Delete dead code. Document live code.

[• espindola]

Summary

The first .got.plt entries are used by the first plt entry to support lazy loading. With that deleted I can run programs with LD_BIND_NOW, but not without.

The value of the first entry does seem unused. I can boostrap and run all tests with this.

ELF/OutputSections.cpp

 }
 
 template <class ELFT> void GotPltSection<ELFT>::finalize() {
   this->Header.sh_size =
       (Target->GotPltHeaderEntriesNum + Entries.size()) * sizeof(uintX_t);
 }
 
 template <class ELFT> void GotPltSection<ELFT>::writeTo(uint8_t *Buf) {
-  Target->writeGotPltHeader(Buf);
   Buf += Target->GotPltHeaderEntriesNum * sizeof(uintX_t);
   for (const SymbolBody *B : Entries) {
     Target->writeGotPlt(Buf, B->getPltVA<ELFT>());
     Buf += sizeof(uintX_t);
   }
 }
 
 template <class ELFT>

ELF/Target.h

 
 class TargetInfo {
 public:
   uint64_t getVAStart() const;
   virtual bool isTlsInitialExecRel(uint32_t Type) const;
   virtual bool isTlsLocalDynamicRel(uint32_t Type) const;
   virtual bool isTlsGlobalDynamicRel(uint32_t Type) const;
   virtual uint32_t getDynRel(uint32_t Type) const { return Type; }
-  virtual void writeGotPltHeader(uint8_t *Buf) const {}
   virtual void writeGotPlt(uint8_t *Buf, uint64_t Plt) const {};
   virtual uint64_t getImplicitAddend(const uint8_t *Buf, uint32_t Type) const;
 
   // If lazy binding is supported, the first entry of the PLT has code
   // to call the dynamic linker to resolve PLT entries the first time
   // they are called. This function writes that code.
   virtual void writePltZero(uint8_t *Buf) const {}
 
public:
   uint32_t PltRel;
   uint32_t RelativeRel;
   uint32_t IRelativeRel;
   uint32_t TlsGotRel = 0;
   uint32_t TlsModuleIndexRel;
   uint32_t TlsOffsetRel;
   unsigned PltEntrySize = 8;
   unsigned PltZeroSize = 0;
+
+  // At least on x86_64 positions 1 and 2 are used by the first plt entry
+  // to support lazy loading. Position 0 seems unused.
   unsigned GotPltHeaderEntriesNum = 3;
+
   uint32_t ThunkSize = 0;
   bool UseLazyBinding = false;
 
   virtual void relaxTlsGdToIe(uint8_t *Loc, uint32_t Type, uint64_t Val) const;
   virtual void relaxTlsGdToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const;
   virtual void relaxTlsIeToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const;
   virtual void relaxTlsLdToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const;
 };

ELF/Target.cpp

 }
 
 namespace {
 class X86TargetInfo final : public TargetInfo {
 public:
   X86TargetInfo();
   RelExpr getRelExpr(uint32_t Type, const SymbolBody &S) const override;
   uint64_t getImplicitAddend(const uint8_t *Buf, uint32_t Type) const override;
-  void writeGotPltHeader(uint8_t *Buf) const override;
   uint32_t getDynRel(uint32_t Type) const override;
   bool isTlsLocalDynamicRel(uint32_t Type) const override;
   bool isTlsGlobalDynamicRel(uint32_t Type) const override;
   bool isTlsInitialExecRel(uint32_t Type) const override;
   void writeGotPlt(uint8_t *Buf, uint64_t Plt) const override;
   void writePltZero(uint8_t *Buf) const override;
   void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   void relocateOne(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
 
   void relaxTlsGdToIe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
   void relaxTlsGdToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
   void relaxTlsIeToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
   void relaxTlsLdToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
 };
 
 class X86_64TargetInfo final : public TargetInfo {
 public:
   X86_64TargetInfo();
   RelExpr getRelExpr(uint32_t Type, const SymbolBody &S) const override;
   uint32_t getDynRel(uint32_t Type) const override;
   bool isTlsLocalDynamicRel(uint32_t Type) const override;
   bool isTlsGlobalDynamicRel(uint32_t Type) const override;
   bool isTlsInitialExecRel(uint32_t Type) const override;
-  void writeGotPltHeader(uint8_t *Buf) const override;
   void writeGotPlt(uint8_t *Buf, uint64_t Plt) const override;
   void writePltZero(uint8_t *Buf) const override;
   void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   void relocateOne(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
 
   void relaxTlsGdToIe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
   void relaxTlsGdToLe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
case R_386_GOTOFF:
     return R_GOTREL;
   case R_386_TLS_LE:
     return R_TLS;
   case R_386_TLS_LE_32:
     return R_NEG_TLS;
   }
 }
 
-void X86TargetInfo::writeGotPltHeader(uint8_t *Buf) const {
-  write32le(Buf, Out<ELF32LE>::Dynamic->getVA());
-}
-
 void X86TargetInfo::writeGotPlt(uint8_t *Buf, uint64_t Plt) const {
   // Entries in .got.plt initially points back to the corresponding
   // PLT entries with a fixed offset to skip the first instruction.
   write32le(Buf, Plt + 6);
 }
 
 uint32_t X86TargetInfo::getDynRel(uint32_t Type) const {
   if (Type == R_386_TLS_LE)
RelExpr X86_64TargetInfo::getRelExpr(uint32_t Type, const SymbolBody &S) const {
   case R_X86_64_GOTPCREL:
   case R_X86_64_GOTPCRELX:
   case R_X86_64_REX_GOTPCRELX:
   case R_X86_64_GOTTPOFF:
     return R_GOT_PC;
   }
 }
 
-void X86_64TargetInfo::writeGotPltHeader(uint8_t *Buf) const {
-  write64le(Buf, Out<ELF64LE>::Dynamic->getVA());
-}
-
 void X86_64TargetInfo::writeGotPlt(uint8_t *Buf, uint64_t Plt) const {
   // See comments in X86TargetInfo::writeGotPlt.
   write32le(Buf, Plt + 6);
 }
 
 void X86_64TargetInfo::writePltZero(uint8_t *Buf) const {
   const uint8_t PltData[] = {
       0xff, 0x35, 0x00, 0x00, 0x00, 0x00, // pushq GOT+8(%rip)