[ELF] Move gotIndex/pltIndex/globalDynIndex to SymbolAux

[MaskRay]

Summary

to decrease sizeof(SymbolUnion) by 8 on ELF64 platforms.

Symbols needing such information are typically 1% or fewer (5134 out of 560520
when linking clang, 19898 out of 5550705 when linking chrome). Storing them
elsewhere can decrease memory usage and symbol initialization time.
There is a ~0.8% saving on max RSS when linking a large program.

Future direction:

• Move some of dynsymIndex/verdefIndex/versionId to SymbolAux
• Support mixed TLSDESC and TLS GD without increasing sizeof(SymbolUnion)

[peter.smith]

I've made some suggestions in inline comments. This type of change does tend to make the implementation slightly harder to read and more fragile, maybe worth getting some data on how much effect this has on linking performance? If it is negligble and there are no plans to follow it up with more changes that depend on it, is it worth doing?

No fundamental objections though. Would be good to hear other opinions too?

lld/ELF/Relocations.cpp

1556

Would it be better to use sym.getPltIdx() here? It is slightly less efficient, but is more consistent with the rest of the code.

2184

These two lines look like they could be replaced with sym->allocIdx()

lld/ELF/Symbols.cpp

62

To reduce the number of dynamic allocations it may be worth using a vector and reserving 1% of the total number of global symbols in one allocation, using your data from clang?

lld/ELF/Symbols.h

92

Will be worth a comment as auxIdx isn't as obvous as gotIndex, pltIndex.

lld/ELF/SyntheticSections.cpp

658–659

Using symAux.back() directly here makes me a bit nervous as the function body of the callee is a long way from the caller. I'd be more comfortable with using symAux[sym.auxIdx].

Maybe an opportunity for sym.setPltIndex(numEntries) that could abstract this away in the same way that the getPltIndex() does.

If that doesn't appeal, perhaps something like assert(sym.auxIdx == symAuxIdx.size() - 1) to make sure that this doesn't go out of synch.

[MaskRay]

There is a ~0.8% saving on max RSS (8-byte saving on SymbolUnion). In addition, when we support mixsed TLSDESC and TLS-GD by introducing a new Symbol member, the change will remove some (estimate: 0.4%~0.8%) memory regression.

# current
% repeat 5 (=time -f "\nCPU: %Us\tReal: %es\tRAM: %MKB" /tmp/c/0 -flavor gnu --threads=4 @response.txt -o /dev/shm/a.out; 'rm' -f /dev/shm/a.out) # current
CPU: 10.26s     Real: 9.96s     RAM: 6894960KB
CPU: 10.77s     Real: 10.01s    RAM: 6896900KB
CPU: 10.62s     Real: 9.95s     RAM: 6892492KB
CPU: 10.73s     Real: 9.97s     RAM: 6892396KB
CPU: 10.86s     Real: 9.97s     RAM: 6892672KB

# with D116281
% repeat 5 (=time -f "\nCPU: %Us\tReal: %es\tRAM: %MKB" /tmp/c/1 -flavor gnu --threads=4 @response.txt -o /dev/shm/a.out; 'rm' -f /dev/shm/a.out) # current                                                                            
CPU: 10.51s     Real: 9.72s     RAM: 6837240KB
CPU: 10.68s     Real: 9.93s     RAM: 6839616KB
CPU: 10.54s     Real: 9.70s     RAM: 6839244KB
CPU: 10.57s     Real: 9.78s     RAM: 6839652KB
CPU: 10.61s     Real: 9.91s     RAM: 6838356KB

On the performance side, there is a tiny improvement:

% hyperfine --warmup 2 --min-runs 40 "numactl -C 20-23 "{/tmp/c/0,/tmp/c/1}" -flavor gnu @response.txt --threads=4 -o lld.8"
Benchmark 1: numactl -C 20-23 /tmp/c/0 -flavor gnu @response.txt --threads=4 -o lld.8
  Time (mean ± σ):      8.882 s ±  0.042 s    [User: 9.752 s, System: 2.557 s]
  Range (min … max):    8.799 s …  8.994 s    40 runs
 
Benchmark 2: numactl -C 20-23 /tmp/c/1 -flavor gnu @response.txt --threads=4 -o lld.8
  Time (mean ± σ):      8.826 s ±  0.044 s    [User: 9.685 s, System: 2.550 s]
  Range (min … max):    8.760 s …  8.940 s    40 runs
 
Summary
  'numactl -C 20-23 /tmp/c/1 -flavor gnu @response.txt --threads=4 -o lld.8' ran
    1.01 ± 0.01 times faster than 'numactl -C 20-23 /tmp/c/0 -flavor gnu @response.txt --threads=4 -o lld.8'

---

When linking chrome, symtab->symbols().size() == 1806181 && symAux.size() == 19898. symAux roughly takes 1%. The dynamic resize does not happen that much.

