[ELF] - Check that we do not mix regular and synthetic sections.

[grimar]

Summary

Our current logic assumes we place synthetic sections
after all regular ones. I lost some time during writing another patch
because placed some code that added sections before code
that adds common section. That broke logic of removeUnusedSyntheticSections
for me.

There are few possible ways to avoid that.
One of them would be to remove dependency on such logic from
removeUnusedSyntheticSections().

The second is this patch, which introduces an assert check to ensure
that does not happen.

[ruiu]

This is probably doing too much to enforce an obscure invariance. Instead, can you remove the assumption that all synthetic sections must be end of the input sections list?

ELF/Writer.cpp

 }
 
 // Initialize Out members.
 template <class ELFT> void Writer<ELFT>::createSyntheticSections() {
   // Initialize all pointers with NULL. This is needed because
   // you can call lld::elf::main more than once as a library.
   memset(&Out::First, 0, sizeof(Out));
 
-  auto Add = [](InputSectionBase *Sec) { InputSections.push_back(Sec); };
+  auto Add = [](InputSectionBase *Sec) {
+    // We assume all synthetic sections are placed to the end,
+    // after regular ones. Do not mix them. We allow merge sections
+    // because them are converted to synthetic mergeable sections later.
+    if (!InputSections.empty()) {
+      auto IsOk = [](InputSectionBase::Kind K) {
+        return K == InputSectionBase::Synthetic || K == InputSectionBase::Merge;
+      };
+      if (IsOk(InputSections.back()->kind()))
+        assert(IsOk(Sec->kind()));
+    }
+    InputSections.push_back(Sec);
+  };
 
   // Create singleton output sections.
   Out::Bss = make<OutputSection>(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
   Out::BssRelRo =
       make<OutputSection>(".bss.rel.ro", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
   In<ELFT>::DynStrTab = make<StringTableSection<ELFT>>(".dynstr", true);
   In<ELFT>::Dynamic = make<DynamicSection<ELFT>>();
   In<ELFT>::RelaDyn = make<RelocationSection<ELFT>>(
       Config->Rela ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
   In<ELFT>::ShStrTab = make<StringTableSection<ELFT>>(".shstrtab", false);
 
   Out::ElfHeader = make<OutputSection>("", 0, SHF_ALLOC);
   Out::ElfHeader->Size = sizeof(Elf_Ehdr);
   Out::ProgramHeaders = make<OutputSection>("", 0, SHF_ALLOC);
   Out::ProgramHeaders->updateAlignment(sizeof(uintX_t));
 
+  InputSection *Common = createCommonSection<ELFT>();
+  if (!Common->Data.empty()) {
+    In<ELFT>::Common = Common;
+    Add(Common);
+  }
+
   if (needsInterpSection<ELFT>()) {
     In<ELFT>::Interp = createInterpSection();
     Add(In<ELFT>::Interp);
   } else {
     In<ELFT>::Interp = nullptr;
   }
 
   if (!Config->Relocatable)
     Add(createCommentSection<ELFT>());
 
   if (Config->Strip != StripPolicy::All) {
     In<ELFT>::StrTab = make<StringTableSection<ELFT>>(".strtab", false);
     In<ELFT>::SymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::StrTab);
   }
 
   if (Config->BuildId != BuildIdKind::None) {
     In<ELFT>::BuildId = make<BuildIdSection<ELFT>>();
     Add(In<ELFT>::BuildId);
   }
 
-  InputSection *Common = createCommonSection<ELFT>();
-  if (!Common->Data.empty()) {
-    In<ELFT>::Common = Common;
-    Add(Common);
-  }
-
   // Add MIPS-specific sections.
   bool HasDynSymTab =
       !Symtab<ELFT>::X->getSharedFiles().empty() || Config->pic() ||
       Config->ExportDynamic;
   if (Config->EMachine == EM_MIPS) {
     if (!Config->Shared && HasDynSymTab) {
       In<ELFT>::MipsRldMap = make<MipsRldMapSection<ELFT>>();
       Add(In<ELFT>::MipsRldMap);