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[ELF] Move gotIndex/pltIndex/globalDynIndex to SymbolAux 

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)

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D116281
This commit is contained in:
Fangrui Song 2022-01-09 13:43:26 -08:00
parent 2bcff220bf
commit 5d3bd7f360
9 changed files with 94 additions and 46 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
lld/ELF/Arch/PPC.cpp
View File

@ -192,7 +192,7 @@ void PPC::writeGotHeader(uint8_t *buf) const {
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,

2
lld/ELF/Arch/PPC64.cpp
View File

@ -1089,7 +1089,7 @@ void PPC64::writePltHeader(uint8_t *buf) const {
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));
}

8
lld/ELF/Arch/X86.cpp
View File

@ -220,7 +220,7 @@ void X86::writePltHeader(uint8_t *buf) const {
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)
@ -502,7 +502,7 @@ 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);
}
@ -600,7 +600,7 @@ void RetpolinePic::writePltHeader(uint8_t *buf) const {
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
@ -659,7 +659,7 @@ void RetpolineNoPic::writePltHeader(uint8_t *buf) const {
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

6
lld/ELF/Arch/X86_64.cpp
View File

@ -417,7 +417,7 @@ void X86_64::writePlt(uint8_t *buf, const Symbol &sym,
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);
}
@ -995,7 +995,7 @@ 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);
}
@ -1107,7 +1107,7 @@ void Retpoline::writePlt(uint8_t *buf, const Symbol &sym,
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);
}

1
lld/ELF/Driver.cpp
View File

@ -96,6 +96,7 @@ bool elf::link(ArrayRef<const char *> args, bool canExitEarly,
sharedFiles.clear();
backwardReferences.clear();
whyExtract.clear();
symAux.clear();
tar = nullptr;
in.reset();

23
lld/ELF/Relocations.cpp
View File

@ -302,8 +302,7 @@ static void replaceWithDefined(Symbol &sym, SectionBase &sec, uint64_t value,
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;
@ -1544,15 +1543,17 @@ static bool handleNonPreemptibleIfunc(Symbol &sym) {
// 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.
@ -1571,6 +1572,10 @@ 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)
@ -1584,9 +1589,10 @@ void elf::postScanRelocations() {
} 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
@ -1653,6 +1659,8 @@ void elf::postScanRelocations() {
if (sym.needsTlsIe && !sym.needsTlsGdToIe)
addTpOffsetGotEntry(sym);
};
assert(symAux.empty());
for (Symbol *sym : symtab->symbols())
fn(*sym);
@ -2173,6 +2181,7 @@ void elf::hexagonTLSSymbolUpdate(ArrayRef<OutputSection *> outputSections) {
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;

11
lld/ELF/Symbols.cpp
View File

@ -59,6 +59,7 @@ 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()) {
@ -153,7 +154,7 @@ uint64_t Symbol::getGotVA() const {
}
uint64_t Symbol::getGotOffset() const {
return gotIndex * target->gotEntrySize;
return getGotIdx() * target->gotEntrySize;
}
uint64_t Symbol::getGotPltVA() const {
@ -164,15 +165,15 @@ uint64_t Symbol::getGotPltVA() const {
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.

45
lld/ELF/Symbols.h
View File

@ -56,6 +56,16 @@ struct StringRefZ {
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:
@ -79,11 +89,10 @@ protected:
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;
@ -191,8 +200,18 @@ public:
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;
@ -301,6 +320,16 @@ public:
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;
@ -504,9 +533,9 @@ union SymbolUnion {
};
// 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>(),

42
lld/ELF/SyntheticSections.cpp
View File

@ -650,14 +650,13 @@ GotSection::GotSection()
}
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;
@ -674,11 +673,11 @@ bool GotSection::addTlsIndex() {
}
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() {
@ -972,12 +971,18 @@ void MipsGotSection::build() {
}
}
// 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) {
@ -1145,7 +1150,8 @@ GotPltSection::GotPltSection()
}
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);
}
@ -1190,7 +1196,7 @@ IgotPltSection::IgotPltSection()
target->gotEntrySize, getIgotPltName()) {}
void IgotPltSection::addEntry(Symbol &sym) {
assert(sym.pltIndex == entries.size());
assert(symAux.back().pltIdx == entries.size());
entries.push_back(&sym);
}
@ -2069,7 +2075,7 @@ static bool sortMipsSymbols(const SymbolTableEntry &l,
// 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();
@ -2546,7 +2552,8 @@ void PltSection::writeTo(uint8_t *buf) {
}
void PltSection::addEntry(Symbol &sym) {
sym.pltIndex = entries.size();
assert(sym.auxIdx == symAux.size() - 1);
symAux.back().pltIdx = entries.size();
entries.push_back(&sym);
}
@ -2592,7 +2599,8 @@ size_t IpltSection::getSize() const {
}
void IpltSection::addEntry(Symbol &sym) {
sym.pltIndex = entries.size();
assert(sym.auxIdx == symAux.size() - 1);
symAux.back().pltIdx = entries.size();
entries.push_back(&sym);
}