[MaskRay]

Thanks for the comments! Addressed

lld/ELF/Symbols.cpp

62

When linking chrome, symtab->symbols().size() == 1806181 && symAux.size() == 19898. symAux roughly takes 1%.

When linking clang (release+asserts, all targets), symtab->symbols().size() == 137246 && symAux.size() == 5184.

symAux may be too small to bother with reserve.

lld/ELF/SyntheticSections.cpp

658–659

Thanks! Using assert(sym.auxIdx == symAuxIdx.size() - 1) for now.

set*Index is not appealing to me as that introduces some called-only-once functions.

[MaskRay]

add an assert

[peter.smith]

Thanks for the updates and the data. I don't have any more comments. I've marked as approved from my side, but please wait a bit to see if the other reviewers have any comments or concerns.

lld/ELF/Arch/PPC.cpp

void PPC::writeGotHeader(uint8_t *buf) const {
   // _GLOBAL_OFFSET_TABLE_[0] = _DYNAMIC
   // glibc stores _dl_runtime_resolve in _GLOBAL_OFFSET_TABLE_[1],
   // link_map in _GLOBAL_OFFSET_TABLE_[2].
   write32(buf, mainPart->dynamic->getVA());
 }
 
 void PPC::writeGotPlt(uint8_t *buf, const Symbol &s) const {
   // Address of the symbol resolver stub in .glink .
-  write32(buf, in.plt->getVA() + in.plt->headerSize + 4 * s.pltIndex);
+  write32(buf, in.plt->getVA() + in.plt->headerSize + 4 * s.getPltIdx());
 }
 
 bool PPC::needsThunk(RelExpr expr, RelType type, const InputFile *file,
                      uint64_t branchAddr, const Symbol &s, int64_t a) const {
   if (type != R_PPC_LOCAL24PC && type != R_PPC_REL24 && type != R_PPC_PLTREL24)
     return false;
   if (s.isInPlt())
     return true;

lld/ELF/Arch/PPC64.cpp

void PPC64::writePltHeader(uint8_t *buf) const {
   // following instruction ('mflr r11'). Here we store the offset from that
   // instruction  to the first entry in the GotPlt section.
   int64_t gotPltOffset = in.gotPlt->getVA() - (in.plt->getVA() + 8);
   write64(buf + 52, gotPltOffset);
 }
 
 void PPC64::writePlt(uint8_t *buf, const Symbol &sym,
                      uint64_t /*pltEntryAddr*/) const {
-  int32_t offset = pltHeaderSize + sym.pltIndex * pltEntrySize;
+  int32_t offset = pltHeaderSize + sym.getPltIdx() * pltEntrySize;
   // bl __glink_PLTresolve
   write32(buf, 0x48000000 | ((-offset) & 0x03FFFFFc));
 }
 
 void PPC64::writeIplt(uint8_t *buf, const Symbol &sym,
                       uint64_t /*pltEntryAddr*/) const {
   writePPC64LoadAndBranch(buf, sym.getGotPltVA() - getPPC64TocBase());
 }

lld/ELF/Arch/X86.cpp

void X86::writePltHeader(uint8_t *buf) const {
   memcpy(buf, pltData, sizeof(pltData));
   uint32_t gotPlt = in.gotPlt->getVA();
   write32le(buf + 2, gotPlt + 4);
   write32le(buf + 8, gotPlt + 8);
 }
 
 void X86::writePlt(uint8_t *buf, const Symbol &sym,
                    uint64_t pltEntryAddr) const {
-  unsigned relOff = in.relaPlt->entsize * sym.pltIndex;
+  unsigned relOff = in.relaPlt->entsize * sym.getPltIdx();
   if (config->isPic) {
     const uint8_t inst[] = {
         0xff, 0xa3, 0, 0, 0, 0, // jmp *foo@GOT(%ebx)
         0x68, 0,    0, 0, 0,    // pushl $reloc_offset
         0xe9, 0,    0, 0, 0,    // jmp .PLT0@PC
     };
     memcpy(buf, inst, sizeof(inst));
     write32le(buf + 2, sym.getGotPltVA() - in.gotPlt->getVA());
public:
   static const unsigned IBTPltHeaderSize = 16;
 };
 } // namespace
 
