[LLD][ELF] Move SHF_LINK_ORDER processing earlier in Writer.cpp [NFC]

[peter.smith]

Summary

By moving this step before thunk creation and other processing that depends on the size of sections, we permit removal of duplicates in the .ARM.exidx section.
This is patch 1 of 3 in sequence:

• Move SHF_LINK_ORDER processing earlier in Writer.cpp [NFC]
• Refactor to remove loop copying all Sections in OS->finalize() [NFC]
• Remove Duplicate .ARM.exidx sections

This review is dependent on D38361: [ELF] Stop setting output section size early; for the addition of the OutSecPos field that permits the SHF_LINK_ORDER sections to be sorted before assignAddresses().

[peter.smith]

Rebased in light of changes to D38361. We know use the OutSecOff field for comparisons of .ARM.exidx sections.

[ruiu]

LGTM

ELF/OutputSections.cpp

template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
 
   // Linker scripts may have BYTE()-family commands with which you
   // can write arbitrary bytes to the output. Process them if any.
   for (BaseCommand *Base : SectionCommands)
     if (auto *Data = dyn_cast<ByteCommand>(Base))
       writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
 }
 
-static bool compareByFilePosition(InputSection *A, InputSection *B) {
-  // Synthetic doesn't have link order dependecy, stable_sort will keep it last
-  if (A->kind() == InputSectionBase::Synthetic ||
-      B->kind() == InputSectionBase::Synthetic)
-    return false;
-  InputSection *LA = A->getLinkOrderDep();
-  InputSection *LB = B->getLinkOrderDep();
-  OutputSection *AOut = LA->getParent();
-  OutputSection *BOut = LB->getParent();
-  if (AOut != BOut)
-    return AOut->SectionIndex < BOut->SectionIndex;
-  return LA->OutSecOff < LB->OutSecOff;
-}
-
 template <class ELFT>
 static void finalizeShtGroup(OutputSection *OS,
                              ArrayRef<InputSection *> Sections) {
   assert(Config->Relocatable && Sections.size() == 1);
 
   // sh_link field for SHT_GROUP sections should contain the section index of
   // the symbol table.
   OS->Link = InX::SymTab->getParent()->SectionIndex;
 
   // sh_info then contain index of an entry in symbol table section which
   // provides signature of the section group.
   ObjFile<ELFT> *Obj = Sections[0]->getFile<ELFT>();
   ArrayRef<Symbol *> Symbols = Obj->getSymbols();
   OS->Info = InX::SymTab->getSymbolIndex(Symbols[Sections[0]->Info]);
 }
 
 template <class ELFT> void OutputSection::finalize() {
-  // Link order may be distributed across several InputSectionDescriptions
-  // but sort must consider them all at once.
-  std::vector<InputSection **> ScriptSections;
   std::vector<InputSection *> Sections;
   for (BaseCommand *Base : SectionCommands) {
     if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
-      for (InputSection *&IS : ISD->Sections) {
-        ScriptSections.push_back(&IS);
+      for (InputSection *&IS : ISD->Sections)
         Sections.push_back(IS);
-      }
-    }
-  }
 
   if (Flags & SHF_LINK_ORDER) {
-    std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
-    for (int I = 0, N = Sections.size(); I < N; ++I)
-      *ScriptSections[I] = Sections[I];
-
     // We must preserve the link order dependency of sections with the
     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
     // need to translate the InputSection sh_link to the OutputSection sh_link,
     // all InputSections in the OutputSection have the same dependency.
     if (auto *D = Sections.front()->getLinkOrderDep())
       Link = D->getParent()->SectionIndex;
   }
 

ELF/Writer.cpp

public:
 
   void run();
 
 private:
   void copyLocalSymbols();
   void addSectionSymbols();
   void forEachRelSec(std::function<void(InputSectionBase &)> Fn);
   void sortSections();
+  void resolveShfLinkOrder();
   void sortInputSections();
   void finalizeSections();
   void addPredefinedSections();
   void setReservedSymbolSections();
 
   std::vector<PhdrEntry *> createPhdrs();
   void removeEmptyPTLoad();
   void addPtArmExidx(std::vector<PhdrEntry *> &Phdrs);
while (NonScriptI != E) {
     });
     std::rotate(Pos, NonScriptI, End);
     NonScriptI = End;
   }
 
   Script->adjustSectionsAfterSorting();
 }
 
+static bool compareByFilePosition(InputSection *A, InputSection *B) {
+  // Synthetic doesn't have link order dependecy, stable_sort will keep it last
+  if (A->kind() == InputSectionBase::Synthetic ||
+      B->kind() == InputSectionBase::Synthetic)
+    return false;
+  InputSection *LA = A->getLinkOrderDep();
+  InputSection *LB = B->getLinkOrderDep();
+  OutputSection *AOut = LA->getParent();
+  OutputSection *BOut = LB->getParent();
+  if (AOut != BOut)
+    return AOut->SectionIndex < BOut->SectionIndex;
+  return LA->OutSecOff < LB->OutSecOff;
+}
+
+template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
+  // Link order may be distributed across several InputSectionDescriptions
+  // but sort must consider them all at once.
+  for (OutputSection *Sec : OutputSections) {
+    if (!(Sec->Flags & SHF_LINK_ORDER))
+      continue;
+    std::vector<InputSection **> ScriptSections;
+    std::vector<InputSection *> Sections;
+    for (BaseCommand *Base : Sec->SectionCommands) {
+      if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
+        for (InputSection *&IS : ISD->Sections) {
+          ScriptSections.push_back(&IS);
+          Sections.push_back(IS);
+        }
+      }
+    }
+    std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
+    for (int I = 0, N = Sections.size(); I < N; ++I)
+      *ScriptSections[I] = Sections[I];
+  }
+}
+
 static void applySynthetic(const std::vector<SyntheticSection *> &Sections,
                            std::function<void(SyntheticSection *)> Fn) {
   for (SyntheticSection *SS : Sections)
     if (SS && SS->getParent() && !SS->empty())
       Fn(SS);
 }
 
 // In order to allow users to manipulate linker-synthesized sections,
applySynthetic(
        InX::IgotPlt,     InX::GotPlt,       InX::RelaDyn,  InX::RelaIplt,
        InX::RelaPlt,     InX::Plt,          InX::Iplt,     InX::EhFrameHdr,
        In<ELFT>::VerSym, In<ELFT>::VerNeed, InX::Dynamic},
       [](SyntheticSection *SS) { SS->finalizeContents(); });
 
   if (!Script->HasSectionsCommand && !Config->Relocatable)
     fixSectionAlignments();
 
+  // After link order processing .ARM.exidx sections can be deduplicated, which
+  // needs to be resolved before any other address dependent operation.
+  resolveShfLinkOrder();
+
   // Some architectures need to generate content that depends on the address
   // of InputSections. For example some architectures use small displacements
   // for jump instructions that is is the linker's responsibility for creating
   // range extension thunks for. As the generation of the content may also
   // alter InputSection addresses we must converge to a fixed point.
   if (Target->NeedsThunks || Config->AndroidPackDynRelocs) {
     ThunkCreator TC;
     bool Changed;