) {
+ if (!ComputedSymtab) {
+ // Compute symbol table information.
+ computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
+ ComputedSymtab = true;
+ }
+ writeRelocations(Asm, *Section.getAssociatedSection());
+ } else {
+ Asm.writeSectionData(&SD, Layout);
+ }
uint64_t SecEnd = OS.tell();
- SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
+ SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
+
+ if (Type == ELF::SHT_GROUP || Type == ELF::SHT_REL || Type == ELF::SHT_RELA)
+ continue;
+
+ const MCSectionELF *RelSection = createRelocationSection(Asm, Section);
+
+ if (SignatureSymbol) {
+ Asm.getOrCreateSymbolData(*SignatureSymbol);
+ unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
+ if (!GroupIdx) {
+ const MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
+ Sections.push_back(Group);
+ GroupIdx = Sections.size();
+ MCSectionData *GroupD = &Asm.getOrCreateSectionData(*Group);
+ GroupD->setAlignment(4);
+ }
+ GroupMembers[SignatureSymbol].push_back(&Section);
+ if (RelSection)
+ GroupMembers[SignatureSymbol].push_back(RelSection);
+ }
+
+ Sections.push_back(&Section);
+ SectionIndexMap[&Section] = Sections.size();
+ if (RelSection) {
+ Sections.push_back(RelSection);
+ SectionIndexMap[RelSection] = Sections.size();
+ }
+ }
+
+ if (!ComputedSymtab) {
+ // Compute symbol table information.
+ computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
+ ComputedSymtab = true;
}
WriteSymbolTable(Asm, Layout, Sections, SectionOffsets);
{
uint64_t SecStart = OS.tell();
- createStringTable(Asm, Sections);
+ const MCSectionELF *Sec = createStringTable(Asm, Sections);
uint64_t SecEnd = OS.tell();
- SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
+ SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
}
{
uint64_t SecStart = OS.tell();
- createSectionHeaderStringTable(Asm, Sections);
+ const MCSectionELF *Sec = createSectionHeaderStringTable(Asm, Sections);
uint64_t SecEnd = OS.tell();
- SectionOffsets[0] = std::make_pair(SecStart, SecEnd);
+ SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
}
uint64_t NaturalAlignment = is64Bit() ? 8 : 4;