 IntelIBT::IntelIBT() { pltHeaderSize = 0; }
 
 void IntelIBT::writeGotPlt(uint8_t *buf, const Symbol &s) const {
   uint64_t va =
-      in.ibtPlt->getVA() + IBTPltHeaderSize + s.pltIndex * pltEntrySize;
+      in.ibtPlt->getVA() + IBTPltHeaderSize + s.getPltIdx() * pltEntrySize;
   write32le(buf, va);
 }
 
 void IntelIBT::writePlt(uint8_t *buf, const Symbol &sym,
                         uint64_t /*pltEntryAddr*/) const {
   if (config->isPic) {
     const uint8_t inst[] = {
         0xf3, 0x0f, 0x1e, 0xfb,       // endbr32
const uint8_t insn[] = {
       0xc3,                                     // 2e:   ret
       0xcc,                                     // 2f:   int3; padding
   };
   memcpy(buf, insn, sizeof(insn));
 }
 
 void RetpolinePic::writePlt(uint8_t *buf, const Symbol &sym,
                             uint64_t pltEntryAddr) const {
-  unsigned relOff = in.relaPlt->entsize * sym.pltIndex;
+  unsigned relOff = in.relaPlt->entsize * sym.getPltIdx();
   const uint8_t insn[] = {
       0x50,                            // pushl %eax
       0x8b, 0x83, 0,    0,    0,    0, // mov foo@GOT(%ebx), %eax
       0xe8, 0,    0,    0,    0,       // call plt+0x20
       0xe9, 0,    0,    0,    0,       // jmp plt+0x12
       0x68, 0,    0,    0,    0,       // pushl $reloc_offset
       0xe9, 0,    0,    0,    0,       // jmp plt+0
       0xcc, 0xcc, 0xcc, 0xcc, 0xcc,    // int3; padding
void RetpolineNoPic::writePltHeader(uint8_t *buf) const {
 
   uint32_t gotPlt = in.gotPlt->getVA();
   write32le(buf + 2, gotPlt + 4);
   write32le(buf + 8, gotPlt + 8);
 }
 
 void RetpolineNoPic::writePlt(uint8_t *buf, const Symbol &sym,
                               uint64_t pltEntryAddr) const {
-  unsigned relOff = in.relaPlt->entsize * sym.pltIndex;
+  unsigned relOff = in.relaPlt->entsize * sym.getPltIdx();
   const uint8_t insn[] = {
       0x50,                         // 0:  pushl %eax
       0xa1, 0,    0,    0,    0,    // 1:  mov foo_in_GOT, %eax
       0xe8, 0,    0,    0,    0,    // 6:  call plt+0x20
       0xe9, 0,    0,    0,    0,    // b:  jmp plt+0x11
       0x68, 0,    0,    0,    0,    // 10: pushl $reloc_offset
       0xe9, 0,    0,    0,    0,    // 15: jmp plt+0
       0xcc, 0xcc, 0xcc, 0xcc, 0xcc, // 1a: int3; padding

lld/ELF/Arch/X86_64.cpp

void X86_64::writePlt(uint8_t *buf, const Symbol &sym,
   const uint8_t inst[] = {
       0xff, 0x25, 0, 0, 0, 0, // jmpq *got(%rip)
       0x68, 0, 0, 0, 0,       // pushq <relocation index>
       0xe9, 0, 0, 0, 0,       // jmpq plt[0]
   };
   memcpy(buf, inst, sizeof(inst));
 
   write32le(buf + 2, sym.getGotPltVA() - pltEntryAddr - 6);
-  write32le(buf + 7, sym.pltIndex);
+  write32le(buf + 7, sym.getPltIdx());
   write32le(buf + 12, in.plt->getVA() - pltEntryAddr - 16);
 }
 
 RelType X86_64::getDynRel(RelType type) const {
   if (type == R_X86_64_64 || type == R_X86_64_PC64 || type == R_X86_64_SIZE32 ||
       type == R_X86_64_SIZE64)
     return type;
   return R_X86_64_NONE;
public:
   static const unsigned IBTPltHeaderSize = 16;
 };
 } // namespace
 
