Make SectionPiece 8 bytes smaller on LP64.

[ruiu]

Summary

This patch makes SectionPiece class 8 bytes smaller on platforms
on which pointer size is 8 bytes. Sean suggested in a post commit
review for r270340 that this could make a differentce, and it
actually is. Time to link clang (with debug info) improved from
6.725 seconds to 6.589 seconds or by 2%.

[• rafael]

LGTM.

The largest speedup I noticed when using two ltoed llds was when
linking scylladb. From 4.246328024 to 4.185731084. 1.014477026541795
times faster.

Cheers,
Rafael

lld/trunk/ELF/InputSection.h

public:
   std::vector<Relocation> Relocations;
 };
 
 template <class ELFT> InputSectionBase<ELFT> InputSectionBase<ELFT>::Discarded;
 
 // SectionPiece represents a piece of splittable section contents.
 struct SectionPiece {
   SectionPiece(size_t Off, ArrayRef<uint8_t> Data)
-      : InputOff(Off), Data(Data), Live(!Config->GcSections) {}
-  size_t size() const { return Data.size(); }
+      : InputOff(Off), Data((uint8_t *)Data.data()), Size(Data.size()),
+        Live(!Config->GcSections) {}
+
+  ArrayRef<uint8_t> data() { return {Data, Size}; }
+  size_t size() const { return Size; }
 
   size_t InputOff;
   size_t OutputOff = -1;
-  ArrayRef<uint8_t> Data; // slice of the input section
-  bool Live;
+
+private:
+  // We use bitfields because SplitInputSection is accessed by
+  // std::upper_bound very often.
+  // We want to save bits to make it cache friendly.
+  uint8_t *Data;
+  uint32_t Size : 31;
+
+public:
+  uint32_t Live : 1;
 };
 
 // Usually sections are copied to the output as atomic chunks of data,
 // but some special types of sections are split into small pieces of data
 // and each piece is copied to a different place in the output.
 // This class represents such special sections.
 template <class ELFT> class SplitInputSection : public InputSectionBase<ELFT> {
   typedef typename ELFT::Shdr Elf_Shdr;

lld/trunk/ELF/OutputSections.cpp

 // CIE records from input object files are uniquified by their contents
 // and where their relocations point to.
 template <class ELFT>
 template <class RelTy>
 CieRecord *EhOutputSection<ELFT>::addCie(SectionPiece &Piece,
                                          EhInputSection<ELFT> *Sec,
                                          ArrayRef<RelTy> &Rels) {
   const endianness E = ELFT::TargetEndianness;
-  if (read32<E>(Piece.Data.data() + 4) != 0)
+  if (read32<E>(Piece.data().data() + 4) != 0)
     fatal("CIE expected at beginning of .eh_frame: " + Sec->getSectionName());
 
   SymbolBody *Personality = nullptr;
   if (const RelTy *Rel = getReloc(Piece.InputOff, Piece.size(), Rels))
     Personality = &Sec->getFile()->getRelocTargetSym(*Rel);
 
   // Search for an existing CIE by CIE contents/relocation target pair.
-  CieRecord *Cie = &CieMap[{Piece.Data, Personality}];
+  CieRecord *Cie = &CieMap[{Piece.data(), Personality}];
 
   // If not found, create a new one.
   if (Cie->Piece == nullptr) {
     Cie->Piece = &Piece;
     Cies.push_back(Cie);
   }
   return Cie;
 }

 template <class RelTy>
 void EhOutputSection<ELFT>::addSectionAux(EhInputSection<ELFT> *Sec,
                                           ArrayRef<RelTy> Rels) {
   const endianness E = ELFT::TargetEndianness;
 
   DenseMap<size_t, CieRecord *> OffsetToCie;
   for (SectionPiece &Piece : Sec->Pieces) {
     // The empty record is the end marker.
-    if (Piece.Data.size() == 4)
+    if (Piece.size() == 4)
       return;
 
     size_t Offset = Piece.InputOff;
-    uint32_t ID = read32<E>(Piece.Data.data() + 4);
+    uint32_t ID = read32<E>(Piece.data().data() + 4);
     if (ID == 0) {
       OffsetToCie[Offset] = addCie(Piece, Sec, Rels);
       continue;
     }
 
     uint32_t CieOffset = Offset + 4 - ID;
     CieRecord *Cie = OffsetToCie[CieOffset];
     if (!Cie)
if ((Enc & 0x70) == DW_EH_PE_pcrel)
     return Addr + this->getVA() + Off;
   fatal("unknown FDE size relative encoding");
 }
 
 template <class ELFT> void EhOutputSection<ELFT>::writeTo(uint8_t *Buf) {
   const endianness E = ELFT::TargetEndianness;
   for (CieRecord *Cie : Cies) {
     size_t CieOffset = Cie->Piece->OutputOff;
-    writeCieFde<ELFT>(Buf + CieOffset, Cie->Piece->Data);
+    writeCieFde<ELFT>(Buf + CieOffset, Cie->Piece->data());
 
     for (SectionPiece *Fde : Cie->FdePieces) {
       size_t Off = Fde->OutputOff;
-      writeCieFde<ELFT>(Buf + Off, Fde->Data);
+      writeCieFde<ELFT>(Buf + Off, Fde->data());
 
       // FDE's second word should have the offset to an associated CIE.
       // Write it.
       write32<E>(Buf + Off + 4, Off + 4 - CieOffset);
     }
   }
 
   for (EhInputSection<ELFT> *S : Sections)
     S->relocate(Buf, nullptr);
 
   // Construct .eh_frame_hdr. .eh_frame_hdr is a binary search table
   // to get a FDE from an address to which FDE is applied. So here
   // we obtain two addresses and pass them to EhFrameHdr object.
   if (Out<ELFT>::EhFrameHdr) {
     for (CieRecord *Cie : Cies) {
-      uint8_t Enc = getFdeEncoding<ELFT>(Cie->Piece->Data);
+      uint8_t Enc = getFdeEncoding<ELFT>(Cie->Piece->data());
       for (SectionPiece *Fde : Cie->FdePieces) {
         uintX_t Pc = getFdePc(Buf, Fde->OutputOff, Enc);
         uintX_t FdeVA = this->getVA() + Fde->OutputOff;
         Out<ELFT>::EhFrameHdr->addFde(Pc, FdeVA);
       }
     }
   }
 }
void MergeOutputSection<ELFT>::addSection(InputSectionBase<ELFT> *C) {
   this->updateAlign(Sec->Align);
   this->Header.sh_entsize = Sec->getSectionHdr()->sh_entsize;
 
   bool IsString = this->Header.sh_flags & SHF_STRINGS;
 
   for (SectionPiece &Piece : Sec->Pieces) {
     if (!Piece.Live)
       continue;
-    uintX_t OutputOffset = Builder.add(toStringRef(Piece.Data));
+    uintX_t OutputOffset = Builder.add(toStringRef(Piece.data()));
     if (!IsString || !shouldTailMerge())
       Piece.OutputOff = OutputOffset;
   }
 }
 
 template <class ELFT>
 unsigned MergeOutputSection<ELFT>::getOffset(StringRef Val) {
   return Builder.getOffset(Val);