 IntelIBT::IntelIBT() { pltHeaderSize = 0; }
 
 void IntelIBT::writeGotPlt(uint8_t *buf, const Symbol &s) const {
   uint64_t va =
-      in.ibtPlt->getVA() + IBTPltHeaderSize + s.pltIndex * pltEntrySize;
+      in.ibtPlt->getVA() + IBTPltHeaderSize + s.getPltIdx() * pltEntrySize;
   write64le(buf, va);
 }
 
 void IntelIBT::writePlt(uint8_t *buf, const Symbol &sym,
                         uint64_t pltEntryAddr) const {
   const uint8_t Inst[] = {
       0xf3, 0x0f, 0x1e, 0xfa,       // endbr64
       0xff, 0x25, 0,    0,    0, 0, // jmpq *got(%rip)
void Retpoline::writePlt(uint8_t *buf, const Symbol &sym,
   };
   memcpy(buf, insn, sizeof(insn));
 
   uint64_t off = pltEntryAddr - in.plt->getVA();
 
   write32le(buf + 3, sym.getGotPltVA() - pltEntryAddr - 7);
   write32le(buf + 8, -off - 12 + 32);
   write32le(buf + 13, -off - 17 + 18);
-  write32le(buf + 18, sym.pltIndex);
+  write32le(buf + 18, sym.getPltIdx());
   write32le(buf + 23, -off - 27);
 }
 
 RetpolineZNow::RetpolineZNow() {
   pltHeaderSize = 32;
   pltEntrySize = 16;
   ipltEntrySize = 16;
 }

lld/ELF/Driver.cpp

errorHandler().cleanupCallback = []() {
     archiveFiles.clear();
     binaryFiles.clear();
     bitcodeFiles.clear();
     lazyBitcodeFiles.clear();
     objectFiles.clear();
     sharedFiles.clear();
     backwardReferences.clear();
     whyExtract.clear();
+    symAux.clear();
 
     tar = nullptr;
     in.reset();
 
     partitions.clear();
     partitions.emplace_back();
 
     SharedFile::vernauxNum = 0;

lld/ELF/Relocations.cpp

 // location.
 static void replaceWithDefined(Symbol &sym, SectionBase &sec, uint64_t value,
                                uint64_t size) {
   Symbol old = sym;
 
   sym.replace(Defined{sym.file, sym.getName(), sym.binding, sym.stOther,
                       sym.type, value, size, &sec});
 
-  sym.pltIndex = old.pltIndex;
-  sym.gotIndex = old.gotIndex;
+  sym.auxIdx = old.auxIdx;
   sym.verdefIndex = old.verdefIndex;
   sym.exportDynamic = true;
   sym.isUsedInRegularObj = true;
   // A copy relocated alias may need a GOT entry.
   sym.needsGot = old.needsGot;
 }
 
 // Reserve space in .bss or .bss.rel.ro for copy relocation.
static bool handleNonPreemptibleIfunc(Symbol &sym) {
 
   sym.isInIplt = true;
 
   // Create an Iplt and the associated IRELATIVE relocation pointing to the
   // original section/value pairs. For non-GOT non-PLT relocation case below, we
   // may alter section/value, so create a copy of the symbol to make
   // section/value fixed.
   auto *directSym = makeDefined(cast<Defined>(sym));
+  directSym->allocateAux();
   addPltEntry(*in.iplt, *in.igotPlt, *in.relaIplt, target->iRelativeRel,
               *directSym);
-  sym.pltIndex = directSym->pltIndex;
+  sym.allocateAux();
+  symAux.back().pltIdx = symAux[directSym->auxIdx].pltIdx;
 
   if (sym.hasDirectReloc) {
     // Change the value to the IPLT and redirect all references to it.
     auto &d = cast<Defined>(sym);
     d.section = in.iplt.get();
-    d.value = sym.pltIndex * target->ipltEntrySize;
+    d.value = d.getPltIdx() * target->ipltEntrySize;
     d.size = 0;
     // It's important to set the symbol type here so that dynamic loaders
     // don't try to call the PLT as if it were an ifunc resolver.
     d.type = STT_FUNC;
 
     if (sym.needsGot)
       addGotEntry(sym);
   } else if (sym.needsGot) {
     // Redirect GOT accesses to point to the Igot.
     sym.gotInIgot = true;
   }
   return true;
 }
 
 void elf::postScanRelocations() {
   auto fn = [](Symbol &sym) {
     if (handleNonPreemptibleIfunc(sym))
       return;
+    if (!sym.needsDynReloc())
+      return;
+    sym.allocateAux();
+
     if (sym.needsGot)
       addGotEntry(sym);
     if (sym.needsPlt)
       addPltEntry(*in.plt, *in.gotPlt, *in.relaPlt, target->pltRel, sym);
     if (sym.needsCopy) {
       if (sym.isObject()) {
         addCopyRelSymbol(cast<SharedSymbol>(sym));
         // needsCopy is cleared for sym and its aliases so that in later
         // iterations aliases won't cause redundant copies.
         assert(!sym.needsCopy);
       } else {
         assert(sym.isFunc() && sym.needsPlt);
         if (!sym.isDefined()) {
-          replaceWithDefined(
-              sym, *in.plt,
-              target->pltHeaderSize + target->pltEntrySize * sym.pltIndex, 0);
+          replaceWithDefined(sym, *in.plt,
+                             target->pltHeaderSize +
+                                 target->pltEntrySize * sym.getPltIdx(),
+                             0);
           sym.needsCopy = true;
           if (config->emachine == EM_PPC) {
             // PPC32 canonical PLT entries are at the beginning of .glink
             cast<Defined>(sym).value = in.plt->headerSize;
             in.plt->headerSize += 16;
             cast<PPC32GlinkSection>(*in.plt).canonical_plts.push_back(&sym);
           }
         }
if (sym.needsGotDtprel) {
       in.got->addEntry(sym);
       in.got->relocations.push_back(
           {R_ABS, target->tlsOffsetRel, sym.getGotOffset(), 0, &sym});
     }
 
     if (sym.needsTlsIe && !sym.needsTlsGdToIe)
       addTpOffsetGotEntry(sym);
   };
+
+  assert(symAux.empty());
   for (Symbol *sym : symtab->symbols())
     fn(*sym);
 
   // Local symbols may need the aforementioned non-preemptible ifunc and GOT
   // handling. They don't need regular PLT.
   for (ELFFileBase *file : objectFiles)
     for (Symbol *sym : file->getLocalSymbols())
       fn(*sym);
if (!sym)
     return;
   bool needEntry = true;
   forEachInputSectionDescription(
       outputSections, [&](OutputSection *os, InputSectionDescription *isd) {
         for (InputSection *isec : isd->sections)
           for (Relocation &rel : isec->relocations)
             if (rel.sym->type == llvm::ELF::STT_TLS && rel.expr == R_PLT_PC) {
               if (needEntry) {
+                sym->allocateAux();
                 addPltEntry(*in.plt, *in.gotPlt, *in.relaPlt, target->pltRel,
                             *sym);
                 needEntry = false;
               }
               rel.sym = sym;
             }
       });
 }

lld/ELF/Symbols.h

 struct StringRefZ {
   StringRefZ(const char *s) : data(s), size(-1) {}
   StringRefZ(StringRef s) : data(s.data()), size(s.size()) {}
 
   const char *data;
   const uint32_t size;
 };
 
+// Some index properties of a symbol are stored separately in this auxiliary
+// struct to decrease sizeof(SymbolUnion) in the majority of cases.
+struct SymbolAux {
+  uint32_t gotIdx = -1;
+  uint32_t pltIdx = -1;
+  uint32_t tlsGdIdx = -1;
+};
+
+extern SmallVector<SymbolAux, 0> symAux;
+
 // The base class for real symbol classes.
 class Symbol {
 public:
   enum Kind {
     PlaceholderKind,
     DefinedKind,
     CommonKind,
     SharedKind,
     UndefinedKind,
     LazyArchiveKind,
     LazyObjectKind,
   };
 
   Kind kind() const { return static_cast<Kind>(symbolKind); }
 
   // The file from which this symbol was created.
   InputFile *file;
 
 protected:
   const char *nameData;
   mutable uint32_t nameSize;
 
 public:
+  // A symAux index used to access GOT/PLT entry indexes. This is allocated in
+  // postScanRelocations().
+  uint32_t auxIdx = -1;
   uint32_t dynsymIndex = 0;
-  uint32_t gotIndex = -1;
-  uint32_t pltIndex = -1;
-
-  uint32_t globalDynIndex = -1;
 
   // This field is a index to the symbol's version definition.
   uint16_t verdefIndex = -1;
 
   // Version definition index.
   uint16_t versionId;
 
   // Symbol binding. This is not overwritten by replace() to track
public:
   //
   // For @@, the name has been truncated by insert(). For @, the name has been
   // truncated by Symbol::parseSymbolVersion().
   const char *getVersionSuffix() const {
     (void)getName();
     return nameData + nameSize;
   }
 
-  bool isInGot() const { return gotIndex != -1U; }
-  bool isInPlt() const { return pltIndex != -1U; }
+  uint32_t getGotIdx() const {
+    return auxIdx == uint32_t(-1) ? uint32_t(-1) : symAux[auxIdx].gotIdx;
+  }
+  uint32_t getPltIdx() const {
+    return auxIdx == uint32_t(-1) ? uint32_t(-1) : symAux[auxIdx].pltIdx;
+  }
+  uint32_t getTlsGdIdx() const {
+    return auxIdx == uint32_t(-1) ? uint32_t(-1) : symAux[auxIdx].tlsGdIdx;
+  }
+
+  bool isInGot() const { return getGotIdx() != uint32_t(-1); }
+  bool isInPlt() const { return getPltIdx() != uint32_t(-1); }
 
   uint64_t getVA(int64_t addend = 0) const;
 
   uint64_t getGotOffset() const;
   uint64_t getGotVA() const;
   uint64_t getGotPltOffset() const;
   uint64_t getGotPltVA() const;
   uint64_t getPltVA() const;
public:
   uint8_t needsTlsDesc : 1;
   uint8_t needsTlsGd : 1;
   uint8_t needsTlsGdToIe : 1;
   uint8_t needsTlsLd : 1;
   uint8_t needsGotDtprel : 1;
   uint8_t needsTlsIe : 1;
   uint8_t hasDirectReloc : 1;
 
+  bool needsDynReloc() const {
+    return needsCopy || needsGot || needsPlt || needsTlsDesc || needsTlsGd ||
+           needsTlsGdToIe || needsTlsLd || needsGotDtprel || needsTlsIe;
+  }
+  void allocateAux() {
+    assert(auxIdx == uint32_t(-1));
+    auxIdx = symAux.size();
+    symAux.emplace_back();
+  }
+
   // The partition whose dynamic symbol table contains this symbol's definition.
   uint8_t partition = 1;
 
   bool isSection() const { return type == llvm::ELF::STT_SECTION; }
   bool isTls() const { return type == llvm::ELF::STT_TLS; }
   bool isFunc() const { return type == llvm::ELF::STT_FUNC; }
   bool isGnuIFunc() const { return type == llvm::ELF::STT_GNU_IFUNC; }
   bool isObject() const { return type == llvm::ELF::STT_OBJECT; }
union SymbolUnion {
   alignas(CommonSymbol) char b[sizeof(CommonSymbol)];
   alignas(Undefined) char c[sizeof(Undefined)];
   alignas(SharedSymbol) char d[sizeof(SharedSymbol)];
   alignas(LazyArchive) char e[sizeof(LazyArchive)];
   alignas(LazyObject) char f[sizeof(LazyObject)];
 };
 
 // It is important to keep the size of SymbolUnion small for performance and
-// memory usage reasons. 80 bytes is a soft limit based on the size of Defined
+// memory usage reasons. 72 bytes is a soft limit based on the size of Defined
 // on a 64-bit system.
-static_assert(sizeof(SymbolUnion) <= 80, "SymbolUnion too large");
+static_assert(sizeof(SymbolUnion) <= 72, "SymbolUnion too large");
 
 template <typename T> struct AssertSymbol {
   static_assert(std::is_trivially_destructible<T>(),
                 "Symbol types must be trivially destructible");
   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
   static_assert(alignof(T) <= alignof(SymbolUnion),
                 "SymbolUnion not aligned enough");
 };

lld/ELF/Symbols.cpp

 Defined *ElfSym::relaIpltStart;
 Defined *ElfSym::relaIpltEnd;
 Defined *ElfSym::riscvGlobalPointer;
 Defined *ElfSym::tlsModuleBase;
 DenseMap<const Symbol *, std::pair<const InputFile *, const InputFile *>>
     elf::backwardReferences;
 SmallVector<std::tuple<std::string, const InputFile *, const Symbol &>, 0>
     elf::whyExtract;
+SmallVector<SymbolAux, 0> elf::symAux;
 
 static uint64_t getSymVA(const Symbol &sym, int64_t addend) {
   switch (sym.kind()) {
   case Symbol::DefinedKind: {
     auto &d = cast<Defined>(sym);
     SectionBase *isec = d.section;
 
     // This is an absolute symbol.

 
 uint64_t Symbol::getGotVA() const {
   if (gotInIgot)
     return in.igotPlt->getVA() + getGotPltOffset();
   return in.got->getVA() + getGotOffset();
 }
 
 uint64_t Symbol::getGotOffset() const {
-  return gotIndex * target->gotEntrySize;
+  return getGotIdx() * target->gotEntrySize;
 }
 
 uint64_t Symbol::getGotPltVA() const {
   if (isInIplt)
     return in.igotPlt->getVA() + getGotPltOffset();
   return in.gotPlt->getVA() + getGotPltOffset();
 }
 
 uint64_t Symbol::getGotPltOffset() const {
   if (isInIplt)
-    return pltIndex * target->gotEntrySize;
-  return (pltIndex + target->gotPltHeaderEntriesNum) * target->gotEntrySize;
+    return getPltIdx() * target->gotEntrySize;
+  return (getPltIdx() + target->gotPltHeaderEntriesNum) * target->gotEntrySize;
 }
 
 uint64_t Symbol::getPltVA() const {
   uint64_t outVA = isInIplt
-                       ? in.iplt->getVA() + pltIndex * target->ipltEntrySize
+                       ? in.iplt->getVA() + getPltIdx() * target->ipltEntrySize
                        : in.plt->getVA() + in.plt->headerSize +
-                             pltIndex * target->pltEntrySize;
+                             getPltIdx() * target->pltEntrySize;
 
   // While linking microMIPS code PLT code are always microMIPS
   // code. Set the less-significant bit to track that fact.
   // See detailed comment in the `getSymVA` function.
   if (config->emachine == EM_MIPS && isMicroMips())
     outVA |= 1;
   return outVA;
 }

lld/ELF/SyntheticSections.cpp

 
 GotSection::GotSection()
     : SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS,
                        target->gotEntrySize, ".got") {
   numEntries = target->gotHeaderEntriesNum;
 }
 
 void GotSection::addEntry(Symbol &sym) {
-  sym.gotIndex = numEntries;
-  ++numEntries;
+  assert(sym.auxIdx == symAux.size() - 1);
+  symAux.back().gotIdx = numEntries++;
 }
 
 bool GotSection::addDynTlsEntry(Symbol &sym) {
-  if (sym.globalDynIndex != -1U)
-    return false;
-  sym.globalDynIndex = numEntries;
+  assert(sym.auxIdx == symAux.size() - 1);
+  symAux.back().tlsGdIdx = numEntries;
   // Global Dynamic TLS entries take two GOT slots.
   numEntries += 2;
   return true;
 }
 
 // Reserves TLS entries for a TLS module ID and a TLS block offset.
 // In total it takes two GOT slots.
 bool GotSection::addTlsIndex() {
   if (tlsIndexOff != uint32_t(-1))
     return false;
   tlsIndexOff = numEntries * config->wordsize;
   numEntries += 2;
   return true;
 }
 
 uint64_t GotSection::getGlobalDynAddr(const Symbol &b) const {
-  return this->getVA() + b.globalDynIndex * config->wordsize;
+  return this->getVA() + b.getTlsGdIdx() * config->wordsize;
 }
 
 uint64_t GotSection::getGlobalDynOffset(const Symbol &b) const {
-  return b.globalDynIndex * config->wordsize;
+  return b.getTlsGdIdx() * config->wordsize;
 }
 
 void GotSection::finalizeContents() {
   if (config->emachine == EM_PPC64 &&
       numEntries <= target->gotHeaderEntriesNum && !ElfSym::globalOffsetTable)
     size = 0;
   else
     size = numEntries * config->wordsize;
for (FileGot &got : gots) {
     for (auto &p: got.tls)
       p.second = index++;
     for (auto &p: got.dynTlsSymbols) {
       p.second = index;
       index += 2;
     }
   }
 
-  // Update Symbol::gotIndex field to use this
+  // Update SymbolAux::gotIdx field to use this
   // value later in the `sortMipsSymbols` function.
-  for (auto &p : primGot->global)
-    p.first->gotIndex = p.second;
-  for (auto &p : primGot->relocs)
-    p.first->gotIndex = p.second;
+  for (auto &p : primGot->global) {
+    if (p.first->auxIdx == uint32_t(-1))
+      p.first->allocateAux();
+    symAux.back().gotIdx = p.second;
+  }
+  for (auto &p : primGot->relocs) {
+    if (p.first->auxIdx == uint32_t(-1))
+      p.first->allocateAux();
+    symAux.back().gotIdx = p.second;
+  }
 
   // Create dynamic relocations.
   for (FileGot &got : gots) {
     // Create dynamic relocations for TLS entries.
     for (std::pair<Symbol *, size_t> &p : got.tls) {
       Symbol *s = p.first;
       uint64_t offset = p.second * config->wordsize;
       // When building a shared library we still need a dynamic relocation
if (config->emachine == EM_PPC) {
     name = ".plt";
   } else if (config->emachine == EM_PPC64) {
     type = SHT_NOBITS;
     name = ".plt";
   }
 }
 
 void GotPltSection::addEntry(Symbol &sym) {
-  assert(sym.pltIndex == entries.size());
+  assert(sym.auxIdx == symAux.size() - 1 &&
+         symAux.back().pltIdx == entries.size());
   entries.push_back(&sym);
 }
 
 size_t GotPltSection::getSize() const {
   return (target->gotPltHeaderEntriesNum + entries.size()) *
          target->gotEntrySize;
 }
 

 // On PowerPC64 the GotPltSection type is SHT_NOBITS so we have to follow suit
 // with the IgotPltSection.
 IgotPltSection::IgotPltSection()
     : SyntheticSection(SHF_ALLOC | SHF_WRITE,
                        config->emachine == EM_PPC64 ? SHT_NOBITS : SHT_PROGBITS,
                        target->gotEntrySize, getIgotPltName()) {}
 
 void IgotPltSection::addEntry(Symbol &sym) {
-  assert(sym.pltIndex == entries.size());
+  assert(symAux.back().pltIdx == entries.size());
   entries.push_back(&sym);
 }
 
 size_t IgotPltSection::getSize() const {
   return entries.size() * target->gotEntrySize;
 }
 
 void IgotPltSection::writeTo(uint8_t *buf) {

 // See "Global Offset Table" in Chapter 5 in the following document
 // for detailed description:
 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
 static bool sortMipsSymbols(const SymbolTableEntry &l,
                             const SymbolTableEntry &r) {
   // Sort entries related to non-local preemptible symbols by GOT indexes.
   // All other entries go to the beginning of a dynsym in arbitrary order.
   if (l.sym->isInGot() && r.sym->isInGot())
-    return l.sym->gotIndex < r.sym->gotIndex;
+    return l.sym->getGotIdx() < r.sym->getGotIdx();
   if (!l.sym->isInGot() && !r.sym->isInGot())
     return false;
   return !l.sym->isInGot();
 }
 
 void SymbolTableBaseSection::finalizeContents() {
   if (OutputSection *sec = strTabSec.getParent())
     getParent()->link = sec->sectionIndex;
void PltSection::writeTo(uint8_t *buf) {
 
   for (const Symbol *sym : entries) {
     target->writePlt(buf + off, *sym, getVA() + off);
     off += target->pltEntrySize;
   }
 }
 
 void PltSection::addEntry(Symbol &sym) {
-  sym.pltIndex = entries.size();
+  assert(sym.auxIdx == symAux.size() - 1);
+  symAux.back().pltIdx = entries.size();
   entries.push_back(&sym);
 }
 
 size_t PltSection::getSize() const {
   return headerSize + entries.size() * target->pltEntrySize;
 }
 
 bool PltSection::isNeeded() const {
void IpltSection::writeTo(uint8_t *buf) {
   }
 }
 
 size_t IpltSection::getSize() const {
   return entries.size() * target->ipltEntrySize;
 }
 
 void IpltSection::addEntry(Symbol &sym) {
-  sym.pltIndex = entries.size();
+  assert(sym.auxIdx == symAux.size() - 1);
+  symAux.back().pltIdx = entries.size();
   entries.push_back(&sym);
 }
 
 // ARM uses mapping symbols to aid disassembly.
 void IpltSection::addSymbols() {
   size_t off = 0;
   for (size_t i = 0, e = entries.size(); i != e; ++i) {
     target->addPltSymbols(*this, off);